Spatial analysis of scrub typhus infection and its association with environmental and socioeconomic factors in Taiwan

Epidemiologic changes of a longitudinal surveillance study spanning 51 years of scrub typhus in mainland China

Scrub typhus may be one of the world's most prevalent, neglected and serious, but easily treatable, febrile diseases. It has become a significant potential threat to public health in China. In this study we used national disease surveillance data to analyze the incidence and spatial-temporal distribution of scrub typhus in mainland China during 1952-1989 and 2006-2018. Descriptive epidemiological methods and spatial-temporal epidemiological methods were used to investigate the epidemiological trends and identify high-risk regions of scrub typhus infection. Over the 51-year period, a total of 182,991 cases and 186 deaths were notified. The average annual incidence was 0.13 cases/100,000 population during 1952-1989. The incidence increased sharply from 0.09/100,000 population in 2006 to 1.93/100,000 population in 2018 and then exponentially increased after 2006. The incidence was significantly higher in females than males (χ2 = 426.32, P < 0.001). Farmers had a higher incidence of scrub typhus than non-farmers (χ2 = 684.58, P < 0.001). The majority of cases each year were reported between July and November with peak incidence occurring during October each year. The trend surface analysis showed that the incidence of scrub typhus increased gradually from north to south, and from east and west to the central area. The spatial autocorrelation analysis showed that a spatial positive correlation existed in the prevalence of scrub typhus on a national scale, which had the characteristic of aggregated distribution (I = 0.533, P < 0.05). LISA analysis showed hotspots (High-High) were primarily located in the southern and southwestern provinces of China with the geographical area expanding annually. These findings provide scientific evidence for the surveillance and control of scrub typhus which may contribute to targeted strategies and measures for the government.

Exploring the effects and interactions of meteorological factors on the incidence of scrub typhus in Ganzhou City, 2008-2021

BACKGROUND

Scrub typhus poses a substantial risk to human life and wellbeing as it is transmitted by vectors. Although the correlation between climate and vector-borne diseases has been investigated, the impact of climate on scrub typhus remains inadequately comprehended. The objective of this study is to investigate the influence of meteorological conditions on the occurrence of scrub typhus in Ganzhou City, Jiangxi Province.  METHODS: From January 1, 2008 to December 31, 2021, we gathered weekly records of scrub typhus prevalence alongside meteorological data in Ganzhou city. In order to investigate the correlation between meteorological factors and scrub typhus incidence, we utilized distributional lag nonlinear models and generalized additive models for our analysis.

RESULTS

Between 2008 and 2021, a total of 5942 cases of scrub typhus were recorded in Ganzhou City. The number of females affected exceeded that of males, with a male-to-female ratio of 1:1.86. Based on the median values of these meteorological factors, the highest relative risk for scrub typhus occurrence was observed when the weekly average temperature reached 26 °C, the weekly average relative humidity was 75%, the weekly average sunshine duration lasted for 2 h, and the weekly mean wind speed measured 2 m/s. The respective relative risks for these factors were calculated as 3.816 (95% CI: 1.395-10.438), 1.107 (95% CI: 1.008-1.217), 2.063 (95% CI: 1.022-4.165), and 1.284 (95% CI: 1.01-1.632). Interaction analyses showed that the risk of scrub typhus infection in Ganzhou city escalates with higher weekly average temperature and sunshine duration.

CONCLUSION

The findings of our investigation provide evidence of a correlation between environmental factors and the occurrence of scrub typhus. As a suggestion, utilizing environmental factors as early indicators could be recommended for initiating control measures and response strategies.

Spatiotemporal epidemiology and risk factors of scrub typhus in Hainan Province, China, 2011-2020

Background

The re-emergence of scrub typhus in the southern provinces of China in recent decades has been validated, thereby attracting the attention of public health authorities. There has been a spatial and temporal expansion of scrub typhus in Hainan Province, but the epidemiological characteristics, environmental drivers, and potential high-risk areas for scrub typhus have not yet been investigated.

Objective

The aims of this study were to characterize the spatiotemporal epidemiology of scrub typhus, identify dominant environmental risk factors, and map potential risk areas in Hainan Province from 2011 to 2020.

Methods

The spatiotemporal dynamics of scrub typhus in Hainan Province between 2011 and 2020 were analyzed using spatial analyses and seasonal-trend decomposition using regression (STR). The maximum entropy (MaxEnt) model was applied to determine the key environmental predictors and environmentally suitable areas for scrub typhus, and the demographic diversity of the predicted suitable zones was evaluated.

Results

During 2011-2020, 3260 scrub typhus cases were recorded in Hainan Province. The number of scrub typhus cases increased continuously each year, particularly among farmers (67.61%) and individuals aged 50-59 years (23.25%) who were identified as high-risk groups. A dual epidemic peak was detected, emerging annually from April to June and from July to October. The MaxEnt-based risk map illustrated that highly suitable areas, accounting for 25.36% of the total area, were mainly distributed in the northeastern part of Hainan Province, where 75.43% of the total population lived. Jackknife tests revealed that ground surface temperature, elevation, cumulative precipitation, evaporation, land cover, population density, and ratio of dependents were the most significant environmental factors.

Conclusion

In this study, we gained insights into the spatiotemporal epidemiological dynamics, pivotal environmental drivers, and potential risk map of scrub typhus in Hainan Province. These results have important implications for researchers and public health officials in guiding future prevention and control strategies for scrub typhus.

The spatio-temporal distribution of acute encephalitis syndrome and its association with climate and landcover in Vietnam

BACKGROUND

Acute encephalitis syndrome (AES) differs in its spatio-temporal distribution in Vietnam with the highest incidence seen during the summer months in the northern provinces. AES has multiple aetiologies, and the cause remains unknown in many cases. While vector-borne disease such as Japanese encephalitis and dengue virus and non-vector-borne diseases such as influenza and enterovirus show evidence of seasonality, associations with climate variables and the spatio-temporal distribution in Vietnam differs between these. The aim of this study was therefore to understand the spatio-temporal distribution of, and risk factors for AES in Vietnam to help hypothesise the aetiology.

METHODS

The number of monthly cases per province for AES, meningitis and diseases including dengue fever; influenza-like-illness (ILI); hand, foot, and mouth disease (HFMD); and Streptococcus suis were obtained from the General Department for Preventive Medicine (GDPM) from 1998-2016. Covariates including climate, normalized difference vegetation index (NDVI), elevation, the number of pigs, socio-demographics, JEV vaccination coverage and the number of hospitals were also collected. Spatio-temporal multivariable mixed-effects negative binomial Bayesian models with an outcome of the number of cases of AES, a combination of the covariates and harmonic terms to determine the magnitude of seasonality were developed.

RESULTS

The national monthly incidence of AES declined by 63.3% over the study period. However, incidence increased in some provinces, particularly in the Northwest region. In northern Vietnam, the incidence peaked in the summer months in contrast to the southern provinces where incidence remained relatively constant throughout the year. The incidence of meningitis, ILI and S. suis infection; temperature, relative humidity with no lag, NDVI at a lag of one month, and the number of pigs per 100,000 population were positively associated with the number of cases of AES in all models in which these covariates were included.

CONCLUSIONS

The positive correlation of AES with temperature and humidity suggest that a number of cases may be due to vector-borne diseases, suggesting a need to focus on vaccination campaigns. However, further surveillance and research are recommended to investigate other possible aetiologies such as S. suis or Orientia tsutsugamushi.

Prediction of risk factors for scrub typhus from 2006 to 2019 based on random forest model in Guangzhou, China

OBJECTIVES

Scrub typhus is an increasingly serious public health problem, which is becoming the most common vector-borne disease in Guangzhou. This study aimed to analyse the correlation between scrub typhus incidence and potential factors and rank the importance of influential factors.

METHODS

We collected monthly scrub typhus cases, meteorological variables, rodent density (RD), Normalised Difference Vegetation Index (NDVI), and land use type in Guangzhou from 2006 to 2019. Correlation analysis and a random forest model were used to identify the risk factors for scrub typhus and predict the importance rank of influencing factors related to scrub typhus incidence.

RESULTS

The epidemiological results of the scrub typhus cases in Guangzhou between 2006 and 2019 showed that the incidence rate was on the rise. The results of correlation analysis revealed that a positive relationship between scrub typhus incidence and meteorological factors of mean temperature (T_mean ), accumulative rainfall (RF), relative humidity (RH), sunshine hours (SH), and NDVI, RD, population density, and green land coverage area (all p < 0.001). Additionally, we tested the relationship between the incidence of scrub typhus and the lagging meteorological factors through cross-correlation function, and found that incidence was positively correlated with 1-month lag T_mean , 2-month lag RF, 2-month lag RH, and 6-month lag SH (all p < 0.001). Based on the random forest model, we found that the T_mean was the most important predictor among the influential factors, followed by NDVI.

CONCLUSIONS

Meteorological factors, NDVI, RD, and land use type jointly affect the incidence of scrub typhus in Guangzhou. Our results provide a better understanding of the influential factors correlated with scrub typus, which can improve our capacity for biological monitoring and help public health authorities to formulate disease control strategies.

Spatiotemporal dynamics and environmental determinants of scrub typhus in Anhui Province, China, 2010-2020

This study aims to describe the epidemiological characteristics of scrub typhus, detect the spatio-temporal patterns of scrub typhus at county level, and explore the associations between the environmental variables and scrub typhus cases in Anhui Province. Time-series analysis, spatial autocorrelation analysis, and space-time scan statistics were used to explore the characteristics and spatiotemporal patterns of the scrub typhus in Anhui Province. Negative binomial regression analysis was used to explore the association between scrub typhus and environmental variables. A total of 16,568 clinically diagnosed and laboratory-confirmed cases were reported from 104 counties of 16 prefecture-level cities. The number of female cases was higher than male cases, with a proportion of 1.32:1. And the proportion of cases over 65 years old was the highest, accounting for 33.8% of the total cases. Two primary and five secondary high-risk clusters were detected in the northwestern, northeastern, and central-eastern parts of Anhui Province. The number of cases in primary and secondary high-risk clusters accounted for 60.27% and 3.00%, respectively. Scrub typhus incidence in Anhui Province was positively correlated with the population density, normalized difference vegetation index, and several meteorological variables. The mean monthly sunshine duration with 3 lags (SSD_lag3), mean monthly ground surface temperature with 1 lag (GST_lag1), and mean monthly relative humidity with 3 lags (RHU_lag3) had the most significant association with increased cases of scrub typhus. Our findings indicate that public health interventions need to be focused on the elderly farmers in north of the Huai River in Anhui Province.

Using ecological niche modeling to predict the potential distribution of scrub typhus in Fujian Province, China

BACKGROUND

Despite the increasing number of cases of scrub typhus and its expanding geographical distribution in China, its potential distribution in Fujian Province, which is endemic for the disease, has yet to be investigated.

METHODS

A negative binomial regression model for panel data mainly comprising meteorological, socioeconomic and land cover variables was used to determine the risk factors for the occurrence of scrub typhus. Maximum entropy modeling was used to identify the key predictive variables of scrub typhus and their ranges, map the suitability of different environments for the disease, and estimate the proportion of the population at different levels of infection risk.

RESULTS

The final multivariate negative binomial regression model for panel data showed that the annual mean normalized difference vegetation index had the strongest correlation with the number of scrub typhus cases. With each 0.1% rise in shrubland and 1% rise in barren land there was a 75.0% and 37.0% increase in monthly scrub typhus cases, respectively. In contrast, each unit rise in mean wind speed in the previous 2 months and each 1% increase in water bodies corresponded to a decrease of 40.0% and 4.0% in monthly scrub typhus cases, respectively. The predictions of the maximum entropy model were robust, and the average area under the curve value was as high as 0.864. The best predictive variables for scrub typhus occurrence were population density, annual mean normalized difference vegetation index, and land cover types. The projected potentially most suitable areas for scrub typhus were widely distributed across the eastern coastal area of Fujian Province, with highly suitable and moderately suitable areas accounting for 16.14% and 9.42%, respectively. Of the total human population of the province, 81.63% reside in highly suitable areas for scrub typhus.

CONCLUSIONS

These findings could help deepen our understanding of the risk factors of scrub typhus, and provide information for public health authorities in Fujian Province to develop more effective surveillance and control strategies in identified high risk areas in Fujian Province.

Seroprevalence of scrub typhus and other rickettsial diseases among the household rodents of Mizoram, North-East India

In the last decade, scrub typhus, a zoonotic disease has emerged as a major health concern in Mizoram, a North-East Indian state that shares international borders with Myanmar and Bangladesh. Mizoram is a biodiversity hotspot and >85% of the state is under forest cover, which provides an ideal ecological niche for the rodents and mites to transmit scrub typhus and other rickettsial infections. Using the Weil-Felix test, a serosurvey of household rodents from 41 villages spread across all the 11 districts in Mizoram was undertaken to gather important insights on their role in disease transmission. Furthermore, the chigger and flea indexes were calculated from the captured rodents. The 163 rodents captured belonged to five species; the highest numbers were from Rattus tanezumi (87), followed by Rattus rattus (41), Mus musculus (17), Suncus murinus (16), and Bandicota bengalensis (2). The rickettsial seropositivity of the captured rodents was 66.26% (108 out of 163 were positive). Among the 163 rodents, sera of 75 (46.01%), 61 (37.42%), and 73 (44.78%) were reactive to OXK, OX19, and OX2 antigens, respectively. The chigger and flea index were 17.92 and 0.16, respectively. Overall, the study has given important insights into the risk of multiple rickettsial infections that household rodents could transmit in Mizoram. These findings indicate the need for the urgent implementation of effective rodent control strategies in Mizoram.

Occupational health surveillance and detection of emerging occupational diseases among Taiwan farmers, through analysis of national-based farmers' and medico-administrative databases

OBJECTIVES

The purpose of this study was to identify occupational injuries and diseases associated with agriculture in Asia, to provide a reference for prevention and hypotheses for future research.

METHODS

We matched data on agricultural workers (n = 963,124) enrolled in Taiwan's national Farmers Health Insurance since its inception in 1989, to general population controls of the same age, gender, and township. The study population was linked to the National Health Insurance Research Database from 2001 to 2016 for inpatient cases. Logistic regression was used to assess odds ratios for outcomes.

RESULTS

Farmers had 2.76 times the risk of mycotic corneal ulcer (95% CI: 1.96-3.87) and 1.65 times the risk of typhus fever infections (95% CI: 1.47-1.85) compared to the general population. The odds ratio for poisonous animal bites was 2.22 (95% CI: 2.07-2.38), for falling into a storm drain or manhole was 2.04 (95% CI: 1.30-3.20), and for toxic effects from pesticides was 2.01 (95% CI: 1.92-2.11). The toxic effects of organophosphate and carbamate insecticides were correlated with the cultivation of rice, fruit trees, and flowers. Q fever and motorcycle accidents were associated with fruit tree cultivation.

CONCLUSIONS

The study identifies agricultural occupational injuries and diseases that may inform occupational health policy and the development of prevention priorities to prevent occupational hazards for farmers.

How meteorological factors impacting on scrub typhus incidences in the main epidemic areas of 10 provinces, China, 2006-2018

Scrub typhus, caused by Orientia tsutsugamushi, is a serious public health problem in the Asia-Pacific region, threatening the health of more than one billion people. China is one of the countries with the most serious disease burden of scrub typhus. Previous epidemiological evidence indicated that meteorological factors may affect the incidence of scrub typhus, but there was limited evidence for the correlation between local natural environment factors dominated by meteorological factors and scrub typhus. This study aimed to evaluate the correlation between monthly scrub typhus incidence and meteorological factors in areas with high scrub typhus prevalence using a distributed lag non-linear model (DLNM). The monthly data on scrub typhus cases in ten provinces from 2006 to 2018 and meteorological parameters were obtained from the Public Health Science Data Center and the National Meteorological Data Sharing Center. The results of the single-variable and multiple-variable models showed a non-linear relationship between incidence and meteorological factors of mean temperature (Tmean), rainfall (RF), sunshine hours (SH), and relative humidity (RH). Taking the median of meteorological factors as the reference value, the relative risks (RRs) of monthly Tmean at 0°C, RH at 46%, and RF at 800 mm were most significant, with RRs of 2.28 (95% CI: 0.95-5.43), 1.71 (95% CI: 1.39-2.09), and 3.33 (95% CI: 1.89-5.86). In conclusion, relatively high temperature, high humidity, and favorable rainfall were associated with an increased risk of scrub typhus.

The current and future risk of spread of Leptotrombidium deliense and Leptotrombidium scutellare in mainland China

BACKGROUND

The chigger mites Leptotrombidium deliense (L. deliense) and Leptotrombidium scutellare (L. scutellare) are two main vectors of mite-borne diseases in China. However, the associated environmental risk factors are poorly understood, and the potential geographic ranges of the two mite species are unknown.

METHODS

We combined an ensemble boosted regression tree modelling framework with contemporary records of mites and multiple environmental factors to explore the effects of environmental variables on both mites, as well as to predict the current and future environmental suitability distributions of both species. Additionally, the human population living in the potential spread risk zones of each species was also estimated across mainland China.

RESULTS

Our results indicated that climate, land cover, and elevation are significantly associated with the spatial distributions of the two mite species. The current environmental suitability distribution of L. deliense is mainly concentrated in southern China, and that of L. scutellare is mainly distributed in southern and eastern coastal areas. With climate warming, the geographical distribution of the two mites generally tends to expand to the north and northwest. In addition, we estimated that 305.1-447.6 and 398.3-430.7 million people will inhabit the future spread risk zones of L. deliense and L. scutellare, respectively, in mainland China.

CONCLUSIONS

Our findings provide novel insights into understanding the current and future risks of spread of these two mite species and highlight the target zones for helping public health authorities better prepare for and respond to future changes in mite-borne disease risk.

Epidemiological characteristics and spatiotemporal patterns of scrub typhus in Fujian province during 2012–2020

Background

Scrub typhus has become a serious public health concern in the Asia-Pacific region including China. There were new natural foci continuously recognized and dramatically increased reported cases in mainland China. However, the epidemiological characteristics and spatiotemporal patterns of scrub typhus in Fujian province have yet to be investigated.

Objective

This study proposes to explore demographic characteristics and spatiotemporal dynamics of scrub typhus cases in Fujian province, and to detect high-risk regions between January 2012 and December 2020 at county/district scale and thereby help in devising public health strategies to improve scrub typhus prevention and control measures.

Method

Monthly cases of scrub typhus reported at the county level in Fujian province during 2012–2020 were collected from the National Notifiable Disease Surveillance System. Time-series analyses, spatial autocorrelation analyses and space-time scan statistics were applied to identify and visualize the spatiotemporal patterns of scrub typhus cases in Fujian province. The demographic differences of scrub typhus cases from high-risk and low-risk counties in Fujian province were also compared.

Results

A total of 11,859 scrub typhus cases reported in 87 counties from Fujian province were analyzed and the incidence showed an increasing trend from 2012 (2.31 per 100,000) to 2020 (3.20 per 100,000) with a peak in 2018 (4.59 per 100,000). There existed two seasonal peaks in June-July and September-October every year in Fujian province. A significant positive spatial autocorrelation of scrub typhus incidence in Fujian province was observed with Moran’s I values ranging from 0.258 to 0.471 (P<0.001). Several distinct spatiotemporal clusters mainly concentrated in north and southern parts of Fujian province. Compared to low-risk regions, a greater proportion of cases were female, farmer, and older residents in high-risk counties.

Conclusions

These results demonstrate a clear spatiotemporal heterogeneity of scrub typhus cases in Fujian province, and provide the evidence in directing future researches on risk factors and effectively assist local health authorities in the refinement of public health interventions against scrub typhus transmission in the high risk regions.

Scrub typhus is a vector-borne zoonotic disease caused by Orientia tsutsugamushi and is popular in the Asia-Pacific area. Nowadays scrub typhus has been recognized as a considerable burden on public health in Fujian province. We explored the epidemiological characteristics, spatiotemporal patterns and diffusion characteristics of scrub typhus, and detected high-risk regions at the county level in Fujian province between January 2012 and December 2020. Our results indicated that the majority of cases were reported in June-July and September-October and that that middle aged and elderly people were more prone to infection every year in Fujian province. The spatial autocorrelation analysis revealed clustering in geographic distribution of cases and several distinct spatiotemporal clusters were identified in north and southern parts of Fujian province. Compared with cases from low-risk areas, a higher proportion of cases were female, farmer, and older residents in high-risk counties.

The Temporal Lagged Relationship Between Meteorological Factors and Scrub Typhus With the Distributed Lag Non-linear Model in Rural Southwest China

Background:

Meteorological factors can affect the emergence of scrub typhus for a period lasting days to weeks after their occurrence. Furthermore, the relationship between meteorological factors and scrub typhus is complicated because of lagged and non-linear patterns. Investigating the lagged correlation patterns between meteorological variables and scrub typhus may promote an understanding of this association and be beneficial for preventing disease outbreaks.

Methods

We extracted data on scrub typhus cases in rural areas of Panzhihua in Southwest China every week from 2008 to 2017 from the China Information System for Disease Control and Prevention. The distributed lag non-linear model (DLNM) was used to study the temporal lagged correlation between weekly meteorological factors and weekly scrub typhus.

Results

There were obvious lagged associations between some weather factors (rainfall, relative humidity, and air temperature) and scrub typhus with the same overall effect trend, an inverse-U shape; moreover, different meteorological factors had different significant delayed contributions compared with reference values in many cases. In addition, at the same lag time, the relative risk increased with the increase of exposure level for all weather variables when presenting a positive association.

Conclusions

The results found that different meteorological factors have different patterns and magnitudes for the lagged correlation between weather factors and scrub typhus. The lag shape and association for meteorological information is applicable for developing an early warning system for scrub typhus.

Scrub Typhus: A Spatial and Temporal Analysis from South India

Scrub typhus is a zoonotic rickettsial disease caused by the bacterium Orientia tsutsugamushi. The non-specificity of presentation, low index of suspicion and the poor availability of diagnostic tests often lead to delayed diagnosis and significant morbidity and mortality. Temperature, humidity, rainfall and Normalized Difference Vegetation Index (NDVI) on the spatio-temporal clustering of scrub typhus cases in children in three contiguous administrative districts in South India over 5 years were studied. A total of 419 children were diagnosed with scrub typhus during the study period. A surge of children with scrub typhus was noted when the NVDI ranged between 0.6 and 0.8 µm. Temperature, humidity and rainfall had a major role in the incidence of scrub typhus.

Epidemiological characteristics and spatiotemporal patterns of scrub typhus in Yunnan Province from 2006 to 2017

Scrub typhus is an acute infectious disease in humans. A temporal, spatial and epidemiologic study was conducted to understand the characteristics of scrub typhus in Yunnan, to assist public health prevention and control measures. Based on the data on all cases reported in Yunnan during 2006–2017, we characterized the epidemiological features. Spatio-temporal patterns and Q-type cluster method were adopted to analyze the incidence of scrub typhus in Yunnan. In total, 27,838 scrub typhus cases were reported in Yunnan during 2006–2017. Of these, 49.53% (13,787) were male and 50.47% (14,051) were female (P > 0.05). Most patients were farmers (71.70%) (P < 0.05) and children aged 0–5 years (13.16%) (P < 0.01), which accounted for 84.86% of the total cases. An almost 20-fold increase in the number of patients was observed in 2017 (6,337 cases) compared to 2006 (307 cases). Baoshan and Lincang had the most cases accounting for 41.94%, while Diqing had the lowest incidence (only 3 cases). Sixteen municipalities infected were classified into three groups numbered in sequence. The incidence of scrub typhus in Yunnan is high and the annual incidence increased noticeably over time. Our results also indicate that surveillance and public education need to be focused on Baoshan, Lincang and Dehong.

Epidemiology and Risk Factors for Notifiable Scrub Typhus in Taiwan during the Period 2010-2019

Scrub typhus is a zoonotic disease caused by the bacterium Orientia tsutsugamushi. In this study, the epidemiological characteristics of scrub typhus in Taiwan, including gender, age, seasonal variation, climate factors, and epidemic trends from 2010 to 2019 were investigated. Information about scrub typhus in Taiwan was extracted from annual summary data made publicly available on the internet by the Taiwan Centers for Disease Control. From 2010 to 2019, there were 4352 confirmed domestic and 22 imported cases of scrub typhus. The incidence of scrub typhus ranged from 1.39 to 2.30 per 100,000 from 2010-2019, and peaked in 2013 and 2015-2016. Disease incidence varied between genders, age groups, season, and residence (all p < 0.001) from 2010 to 2019. Risk factors were being male (odds ratio (OR) =1.358), age 40 to 64 (OR = 1.25), summer (OR = 1.96) or fall (OR = 1.82), and being in the Penghu islands (OR = 1.74) or eastern Taiwan (OR = 1.92). The occurrence of the disease varied with gender, age, and place of residence comparing four seasons (all p < 0.001). Weather, average temperature (°C) and rainfall were significantly correlated with confirmed cases. The number of confirmed cases increased by 3.279 for every 1 °C (p = 0.005) temperature rise, and 0.051 for every 1 mm rise in rainfall (p = 0.005). In addition, the total number of scrub typhus cases in different geographical regions of Taiwan was significantly different according to gender, age and season (all p < 0.001). In particular, Matsu islands residents aged 20-39 years (OR = 2.617) and residents of the Taipei area (OR = 3.408), northern Taiwan (OR = 2.268) and eastern Taiwan (OR = 2.027) were affected during the winter. Males and females in the 50-59 age group were at high risk. The total number of imported cases was highest among men, aged 20-39, during the summer months, and in Taipei or central Taiwan. The long-term trend of local cases of scrub typhus was predicted using the polynomial regression model, which predicted the month of most cases in a high-risk season according to the seasonal index (1.19 in June by the summer seasonal index, and 1.26 in October by the fall seasonal index). The information in this study will be useful for policy-makers and clinical experts for direct prevention and control of chigger mites with O. tsutsugamushi that cause severe illness and are an economic burden to the Taiwan medical system. These data can inform future surveillance and research efforts in Taiwan.

Commensal Rodent Habitat Expansion Enhances Arthropod Disease Vectors on a Tropical Volcanic Island

On volcanic islands, the release of animals from predators and competitors can lead to increased body size and population density as well as the expanded habitat use of introduced animals relative to their mainland counterparts. Such alterations might facilitate the spread of diseases on islands when these exotic animals also carry pathogenic agents; however, this has rarely been investigated. The commensal Asian house rat (Rattus tanezumi) is confined to human residential surroundings in mainland Taiwan but can be observed in the forests of nearby Orchid Island, which is a tropical volcanic island. Orchid Island is also a hot spot for scrub typhus, a lethal febrile disease transmitted by larval trombiculid mites (chiggers) that are infected primarily with the rickettsia Orientia tsutsugamushi (OT). We predicted an increase in chigger abundance when rodents (the primary host of chiggers) invade forests from human settlements since soils are largely absent in the latter habitat but necessary for the survival of nymphal and adult mites. A trimonthly rodent survey at 10 sites in three habitats (human residential, grassland, and forest) found only R. tanezumi and showed more R. tanezumi and chiggers in forests than in human residential sites. There was a positive association between rodent and chigger abundance, as well as between rodent body weight and chigger load. Lastly, >95% of chiggers were Leptotrombidium deliense and their OT infection rates were similar among all habitats. Our study demonstrated potentially elevated risks of scrub typhus when this commensal rat species is allowed to invade natural habitats on islands. Additionally, while the success of invasive species can be ascribed to their parasites being left behind, island invaders might instead obtain more parasites if the parasite requires only a single host (e.g., trombiculid mite), is a host generalist (e.g., L. deliense), and is transferred from unsuitable to suitable habitats (i.e., human settlements on the mainland to forests on an island).

Scrub typhus and antibiotic-resistant Orientia tsutsugamushi

Introduction: Scrub typhus is one of the most underdiagnosed and under-reported febrile illnesses requiring hospitalization, mainly occurring in Southeast and East Asia and the Pacific Islands, in an area referred to as the 'Tsutsugamushi Triangle.' Scrub typhus is a zoonotic rickettsial disease that is transmitted to humans by trombiculid mites.Areas covered: A MEDLINE/PubMed search of the available literature was performed to describe the role of antibiotic-resistant scrub typhus in therapy failure.Expert opinion: Scrub typhus is characterized by an eschar that may appear 2-3 days before sudden-onset fever with chills, headache, backache, myalgia, profuse sweating, vomiting, and enlarged lymph nodes. A macular or maculopapular skin rash can develop within 3-8 days after the onset of fever. Various antibiotics, such as chloramphenicol, tetracycline, doxycycline, macrolides, quinolones, and rifampicin, have been used to treat scrub typhus. Resistance to tetracycline has been proposed to underlie delayed clinical improvement since 1996, but recent reports have questioned the existence of doxycycline resistance. Nevertheless, the existence and importance of antibiotic-resistant scrub typhus remain uncertain and require further study.

Assessing scrub typhus and rickettsioses transmission risks in the Chiang Rai province of northern Thailand

BACKGROUND

Scrub typhus is an important disease in the Asia-Pacific countries with increasing relevance for public health worldwide. Entomological risk assessment for scrub typhus and rickettsial disease in Phu Chi Fah village-Chiang Rai was performed to determine areas at greatest risk for disease transmission in order to increase awareness of diseases to travelers and healthcare workers.

METHODS

From 2016 to 2018, rodents and chiggers were collected from 7 sites covering residential, grassland, and forest areas to determine the prevalence of pathogen of interest. The correlation between land type and vector-host-pathogen interaction system was investigated.

RESULT

High prevalence of O. tsutsugamushi-infected and Rickettsia-infected chiggers was observed especially in areas with grassland and forest ecotones. Chigger and rodent species composition were negatively associated with the level of human disturbance. Increased density of rodents was responsible for a higher abundance of chigger population and increased prevalence of O. tsutsugmaushi infection in chigger in the following year.

CONCLUSION

Communities in the study areas have an increased exposure risk to scrub typhus and potentially spotted fever group rickettsiosis. Travellers to this endemic area should pay more attention pathogen risks so as to avoid vector and disease exposure. Seasonal rodenticidal activity may help migitate the risk of pathogen transmission.

Spatiotemporal Dynamics of Scrub Typhus in Jiangxi Province, China, from 2006 to 2018

Background: Scrub typhus (ST) has become a significant potential threat to public health in Jiangxi. Further investigation is essential for the control and management of the spatiotemporal patterns of the disease. Methods: Time-series analyses, spatial distribution analyses, spatial autocorrelation analysis, and space-time scan statistics were performed to detect spatiotemporal dynamics distribution of the incidence of ST. Results: From 2006 to 2018, a total of 5508 ST cases occurred in Jiangxi, covering 79 counties. The number of ST cases increased continuously from 2006 to 2018, and there was obvious seasonality during the variation process in each year, with a primary peak in autumn (September to October) and a smaller peak in summer (June to August). From 2007 to 2018, the spatial distribution of the ST epidemic was significant heterogeneity, and Nanfeng, Huichang, Xunwu, Anyuan, Longnan, and Xinfeng were hotspots. Seven spatiotemporal clusters were observed using Kulldorff's space-time scan statistic, and the most likely cluster only included one county, Nanfeng county. The high-risk areas of the disease were in the mountainous, hilly region of Wuyi and the southern mountainous region of Jiangxi. Conclusions: Targeted interventions should be executed in high-risk regions for the precise prevention and control of ST.

Spatiotemporal patterns and spatial risk factors for visceral leishmaniasis from 2007 to 2017 in Western and Central China: A modelling analysis

Visceral leishmaniasis (VL) is a neglected disease caused by trypanosomatid protozoa in the genus Leishmania, which is transmitted by phlebotomine sandflies. Although this vector-borne disease has been eliminated in several regions of China during the last century, the reported human VL cases have rebounded in Western and Central China in recent decades. However, understanding of the spatial epidemiology of the disease remains vague, as the spatial risk factors driving the spatial heterogeneity of VL. In this study, we analyzed the spatiotemporal patterns of annual human VL cases in Western and Central China from 2007 to 2017. Based on the related spatial maps, the boosted regression tree (BRT) model was adopted to explore the relationships between VL and spatial correlates as well as predicting both the existing and potential infection risk zones of VL in Western and Central China. The mined links reveal that elevation, minimum temperature, relative humidity, and annual accumulated precipitation make great contributions to the spatial heterogeneity of VL. The maps show that Xinjiang Uygur Autonomous Region, Gansu, western Inner Mongolia Autonomous Region, and Sichuan are predicted to fall in the highest infection risk zones of VL. Approximately 61.60 million resident populations lived in the high-risk regions of VL in Western and Central China. Our results provide a better understanding of how spatial risk factors driving VL spread as well as identifying the potential endemic risk region of VL, thereby enhancing the biosurveillance capacity of public health authorities.

Non-linear effect of different humidity types on scrub typhus occurrence in endemic provinces, Thailand

Background

Reported monthly scrub typhus (ST) cases in Thailand has an increase in the number of cases during 2009–2014. Humidity is a crucial climatic factor for the survival of chiggers, which is the disease vectors. The present study was to determine the role of humidity in ST occurrence in Thailand and its delayed effect.

Methods

We obtained the climate data from the Department of Meteorology, the disease data from Ministry of Public Health. Negative binomial regression combined with a distributed lag non-linear model (NB-DLNM) was employed to determine the non-linear effects of different types of humidity on the disease. This model controlled overdispersion and confounder, including seasonality, minimum temperature, and cumulative total rainwater.

Results

The occurrence of the disease in the 6-year period showed the number of cases gradually increased summer season (Mid-February – Mid-May) and then reached a plateau during the rainy season (Mid-May – Mid-October) and then steep fall after the cold season (Mid-October – Mid-February). The high level (at 70%) of minimum relative humidity (RHmin) was associated with a 33% (RR 1.33, 95% CI 1.13–1.57) significant increase in the number of the disease; a high level (at 14 g/m³) of minimum absolute humidity (AHmin) was associated with a 30% (RR 1.30, 95% CI 1.14–1.48); a high level (at 1.4 g/kg) of minimum specific humidity (SHmin) was associated with a 28% (RR 1.28, 95% CI 1.04–1.57). The significant effects of these types of humidity occurred within the past month.

Conclusion

Humidity played a significant role in enhancing ST cases in Thailand, particularly at a high level and usually occurred within the past month. NB-DLNM had good controlled for the overdispersion and provided the precise estimated relative risk of non-linear associations. Results from this study contributed the evidence to support the Ministry of Public Health on warning system which might be useful for public health intervention and preparation in Thailand.

Risk mapping of scrub typhus infections in Qingdao city, China

BACKGROUND

The emergence and re-emergence of scrub typhus has been reported in the past decade in many global regions. In this study, we aim to identify potential scrub typhus infection risk zones with high spatial resolution in Qingdao city, in which scrub typhus is endemic, to guide local prevention and control strategies.

METHODOLOGY/PRINCIPAL FINDINGS

Scrub typhus cases in Qingdao city during 2006-2018 were retrieved from the Chinese National Infectious Diseases Reporting System. We divided Qingdao city into 1,101 gridded squares and classified them into two categories: areas with and without recorded scrub typhus cases. A boosted regression tree model was used to explore environmental and socioeconomic covariates associated with scrub typhus occurrence and predict the risk of scrub typhus infection across the whole area of Qingdao city. A total of 989 scrub typhus cases were reported in Qingdao from 2006-2018, with most cases located in rural and suburban areas. The predicted risk map generated by the boosted regression tree models indicated that the highest infection risk areas were mainly concentrated in the mid-east and northeast regions of Qingdao, with gross domestic product (20.9%±1.8% standard error) and annual cumulative precipitation (20.3%±1.1%) contributing the most to the variation in the models. By using a threshold environmental suitability value of 0.26, we identified 757 squares (68.7% of the total) with a favourable environment for scrub typhus infection; 66.2% (501/757) of the squares had not yet recorded cases. It is estimated that 6.32 million people (72.5% of the total population) reside in areas with a high risk of scrub typhus infection.

CONCLUSIONS/SIGNIFICANCE

Many locations in Qingdao city with no recorded scrub typhus cases were identified as being at risk for scrub typhus occurrence. In these at-risk areas, awareness and capacity for case diagnosis and treatment should be enhanced in the local medical service institutes.

Epidemiology of tsutsugamushi disease and its relationship with meteorological factors in Xiamen city, China

Tsutsugamushi disease (TD) is an acute infectious disease caused by Orientia tsutsugamushi. This study aimed to analyze the epidemiological features of TD, investigate chigger mites and their hosts, and investigate the meteorological factors affecting TD incidence and the host of O. tsutsugamushi in Xiamen city, China. Data on reported TD cases were collected from 2006 to 2018. Spearman’s correlation test were used for identifying the relationship between meteorological factors and TD incidence and whether meteorological factors affect the host of O. tsutsugamushi. The incidence of reported TD increased gradually from 2006, reached a peak of 4.59 per 100,000 persons in 2014, and then decreased gradually. The TD incidence was seasonal, with epidemic periods occurred mainly in summer and autumn. Patients aged 40–60 years had the highest proportion of cases, accounting for 44.44% of the total cases. Farmers had the largest number of cases among all occupational groups. Rattus Norvegicus was the most common host, accounting for the largest proportion of rats (73.00%), and the highest rat density was observed in March and October every year. There were significant positive correlations between the number of reported cases and average temperature, sunshine duration, and rainfall as well as between rat density and average temperature. On phylogenetic analysis, 7 sequences of hosts and human TD cases obtained from health records demonstrated the highest similarities to the Kato, Karp, and Gilliam strains. No correlations were observed between rat density, and sunshine duration and rainfall. The transmission of TD in Xiamen city, China, was seasonal, and its incidence was affected by several meteorological factors including average temperature, sunshine duration, and rainfall. However, the host of O. tsutsugamushi was only affected by average temperature.

Tsutsugamushi disease (TD) is a natural focal disease caused by Orientia tsutsugamushi, which is widespread in some areas. The incidence of TD is greatly influenced by meteorological factors. Therefore, it is essential to analyze the epidemiological features of TD, investigate chigger mites and their hosts, and explore the relationship between meteorological factors and TD incidence and whether meteorological factors affect the host of O. tsutsugamushi in Xiamen city, China. The results showed significant positive correlations between the number of reported cases and average temperature, sunshine duration, and rainfall. A positive correlation between rat density and average temperature. No correlations were observed between the rat density and sunshine duration and rainfall. We believe our findings may provide scientific basis for studies or health strategies conducted in areas that are meteorologically similar in characteristics with Xiamen city, China.

Spatiotemporal Patterns and Risk Factors for Scrub Typhus From 2007 to 2017 in Southern China

BACKGROUND

Substantial outbreaks of scrub typhus, coupled with the discovery of this vector-borne disease in new areas, suggest that the disease remains remarkably neglected. The objectives of this study were to map the contemporary and potential transmission risk zones of the disease and to provide novel insights into the health burden imposed by scrub typhus in southern China.

METHODS

Based on the assembled data sets of annual scrub typhus cases and maps of environmental and socioeconomic correlates, a boosted regression tree modeling procedure was used to identify the environmental niche of scrub typhus and to predict the potential infection zones of the disease. Additionally, we estimated the population living in the potential scrub typhus infection areas in southern China.

RESULTS

Spatiotemporal patterns of the annual scrub typhus cases in southern China between 2007 and 2017 reveal a tremendous, wide spread of scrub typhus. Temperature, relative humidity, elevation, and the normalized difference vegetation index are the main factors that influence the spread of scrub typhus. In southern China, the predicted highest transmission risk areas of scrub typhus are mainly concentrated in several regions, such as Yunnan, Guangxi, Guangdong, Hainan, and Fujian. We estimated that 162 684 million people inhabit the potential infection risk zones in southern China.

CONCLUSIONS

Our results provide a better understanding of the environmental and socioeconomic factors driving scrub typhus spread, and estimate the potential infection risk zones beyond the disease's current, limited geographical extent, which enhances our capacity to target biosurveillance and help public health authorities develop disease control strategies.

Diagnosis of scrub typhus: recent advancements and challenges

Scrub typhus is a mite-borne, acute febrile illness caused by the bacterium Orientia tsutsugamushi. It is a re-emerging infectious disease of the tsutsugamushi triangle. Scrub typhus is transmitted through bites of contaminated chiggers (larval stage). Diagnosis of scrub typhus is challenging as its symptoms mimic with other acute febrile illnesses. Several methods are effectual for diagnosis of scrub typhus that includes enzyme-linked immunosorbent assay (ELISA), immunofluorescence assay (IFA), immunochromatographic test (ICT), Weil-Felix, polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP). Weil-Felix test was initially used for the diagnosis of scrub typhus in underdeveloped countries but not preferred due to a lack of both specificity and sensitivity. Other immuno-based methods like IFA and ELISA are most outrank for detection of scrub typhus due to their higher sensitivity and specificity, but not vigorous to lay bare the infection at early stages and need the convalescent sampling for verification of positive samples. On another deed, PCR based methods becoming acceptable over era due to its dexterity of early-stage diagnosis with higher specificity and sensitivity but lack its applicability in circumstances of scrub typhus due to the variegated genetic makeup of Orientia tsutsugamushi among its serotypes. The present review focused on various detection methods along with their advantages and disadvantages used in the diagnosis of scrub typhus. A comparison between available methods of diagnosis with challenges in the detection of scrub typhus is also summarized.

The diversity and distribution of chigger mites associated with rodents in the South African savanna

Chigger mites (Trombiculidae) are temporary habitat-specific ectoparasites that often occur on rodents. Little ecological data are available on chiggers associated with rodents in South Africa. The study aims were to (1) record the chigger species associated with rodents in the savanna, (2) assess if chigger species display parasitope preference on the rodent body and (3) compare the distribution of chigger species in natural, agricultural and urban habitats. Rodents (n = 314) belonging to eight genera were trapped in the savanna biome during 2014 and 2015. Twelve chigger species, of which five are recently described species, were recorded from 161 rodent hosts. The data include three new country locality records. Microtrombicula mastomyia was the most prevalent species across sampling seasons and habitat types. Significant parasitope preference was recorded for two species, with the ear, face and tail base some of the preferred attachment sites. Sampling season and habitat type had a significant effect on chigger communities with summer and agricultural habitats recording the highest species richness, while the highest species diversity was recorded in natural habitats. The study contributes to our current knowledge regarding rodent-associated chigger diversity and distribution in South Africa and further highlights the importance of environmental characteristics in shaping chigger communities.

Mapping Seriphium plumosum encroachment and interaction with wildfire and environmental factors in a protected mountainous grassland

Accurate information on the distribution of invasive native species could provide important and effective procedures for managing savannah environment, especially in sensitive mountainous grasslands. The study detected and mapped Seriphium plumosum within a mountainous landscape and linked the georeferenced occurrence data with the corresponding site-specific environmental factors to predict the locations of unknown populations using a MaxEnt niche model. We also explored the relative contribution in terms of species interaction with its surrounding biophysical environment. The AUC value of 0.876 estimated for the species distribution is an indication of a good model fit. Our findings indicated that Seriphium plumosum preferred areas with higher temperature associated with recurrence fire events and limited soil moisture. It was concluded that the projected conditions of increasing temperature and fire events could promote widespread gain of niche space for Seriphium plumosum while at the same time altering community structure and composition, hydrological properties, and other vital ecosystem services in the study area.

The estimated burden of scrub typhus in Thailand from national surveillance data (2003-2018)

BACKGROUND

Scrub typhus is a major cause of acute febrile illness in the tropics and is endemic over large areas of the Asia Pacific region. The national and global burden of scrub typhus remains unclear due to limited data and difficulties surrounding diagnosis.

METHODOLOGY/PRINCIPAL FINDINGS

Scrub typhus reporting data from 2003-2018 were collected from the Thai national disease surveillance system. Additional information including the district, sub-district and village of residence, population, geographical, meteorological and satellite imagery data were also collected for Chiangrai, the province with the highest number of reported cases from 2003-2018. From 2003-2018, 103,345 cases of scrub typhus were reported with the number of reported cases increasing substantially over the observed period. There were more men than women, with agricultural workers the main occupational group affected. The majority of cases occurred in the 15-64 year old age group (72,144/99,543, 72%). Disease burden was greatest in the northern region, accounting for 53% of the total reported cases per year (mean). In the northern region, five provinces-Chiangrai, Chiangmai, Tak, Nan and Mae Hong Son-accounted for 84% (46,927/55,872) of the total cases from the northern region or 45% (46,927/103,345) of cases nationally. The majority of cases occurred from June to November but seasonality was less marked in the southern region. In Chiangrai province, elevation, rainfall, temperature, population size, habitat complexity and diversity of land cover contributed to scrub typhus incidence.

INTERPRETATION

The burden of scrub typhus in Thailand is high with disease incidence rising significantly over the last two decades. However, disease burden is not uniform with northern provinces particularly affected. Agricultural activity along with geographical, meteorological and land cover factors are likely to contribute to disease incidence. Our report, along with existing epidemiological data, suggests that scrub typhus is the most clinically important rickettsial disease globally.

Fleas of Shrews and Rodents in Rural Lowland Taiwan

Fleas transmit a variety of pathogens to humans but are relatively understudied in comparison to mosquitoes and ticks, including in Taiwan, where fleas in rural lowlands have never been systematically surveyed. In total, 700 fleas of four species were collected from 1,260 shrews and rodents at nine counties across lowland Taiwan. Nosopsyllus nicanus Jordan (Siphonaptera: Ceratophyllidae) and Xenopsylla cheopis Rothschild (Siphonaptera: Pulicidae) were the most abundant flea species (79.0 and 14.6% of total fleas, respectively); the former was largely limited to the islets, while the latter was restricted to the Taiwan main island. Rattus losea Swinhoe (Rodentia: Muridae) was the most common small mammal species (49.3% of total) and hosted the majority of fleas (88.3% of total). Five Rickettsia spp., including Rickettsia conorii Brumpt (Rickettsiales: Rickettsiaceae), Rickettsia felis Bouyer et al. Rickettsia japonica Uchida, Rickettsia raoultii Mediannikov, and Rickettsia rickettsii Brumpt or closely related species, were identified from 67 individually assayed fleas based on ompB and gltA genes. Rickettsia felis, mainly transmitted by fleas, was detected in one X. cheopis in southern Taiwan where a confirmed human case of infection with R. felis has been reported. The presence of R. felis, along with the other four tick-borne Rickettsia spp., demonstrates that a variety of rickettsiae circulate in rural lowland Taiwan and could pose risks to human health.

Mapping Environmental Suitability of Scrub Typhus in Nepal Using MaxEnt and Random Forest Models

Being a globally emerging mite-borne zoonotic disease, scrub typhus is a serious public health concern in Nepal. Mapping environmental suitability and quantifying the human population under risk of the disease is important for prevention and control efforts. In this study, we model and map the environmental suitability of scrub typhus using the ecological niche approach, machine learning modeling techniques, and report locations of scrub typhus along with several climatic, topographic, Normalized Difference Vegetation Index (NDVI), and proximity explanatory variables and estimated population under the risk of disease at a national level. Both MaxEnt and RF technique results reveal robust predictive power with test The area under curve (AUC) and true skill statistics (TSS) of above 0.8 and 0.6, respectively. Spatial prediction reveals that environmentally suitable areas of scrub typhus are widely distributed across the country particularly in the low-land Tarai and less elevated river valleys. We found that areas close to agricultural land with gentle slopes have higher suitability of scrub typhus occurrence. Despite several speculations on the association between scrub typhus and proximity to earthquake epicenters, we did not find a significant role of proximity to earthquake epicenters in the distribution of scrub typhus in Nepal. About 43% of the population living in highly suitable areas for scrub typhus are at higher risk of infection, followed by 29% living in suitable areas of moderate-risk, and about 22% living in moderately suitable areas of lower risk. These findings could be useful in selecting priority areas for surveillance and control strategies effectively.

Spatio-temporal patterns of scrub typhus in mainland China, 2006-2017

Background

Scrub typhus, a serious public health problem in the Asia-Pacific area, is endemic in the “tsutsugamushi triangle” area. Scrub typhus has been widespread and has become a significant health concern in China. However, spatiotemporal patterns need to be investigated further.

Objective

This study aims to explore spatiotemporal patterns, diffusion characteristics and regional distribution differences of scrub typhus cases in mainland China from January 2006 to December 2017.

Method

Monthly cases of scrub typhus reported at the county level during 2006–2017 were obtained. Time-series analyses, spatial distribution analyses, spatial diffusion analyses, spatial autocorrelation analyses and space-time scan statistic analyses were used to explore spatiotemporal characteristics of scrub typhus.

Results

A total of 121 251 scrub typhus cases were reported in 30 provinces (or municipalities) of mainland China during 2006–2017, which rose exponentially. There were seasonal characteristics from June to November for scrub typhus. Scrub typhus had been diffused from south, southwest, southeast and eastern coasts to center, north, northeast and northwest in mainland China. Scrub typhus occurrences were from point to surrounding regions, and from south to north every year. The peak periods of scrub typhus became longer and longer from north to southwest to south in mainland China. There existed a single peak in Southwest region and North region, respectively, but existed a bimodal peak for South region. Scrub typhus cases were clustered in Yunnan, Guangdong, Guangxi, Fujian and Anhui among June to November. The scrub typhus epidemics in Guangdong and Yunnan were the most serious.

Conclusions

The results in this study can be guide targeted public health interventions against scrub typhus at the county level.

Scrub typhus is a bacterial zoonosis caused by Orientia tsutsugamushi and is popular in the Asia-Pacific area. Nowadays scrub typhus has been widespread and causes a considerable burden on public health and economy in China. We investigated spatiotemporal patterns, diffusion characteristics and regional distribution differences of scrub typhus cases in mainland China from 2006 to 2017. The majority of scrub typhus cases were reported in June to November, and 98.3% of the total scrub typhus cases during 2006–2017 were reported in South region, Southwest region and North region. Scrub typhus had been diffused in mainland China except Qinghai. Scrub typhus occurrences were from point to surrounding regions, and from south to north every year. The peak periods of scrub typhus turn longer, and there existed a single peak to a bimodal peak from north to south. Scrub typhus cases were clustered in Yunnan, Guangdong, Guangxi, Fujian and Anhui during June to November. The scrub typhus epidemics in Guangdong and Yunnan were the most serious.

The scrub typhus in mainland China: spatiotemporal expansion and risk prediction underpinned by complex factors

In mainland China, a geographic northward expansion of scrub typhus has been seen, highlighting the need to understand the factors and identify the risk for disease prevention. Incidence data from 1980 to 2013 were used. A Cox proportional hazard model was used to identify drivers for spatial spread, and a boosted regression tree (BRT) model was constructed to predict potential risk areas. Since the 1980s, an invasive expansion from South Natural Foci towards North Natural Foci was clearly identified, with the epidemiological heterogeneity observed between two regions, mainly in spatial distribution, seasonality, and demographic characteristics. Survival analysis disclosed significant factors contributing to the spatial expansion as following: being intersected by freeway (HR = 1.31, 95% CI: 1.11-1.54), coverage percentage of broadleaf forest (HR = 1.10, 95% CI: 1.06-1.15), and monthly average temperature (HR = 1.27, 95% CI: 1.25-1.30). The BRT models showed that precipitation, sunshine hour, temperature, crop field, and relative humidity contributed substantially to the spatial distribution of scrub typhus. A county-scale risk map was created to predict the regions with high probability of the disease. The current study enabled a comprehensive overview of epidemiological characteristics of scrub typhus in mainland China.

Re-emergence of scrub typhus in Zhejiang Province, southern China: A 45-year population-based surveillance study

BACKGROUND

Scrub typhus is the leading cause of treatable unidentified febrile illnesses in Southeast Asia. This study was conducted to document the epidemiological characteristics of scrub typhus and its change in Zhejiang, one of traditional epidemic provinces in China.

METHODS

Scrub typhus surveillance data in Zhejiang province during 1957-1989 and 2006-2012 were obtained. Descriptive analysis was conducted to characterize the epidemiology of scrub typhus. The spatial distributions over the periods were explored using spatial autocorrelation analysis and spatiotemporal cluster analysis.

RESULTS

A total of 4104 cases and 7 deaths were reported from 1957 to 1989 and 2006 to 2017. The incidence declined since 1959, remained low from 1967 to 1989, and then exponentially increased after 2006. The seasonality changed from a summer pattern between 1957 and 1989 to a bimodal peak pattern in July to August and October to November from 2006 to 2017. One primary and three secondary high-risk clusters were affirmed in both periods from 1980 to 1989 and 2006 to 2017. The primary cluster expanded southwestward and the time span of the secondary clusters extended in the later period compared to the clusters in the previous time frame.

CONCLUSION

Zhejiang recently underwent a seasonality change, geographic extension, and incidence increase in scrub typhus. More attention should be paid to controlling scrub typhus.

Serologic Evidence for Orientia Exposure in the Democratic Republic of Sao Tome and Principe

Orientia tsutsugamushi is an obligate intracellular bacterium that causes scrub typhus in humans. Formerly thought to be confined to the "tsutsugamushi triangle" within the Asia-Pacific region, scrub typhus was recently identified in the Western Hemisphere. Moreover, a new species of Orientia bacterial genus was isolated from a patient in Dubai. This study investigated Orientia exposure in an African country, the Democratic Republic of Sao Tome and Principe. Two sets of samples were analyzed in the study: 240 dried blood spots (DBSs) collected in 2016 and 863 serum samples from 570 pregnant women in 2003. Antibodies against O. tsutsugamushi were examined by immunofluorescence assay (IFA) and enzyme-linked immunosorbent assay (ELISA). The positive samples were further confirmed by Western blot. The results of IFA showed that 5.8% (14/240) of DBSs and 20.4% (116/570) of the serum samples contained reactive antibodies, whereas IgG ELISA yielded a positive rate of 15.4% (88/570) for the serum samples. These findings provided serologic evidence of potential Orientia exposure even though case of scrub typhus has never been diagnosed in the nation. Further studies are needed to determine the epidemiology and the burden of this neglected tropical disease in Africa.

Epidemiology, Risk Factors and Seasonal Variation of Scrub Typhus Fever in Central Nepal

Scrub typhus is a mite-borne acute febrile illness caused by Orientia. tsutsugamushi, a zoonotic bacterial infection common in the region known as the tsutsugamushi triangle. This study aims to determine the seroprevalence, seasonal variation, and risk factors of scrub typhus among the acute febrile illness patients attending different hospitals of central Nepal. Blood samples were collected from hospitalized patients of acute febrile illness suspected of scrub typhus infection attending different hospitals of central Nepal from April 2017 to March 2018. The IgM antibody to Orientia tsutsugamushi was detected by using the Scrub Typhus Detect™ Kit. Among the total cases (1585), 358 (22.58%) were positive for IgM Antibodies. Multivariate analysis identified several risks factors to be significantly associated with the scrub typhus infection, including gender (female) (odds ratio [OR] = 1.976, p ≤ 0.001, confidence interval [CI] = 1.417–2.756), rural residential location (odds ratio [OR] = 0.431, p = 0.001, confidence interval [CI] = 0.260–0.715), house near grassland (odds ratio [OR] = 3.288, p ≤ 0.001, confidence interval [CI] = 1.935–5.587), and working in the field (odds ratio [OR] = 9.764, p = 0.004, confidence interval [CI] = 2.059–46.315). The study findings indicate scrub typhus infection to be a significant health problem in Nepal. The proper diagnosis of infection cases, timely institution of therapy, public awareness, and vector control are important measures to be taken for the prevention and management of scrub typhus.

Distribution of Orientia tsutsugamushi in rodents and mites collected from Central India

Orientia tsutsugamushi, the causative agent of scrub typhus, is an obligate intracytosolic bacterium transmitted among humans and small mammals by some species of larval trombiculid mites (chiggers). It has been recognized as a pathogen of major public health concern in the Asia-Pacific region. As disease is considered as a neglected, there exists a gap in our knowledge of the disease with regard to the sporadic epidemiologic data in endemic areas. The purpose of the study was to find out the vector as well as pathogen distribution in rodents present in the scrub typhus-reported areas in central India. We studied the seasonal variations of occurrence in O. tsutsugamushi in rodents and mites by molecular detection targeting the 56-kDa and 47-kDa genes. Rodent and mite samples were collected during December 2015 to July 2017. A total of 127 samples from rodents, seven pools of mites, and four pools of fleas were collected and processed for DNA isolation. Nested PCRs targeting the 56-kDa and 47-kDa surface antigen genes were performed. In addition, quantification of bacterial load was done by qPCR targeting the 47-kDa gene. During the pre-monsoon season, O. tsutsugamushi was detected in 12% and 10% samples employing the 56-kDa and 47-kDa nested PCRs, respectively, whereas, during post-monsoon season, the respective detection rates were 13.33% and 26.66%. This study predicted a bimodal pattern during the months of pre-monsoon and post-monsoon season with a peak in post-monsoon. Thus, the impact of season on the perpetuation of O. tsutsugamushi in the host was observed.

Scrub Typhus: Seven-Year Experience and Literature Review

BACKGROUND

Scrub typhus, an emerging rickettsial disease caused by Orientia tsutsugamushi, is a clinically important endemic disease on Taiwan.

METHODS

From January 1, 2007, to December 31, 2013, 156 patients diagnosed with scrub typhus were admitted to Taitung MacKay Memorial Hospital. Demographic data, clinical features, laboratory results, and outcomes of patients were retrospectively analyzed.

RESULTS

Among 156 cases, 150 survived (96%) and six died (4%). There were 111 males (71%) and 45 females (29%) with a mean age of 47.8 years. The most common clinical features were fever (100%), general malaise (77%), chill (74%), headache (59%), and eschar (55%). No seasonal pattern was observed, with peaks in November and January. The average time to defervescence after appropriate antibiotics for the 150 surviving patients was 2.45 days. The significant risk factors were a delay of initial appropriate antibiotics use within 24 hrs, increased C-reactive protein (CRP), and liver cirrhosis.

CONCLUSION

Scrub typhus, a growing and emerging disease, is considered in the differential diagnosis of fever of unknown origin, and its diagnosis may be missed if it is not considered owing to its nonspecific clinical presentation. It is important to have a high index of suspicion and to increase awareness in endemic areas. Prompt diagnosis and early treatment with appropriate antibiotics are vital.

Scrub typhus in Jiangsu Province, China: epidemiologic features and spatial risk analysis

Background

With the increasing incidence of scrub typhus in recent years, it is of great value to analyse the spatial and temporal distribution of scrub typhus by applying micro-geographical studies at a reasonably fine scale, and to guide the control and management.

Methods

We explored the use of maximum entropy modelling method to confirm the spatial and temporal distribution of scrub typhus according to the occurrence locations of human cases in Jiangsu Province. The risk prediction map under specific environmental factors was therefore drawn by projecting the training model across China. The area under the curve and the omission rate were used to validate the model. Meanwhile, Jackknife tests were applied to enumerate the contribution of different environmental variables, then to predict the final model. The predicted results were validated by using China’s known occurrence locations.

Results

A total of 566 occurrence locations with known 4865 scrub typhus occurrence records were used in our study. The number of female cases was higher than male cases, with a proportion of 1.17:1, and people in any age group could be infected. The number of cases presented an inverted-U relation with age. The percentage of cases aged from 60 to 69 years old was the highest, accounting for 30.50% of all cases. Ecological niche modelling results indicated that the locations of scrub typhus cases, which was of great importance in the disease transmission cycle, had a certain ecological niche with environmental elements in many dimensions. Moreover, the key environmental factors for determining scrub typhus occurrence were temperature (including temperature seasonality, min temperature of coldest month, mean diurnal range, and monthly mean temperature), precipitation of wettest month, and land cover types. The risk prediction maps indicated that mid-eastern China was the potential risk areas for scrub typhus of “autumn type”. Meanwhile, in our results, Guangdong Province was the high-risk region for “autumn type” scrub typhus, where cases were mainly reported as “summer type”.

Conclusion

The combination of climatic and geographic factors with GIS methods is an appropriate option to analyse and estimate the spatial and temporal distribution of scrub typhus.

The Re-Emergence and Emergence of Vector-Borne Rickettsioses in Taiwan

Rickettsial diseases, particularly vector-borne rickettsioses (VBR), have a long history in Taiwan, with studies on scrub typhus and murine typhus dating back over a century. The climatic and geographic diversity of Taiwan's main island and its offshore islands provide many ecological niches for the diversification and maintenance of rickettsiae alike. In recent decades, scrub typhus has re-emerged as the most prevalent type of rickettsiosis in Taiwan, particularly in eastern Taiwan and its offshore islands. While murine typhus has also re-emerged on Taiwan's western coast, it remains neglected. Perhaps more alarming than the re-emergence of these rickettsioses is the emergence of newly described VBR. The first case of human infection with Rickettsia felis was confirmed in 2005, and undetermined spotted fever group rickettsioses have recently been detected. Taiwan is at a unique advantage in terms of detecting and characterizing VBR, as it has universal health coverage and a national communicable disease surveillance system; however, these systems have not been fully utilized for this purpose. Here, we review the existing knowledge on the eco-epidemiology of VBR in Taiwan and recommend future courses of action.

Epidemiology & risk factors of scrub typhus in south India

Background & objectives:

Scrub typhus is a major public health threat in South and Southeastern Asian countries including India. Understanding local patterns of disease and factors that place individuals at risk is pivotal to future preventive measures against scrub typhus. The primary aim of this study was to identify specific epidemiological and geographical factors associated with an increased risk of developing scrub typhus in this region.

Methods:

We mapped 709 patients from Tamil Nadu, Andhra Pradesh and Telangana who were admitted to the Christian Medical College (CMC) Hospital, Vellore, Tamil Nadu, India, for the period 2006-2011, assessed seasonality using monthly counts of scrub typhus cases, and conducted a case-control study among a subset of patients residing in Vellore.

Results:

The geographic distribution of cases at CMC Hospital clusters around the Tamil Nadu-Andhra Pradesh border. However, distinct hotspots clearly exist distal to this area, near Madurai and the coast in Tamil Nadu, and in the Northeast of Andhra Pradesh. Seasonally, the highest numbers of cases were observed in the cooler months of the year, i.e. September to January. In the case-control analysis, cases were more likely to be agricultural laborers (OR 1.79, 95% CI 1.01 - 3.15), not wear a shirt at home (OR 4.23, 95% CI 1.12 - 16.3), live in houses adjacent to bushes or shrubs (OR 1.95, 95% CI 1.08 - 3.53), and live in a single room home (OR 1.75, 95% CI 1.02 - 3.01). On binary logistic regression, the first three of these variables were statistically significant.

Interpretation & conclusions:

With the growing number of cases detected in India, scrub typhus is fast emerging as a public health threat and further research to protect the population from this deadly infection is essential. Health education campaigns focusing on the agricultural workers of Southern India, especially during the cooler months of the year, can serve as an important public health measure to control infection.

Climate variability, animal reservoir and transmission of scrub typhus in Southern China

Objectives

We aimed to evaluate the relationships between climate variability, animal reservoirs and scrub typhus incidence in Southern China.

Methods

We obtained data on scrub typhus cases in Guangzhou every month from 2006 to 2014 from the Chinese communicable disease network. Time-series Poisson regression models and distributed lag nonlinear models (DLNM) were used to evaluate the relationship between risk factors and scrub typhus.

Results

Wavelet analysis found the incidence of scrub typhus cycled with a period of approximately 8–12 months and long-term trends with a period of approximately 24–36 months. The DLNM model shows that relative humidity, rainfall, DTR, MEI and rodent density were associated with the incidence of scrub typhus.

Conclusions

Our findings suggest that the incidence scrub typhus has two main temporal cycles. Determining the reason for this trend and how it can be used for disease control and prevention requires additional research. The transmission of scrub typhus is highly dependent on climate factors and rodent density, both of which should be considered in prevention and control strategies for scrub typhus.

Scrub typhus has been endemic in southern China for several decades. In recent years, it has been increasingly reported and has become a significant health concern in China. The incidence of scrub typhus, a vector-borne disease, is influenced by the density of rats and changes in climate. Several studies have focused on the influence of climate and rat density on scrub typhus independent of one another; however, few studies investigate such factors simultaneously. Furthermore, global climate events such as El Niño have not been considered in any study of scrub typhus risk factors. This study reports novel factor research of scrub typhus in southern China. Data of climate, rat density and cases were collected on a monthly basis. Time-series Poisson regression models and distributed lag nonlinear models (DLNM) were used to evaluate the relationship between risk factors and scrub typhus. Finally, relative humidity, rainfall, DTR, MEI and rodent density were identified as risk factors of the incidence of scrub typhus.

Significance of major international seaports in the distribution of murine typhus in Taiwan

Background

International seaports are hotspots for disease invasion and pathogens can persist in seaports even after ports are abandoned. Transmitted by fleas infected by Rickettsia typhi, murine typhus, a largely neglected and easily misdiagnosed disease, is known to occur primarily in large seaports. However, the significance of seaports in the occurrence of murine typhus has never been validated quantitatively.

Methodology/Principal findings

We studied the spatial distribution of murine typhus, a notifiable disease, in Taiwan. We investigated whether risk of infection was correlated with distance to international seaports and a collection of environmental and socioeconomic factors, using a Bayesian negative binomial conditionally autoregressive model, followed with geographically weighted regression. Seaports that are currently in use and those that operated in the 19^th century for trade with China, but were later abandoned due to siltation were analyzed. A total of 476 human cases of murine typhus were reported during 2000–2014 in the main island of Taiwan, with spatial clustering in districts in southwest and central-west Taiwan. A higher incidence rate (case/population) was associated with a smaller distance to currently in-use international seaports and lower rainfall and temperature, but was uncorrelated with distance to abandoned ports. Geographically weighted regression revealed a geographic heterogeneity in the importance of distance to in-use seaports near the four international seaports of Taiwan.

Conclusions/Significance

Our study suggests that murine typhus is associated with international seaports, especially for those with large trading volume. Thus, one of the costs of global trade in Taiwan might be elevated risks of murine typhus. Globalization has accelerated the spread of infectious diseases, but the burden of disease varies geographically, with regions surrounding major international seaports warranting particular surveillance.

Globalization has hastened the spread of infectious diseases, with seaports as hotspots for disease invasion. Transmitted by fleas infected with the rickettsia Rickettsia typhi, murine typhus occurs worldwide, but its significance as a common causative agent of illness in tropical regions remains largely neglected. Although it is recognized that murine typhus is prevalent primarily in large seaports, the significance of seaports in the occurrence of murine typhus has never been validated quantitatively. We thus investigated whether distribution of murine typhus in Taiwan was associated with international seaports. Notably, abandoned international seaports (abandoned in the 19^th century due to siltation) were also studied to see whether the causative agent of murine typhus might still circulate around the ports even after being abandoned. We found that infection risk of murine typhus was negatively associated with distance to operating seaports but was uncorrelated with nearness to abandoned seaports. In addition, the importance of distance to operating seaports for risk of murine typhus infection varied spatially. Our study highlights elevated disease risk as a cost of international trade and suggests particular surveillance in regions surrounding major international seaports.

Prevalence and risk factors for scrub typhus in South India

OBJECTIVE

To determine the prevalence and risk factors of scrub typhus in Tamil Nadu, South India.

METHODS

We performed a clustered seroprevalence study of the areas around Vellore. All participants completed a risk factor survey, with seropositive and seronegative participants acting as cases and controls, respectively, in a risk factor analysis. After univariate analysis, variables found to be significant underwent multivariate analysis.

RESULTS

Of 721 people participating in this study, 31.8% tested seropositive. By univariate analysis, after accounting for clustering, having a house that was clustered with other houses, having a fewer rooms in a house, having fewer people living in a household, defecating outside, female sex, age >60 years, shorter height, lower weight, smaller body mass index and smaller mid-upper arm circumference were found to be significantly associated with seropositivity. After multivariate regression modelling, living in a house clustered with other houses, female sex and age >60 years were significantly associated with scrub typhus exposure.

CONCLUSIONS

Overall, scrub typhus is much more common than previously thought. Previously described individual environmental and habitual risk factors seem to have less importance in South India, perhaps because of the overall scrub typhus-conducive nature of the environment in this region.

Rapid increase of scrub typhus incidence in Guangzhou, southern China, 2006-2014

BACKGROUND

In the last decade, scrub typhus (ST) has been emerging or re-emerging in some areas of Asia, including Guangzhou, one of the most affected endemic areas of ST in China.

METHODS

Based on the data on all cases reported in Guangzhou from 2006 to 2014, we characterized the epidemiological features, and identified environmental determinants for the spatial distribution of ST using a panel negative binomial model.

RESULTS

A total of 4821 scrub typhus cases were reported in Guangzhou during 2006-2014. The annual incidence increased noticeably and the increase was relatively high and rapid in rural townships and among elderly females. The majority of cases (86.8%) occurred during May-October, and farmers constituted the majority of the cases, accounting for 33.9% in urban and 61.6% in rural areas. The number of housekeeper patients had a rapid increment in both rural and urban areas during the study period. Atmospheric pressure and relative humidity with lags of 1 or 2 months, distributions of broadleaved forest and rural township were identified as determinants for the spatiotemporal distribution of scrub typhus.

CONCLUSION

Our results indicate that surveillance and public education need to be focused on the elderly farmers in rural areas covered with broadleaf forest in southern China.

Meteorological factors affecting scrub typhus occurrence: a retrospective study of Yamagata Prefecture, Japan, 1984–2014

Climate change, by its influence on the ecology of vectors might affect the occurrence of vector-borne diseases. This study examines the effects of meteorological factors in Japan on the occurrence of scrub typhus, a mite-borne zoonosis caused byOrientia tsutsugamushi. Using negative binomial regression, we analysed the relationships between meteorological factors (including temperature, rainfall, snowfall) and spring–early summer cases of scrub typhus in Yamagata Prefecture, Japan, during 1984–2014. The average temperature in July and August of the previous year, cumulative rainfall in September of the previous year, snowfall throughout the winter, and maximum depth of snow cover in January and February were positively correlated with the number of scrub typhus cases. By contrast, cumulative rainfall in July of the previous year showed a negative relationship to the number of cases. These associations can be explained by the life-cycle ofLeptotrombidium pallidum, a predominant vector of spring–early summer cases of scrub typhus in northern Japan. Our findings show that several meteorological factors are useful to estimate the number of scrub typhus cases before the endemic period. They are applicable to establish an early warning system for scrub typhus in northern Japan.

Scrub Typhus and Its Causative Agent, Orientia tsutsugamushi

The obligate intracellular bacterium Orientia tsutsugamushi is responsible for more than one million cases of scrub typhus annually throughout the Asia-Pacific region. Human infection occurs via the bite of the larval form (chigger) of several species of trombiculid mites. While in some patients the result of infection is a mild, febrile illness, others experience severe complications, which may even be fatal. This review discusses the genome and biology of the causative agent, the changing epidemiology of scrub typhus, the challenges of its diagnosis, and current treatment recommendations.

Spatiotemporal Dynamics of Scrub Typhus Transmission in Mainland China, 2006-2014

BACKGROUND

Scrub typhus is endemic in the Asia-Pacific region including China, and the number of reported cases has increased dramatically in the past decade. However, the spatial-temporal dynamics and the potential risk factors in transmission of scrub typhus in mainland China have yet to be characterized.

OBJECTIVE

This study aims to explore the spatiotemporal dynamics of reported scrub typhus cases in mainland China between January 2006 and December 2014, to detect the location of high risk spatiotemporal clusters of scrub typhus cases, and identify the potential risk factors affecting the re-emergence of the disease.

METHOD

Monthly cases of scrub typhus reported at the county level between 2006 and 2014 were obtained from the Chinese Center for Diseases Control and Prevention. Time-series analyses, spatiotemporal cluster analyses, and spatial scan statistics were used to explore the characteristics of the scrub typhus incidence. To explore the association between scrub typhus incidence and environmental variables panel Poisson regression analysis was conducted.

RESULTS

During the time period between 2006 and 2014 a total of 54,558 scrub typhus cases were reported in mainland China, which grew exponentially. The majority of cases were reported each year between July and November, with peak incidence during October every year. The spatiotemporal dynamics of scrub typhus varied over the study period with high-risk clusters identified in southwest, southern, and middle-eastern part of China. Scrub typhus incidence was positively correlated with the percentage of shrub and meteorological variables including temperature and precipitation.

CONCLUSIONS

The results of this study demonstrate areas in China that could be targeted with public health interventions to mitigate the growing threat of scrub typhus in the country.

Prevalence of antibodies against Ehrlichia spp. and Orientia tsutsugamushi in small mammals around harbors in Taiwan

BACKGROUND

Tick-borne ehrlichiosis and mite-borne scrub typhus represent important emerging zoonotic rickettsial diseases. Although scrub typhus has been recognized by the Taiwanese public health system, information on ehrlichial infections is scarce in Taiwan. In this study, the risk of spread of ectoparasites on rodents through aerial and marine transportation was assessed in international and domestic harbors. Here, we report the first systematic surveillance of seroprevalence against Ehrlichia spp. in small mammals on the main island of Taiwan.

METHODS

In total, 1648 small mammals were trapped from 8 international ports, 18 domestic fishing harbors, and 7 local public health centers around Taiwan from November 2004 to December 2008. Sera were analyzed using indirect immunofluorescence assays to detect IgG antibodies against Ehrlichia chaffeensis and Orientia tsutsugamushi. A serum titer of ≧1:80 was considered positive.

RESULTS

Antibodies against Ehrlichia spp. and O. tsutsugamushi were detected in 3.28% and 4.92% of small mammals active around harbors, respectively. The seropositive rate against Ehrlichia was higher in northern Taiwan from 2005 to 2008. However, O. tsutsugamushi infections increased in southern Taiwan during this period. The serological evidence of ehrlichial and O. tsutsugamushi infections in all international ports were included in the study. No significant differences were found among the seropositive rates of Ehrlichia spp. and O. tsutsugamushi in small mammals trapped between international and local harbors.

CONCLUSIONS

The overall prevalence of Ehrlichia spp. and O. tsutsugamushi infections in small mammals active around harbors was 3.28% and 4.92%, respectively. The results provided serological evidence supporting the potential risks of transporting pathogens through air and maritime traffic. This study highlights serious issues of the emergence and spread of rickettsial diseases in Taiwan. The incidence of human ehrlichiosis requires further investigation.