Risk Factors for Scrub Typhus, Murine Typhus, and Spotted Fever Seropositivity in Urban Areas, Rural Plains, and Peri-Forest Hill Villages in South India: A Cross-Sectional Study

A Case Series on Spotted Fever and Typhus Fever Seropositivity at National Center for Disease Control and Epidemiological Perspective

Background: The rickettsioses, except for typhus fever and scrub typhus (ST), were not really recognized as distinct clinical entities until the early 20th century. Only when specific rickettsial serologic testing was introduced in the 1940s could the precise etiologies of various rickettsial diseases (RDs) be determined with certainty. Although ST is a well- recognized zoonotic disease entity, but non-scrub typhus rickettsial infection like spotted fever group and typhus group are not well studied in India and are still underestimated. Methods: We report cases who had shown seropositivity of spotted fever and typhus fever RD in IgM and IgG ELISA whose samples were referred from various hospitals of Delhi/National Capital Region in which clinicians had strong suspicion of rickettsiosis other than ST or Weil-Felix test found positive for any of the OX2, 19, and K antigens. Results: We reported 18 cases of SFG and TGRD with mostly cases presented with fever followed by hepato-intestinal symptoms. Conclusion: The vast variability and nonspecific presentation of rickettsiosis in spotted and typhus fever at times have often made it difficult to diagnose clinically. Prompt antibiotic therapy shortens the course of the disease, lowers the risk of complications, and in turn, reduces morbidity and mortality owing to RDs. There is a distinct need for physicians and health care workers at all levels of care in India to be aware of the clinical features, available diagnostic tests and their interpretation, and the therapy for these infections.

Do Aedes triseriatus Respect State Boundaries?: A Paucity of La Crosse Virus in the South Carolina Appalachian Mountains

Background: La Crosse virus is an important endemic public health concern in the North Carolina Appalachian Mountains; however, human incidence is not commonly noted in this region on the South Carolina side of the mountain range border. No relevant studies have been performed in South Carolina evaluating mosquito vector populations for La Crosse virus (LACV) infection; thus, a pilot mosquito surveillance study was executed in summer 2020. Material and Methods: Vector surveillance occurred at five South Carolina state parks bordering neighboring state endemic counties from May to August 2020. Collections were approved by the state park authority, as noted in Permit No. N-8-20. Results: All three competent mosquito vectors were collected during the study duration; however, these vectors were collected in low abundance: Aedes triseriatus (4.5% of all collected mosquitos); Aedes albopictus (2.0%); Aedes japonicus (1.4%). Principal mosquito vector specimens, Ae. triseriatus, were sent to Centers for Disease Control and Prevention for testing of LACV by real-time reverse transcription PCR-all were negative. Discussion: While entomologic evidence suggests low transmission risk for this arbovirus in the South Carolina Appalachian Mountain region, further eco-epidemiologic investigations are warranted to understand this endemicity variance within a relatively small geographic area.

Climate influences scrub typhus occurrence in Vellore, Tamil Nadu, India: analysis of a 15-year dataset

Climate is one of the major factors determining the prevalence and seasonality of vector borne diseases like scrub typhus (ST). We analyzed, the association of the meteorological factors like temperature, rainfall and humidity with scrub typhus using the 15 years scrub typhus data from a tertiary care hospital in Vellore, South India. Demographic data of permanent residents of Vellore, who had IgM ELISA results for scrub typhus for the time period of May 2005 to April 2020 were included. Meteorological data was correlated with the monthly scrub typhus cases; negative binomial regression model was used to predict the relation between scrub typhus occurrence and climate factors. Maximum number of ST cases were reported between the months August and February with October recording the highest number of cases. Elderly people, farmers, agricultural workers and housewives were at higher risk for scrub typhus. For an increase of 1 °C in mean temperature, the monthly ST cases reduced by 18.8% (95% CI - 24.1, - 13.2%). On the contrary, for 1 percent increase in mean relative humidity (RH), there is an increase of 7.6% (95% CI 5.4, 9.9%) of monthly ST cases. Similarly, an increase of 1 mm of rainfall contributed to 0.5 to 0.7% of monthly ST cases (after 2 months) depending on the variables included in the analysis. This study provides information that meteorological factors influence ST occurrence in Vellore. The rise of scrub typhus cases is maximal 2 months post rainfall. Whereas a rise in relative humidity, causes a rise in scrub typhus cases in same month, while rise in temperature has a negative impact on scrub typhus during the same month. These findings based on a retrospective analysis need validation by prospective studies.

Human rickettsial infections in India - A review

Rickettsial infections are emerging and/or re-emerging disease that poses a serious global threat to humans and animals. Transmission to humans and animals is through the bite of the ectoparasites including ticks, fleas and chigger mites. Most of the rickettsial diseases are endemic in India, but underdiagnosed. This review is aimed at analyzing the prevalence of rickettsiosis in India and the advancement of rickettsial diagnosis. We have conducted a systematic review on the prevalence of rickettsial disease in India ranging from 1.3% to 46.6% for spotted fever, 2.4% to 77.8% for scrub typhus and 1% to 46.4% for Q fever, based on the literature published with the evidence of isolation, serological, and molecular diagnostics. Search engines Medline/PubMed, Science Direct, ProQuest, and EBSCO were used to retrieve the articles from electronic databases by using appropriate keywords to track the emergence of these rickettsial diseases in India for the period of 1865 to till date. We retrieved 153 published rickettsial articles on hospital-based studies from India that were purely made on the basis of prevalence and the laboratory parameters viz., Weil-Felix test (WF) and Rapid Immunochromatographic tests (RICT) with reference to the gold standard IFA and ELISA. More epidemiological studies are required for epidemic typhus to know the exact prevalence status of this louse-borne rickettsiosis in India. Currently, there is no confirmed specific inflammatory marker for rickettsial diseases. Moreover, serological cross-reactivity is an important aspect, and it should be investigated in endemic areas, there is also a need to include molecular diagnostic techniques for further confirmation in healthcare settings.

Prevalence of and factors associated with scrub typhus exposure among the hill tribe population living in high incidence areas in Thailand: a cross-sectional study

BACKGROUND

Scrub typhus is a significant tropical disease, occurring in rural settings and therefore usually afflicting remote agricultural populations who have lower socioeconomic status and limited access to medical care. A large proportion of the hill tribe people in Thailand are financially poor, have limited education, and do not have adequate health care access. This study aimed to estimate the prevalence of and determine factors associated with scrub typhus exposure among the hill tribe population living in high-incidence areas in northern Thailand.

METHODS

A cross-sectional study design was used to gather information from hill tribe people aged 18 years and over living in ten hill tribe villages in Mae Fah Luang, Chiang Rai Province, Thailand. Participants who met the inclusion criteria were invited to participate in the study. A validated questionnaire was used as the research instrument, and 5 mL blood samples were taken. Orientia tsutsugamushi IgM and IgG antibodies were detected by enzyme-linked immunosorbent assay (ELISA) and then confirmed by immunofluorescence assay (IFA). Logistic regression was used to detect associations between variables at a significance level of α = 0.05.

RESULTS

A total of 485 hill tribe people participated in the study; 57.1% were female, 29.9% were over 60 years of age, 46.4% were from the Akha tribe, and 74.2% had never attended school. The overall prevalence of scrub typhus exposure was 48.0%. In the multivariate model, five variables were found to be associated with scrub typhus exposure. Participants aged over 60 years had a 4.31-fold increased risk (95% CI = 1.73-10.72) of scrub typhus exposure compared to those who were younger than 30 years. Those who were illiterate had a 3.46-fold increased risk (95% CI = 1.93-6.21) of scrub typhus exposure than those who had at least a primary education level. Participants from the Akha tribe had a 2.20-fold increased risk (95% CI = 1.31-3.72) of scrub typhus exposure than those from the Lahu tribe. Subjects who had a history of cutting grass had a 1.85-fold increased risk (95% CI = 1.20-2.84) of scrub typhus exposure. Those who never wore gloves for farming had a 2.12-fold increased risk (95% CI = 1.28-3.49) of scrub typhus exposure than those who wore gloves daily.

CONCLUSIONS

There is a high prevalence of scrub typhus exposure among the hill tribe in Thailand. Effective public health interventions to promote scrub typhus awareness and prevention are urgently needed in these populations.

Murine Typhus: A Review of a Reemerging Flea-Borne Rickettsiosis with Potential for Neurologic Manifestations and Sequalae

Murine typhus is an acute febrile illness caused by Rickettsia typhi, an obligately intracellular Gram-negative coccobacillus. Rats (Rattus species) and their fleas (Xenopsylla cheopis) serve as the reservoir and vector of R. typhi, respectively. Humans become infected when R. typhi-infected flea feces are rubbed into flea bite wounds or onto mucous membranes. The disease is endemic throughout much of the world, especially in tropical and subtropical seaboard regions where rats are common. Murine typhus is reemerging as an important cause of febrile illness in Texas and Southern California, where an alternate transmission cycle likely involves opossums (Didelphis virginiana) and cat fleas (Ctenocephalides felis). Although primarily an undifferentiated febrile illness, a range of neurologic manifestations may occur, especially when treatment is delayed. Serology is the mainstay of diagnostic testing, but confirmation usually requires demonstrating seroconversion or a fourfold increase in antibody titer from acute- and convalescent-phase sera (antibodies are seldom detectable in the first week of illness). Thus, early empiric treatment with doxycycline, the drug of choice, is imperative. The purpose of this review is to highlight murine typhus as an important emerging and reemerging infectious disease, review its neurologic manifestations, and discuss areas in need of further study.

Seroprevalence of Rickettsial disease: A population-based survey from kashmir valley, North India

Background

Limited studies have been done regarding the prevalence of Rickettsial diseases in India and as far as UT of Jammu and Kashmir is concerned, only a few hospital-based studies are available.

Objectives

The present study was therefore planned to find the seroprevalence of Rickettsial diseases in Kashmir Valley.

Materials and Methods

A multistage sampling procedure was used for the collection of samples from 10 districts of Kashmir Valley and a total of 1740 samples were collected. In addition, 802 healthy blood donors were included to establish baseline titers for Weil-Felix (WF) Test.

Results

Of 1734 subjects, 73 were positive by the WF test. The overall seroprevalence of Rickettsial diseases was 4.1% with the highest prevalence of scrub typhus (2.30%) followed by the spotted fever group (1.5%) and typhus group (0.40%). Maximum seropositive subjects were from district Kulgam (6.97%) followed by Pulwama (5.92%), Shopian (5.79%), Anantnag (5.47%), Ganderbal (5.00%), Kupwara (4.72%), Baramulla (4.62%), Srinagar (2.63%), Bandipora (2.41%), and Budgam (0.54%), respectively. Seropositivity was higher in females and subjects who had contact with ticks and mites like those involved in the collection of firewood and grass or had contact with uncut grass or shrub. The seropositivity was also significantly higher in those working in paddy fields and those living near the forest (P < 0.05).

Conclusion

The results of the present study confirm the existence of Rickettsial diseases in this region. This data would promote awareness of rickettsioses among local physicians and will also serve as a baseline to detect changing prevalence in the future.

Nonnegligible Seroprevalence and Predictors of Murine Typhus, Japan

To elucidate the epidemiology of murine typhus, which is infrequently reported in Japan, we conducted a cross-sectional study involving 2,382 residents of rickettsiosis-endemic areas in Honshu Island during August-November 2020. Rickettsia typhi seroprevalence rate was higher than that of Orientia tsutsugamushi, indicating that murine typhus is a neglected disease.

A Review of Rickettsial Diseases Other Than Scrub Typhus in India

Rickettsial diseases (RD) are widely reported all over the world. Scrub typhus (ST) is a major tropical infection which is well documented all over India. Therefore, the index of suspicion of scrub typhus is high among physicians with regard to patients presenting with acute febrile illness (AFI) and acute undifferentiated febrile illness (AUFI) in India. Rickettsial diseases other than ST (non-ST RDs), which include spotted fever group (SFG) rickettsioses and typhus group (TG) rickettsioses are not uncommon in India, but the index of suspicion is not as high as ST unless there is a history of the presence of fever with rashes and/or recent arthropod bites. This review aims to look into the Indian scenario on the epidemiology of non-ST RDs, especially the SFG and TG rickettsioses based on various investigations, spectrum of clinical presentation, challenges and gaps in knowledge to suspect and diagnose these infections.

Geography and prevalence of rickettsial infections in Northern Tamil Nadu, India: a cross-sectional study

Rickettsial infections and Q fever are a common cause of acute febrile illness globally. Data on the role of climate and altitude on the prevalence of these infections in lacking from Southern India. In this study, we determined the sero-prevalence of scrub typhus (ST), spotted fever (SF), murine typhus (MT) and Q Fever (QF) in 8 eight geographical regions of North Tamil Nadu by detecting IgG antibodies using ELISA. Totally we tested 2565 people from 86 localities. Among the 27.3% positives, approximately 5% were IgG positive for two or more infections. Sero-prevalence to rickettsioses and Q fever was highest for individuals from rural areas and increased with age (> 30 years). Those in the Nilgiris highlands (wetter and cooler) and Erode, which has the most land under irrigation, demonstrated the least exposure to rickettsioses and Q fever. Lowland plains (AOR: 8.4-22.9; 95% CI 3.1-55.3) and highland areas up to 1000 m (AOR: 6.1-10.3; 95% CI 2.4-23.9) showed the highest risk of exposure to scrub typhus. For spotted fever, the risk of exposure was highest in Jawadhi (AOR:10.8; 95% CI 2.6-44.3) and Kalrayan (AOR:16.6; 95% CI 4.1-66.2). Q fever positivity was most likely to be encountered in Salem (AOR: 5.60; 95% CI 1.01-31.08) and Kalrayan hills (AOR:12.3; 95% CI 2.9-51.6). Murine typhus risk was significant only in Tiruvannamalai (AOR:24.2; 95% CI 3.3-178.6). Our study suggests that prevalence of rickettsial infections and Q fever is low in areas which receive rainfall of ≥ 150 cm/year, with average minimum and maximum temperatures between 15 and 25 °C and elevation in excess of 2000 m. It is also less in well irrigated lowlands with dry climate. These preliminary findings need confirmation by active surveillance in these areas.

Antibody Prevalence and Risk Factors Associated with Rickettsia spp. in a Pediatric Cohort: SFGR Remains Underdiagnosed and Underreported in El Salvador

Spotted fever group rickettsioses (SFGR) are caused by a group of tick-borne pathogens that are increasing in incidence globally. These diseases are typically underreported and undiagnosed in low- and middle-income countries, and thus, have been classified as neglected bacterial pathogens. Countries with high poverty, low human development index score, and limited health infrastructure—like El Salvador in Central America—lack necessary surveillance for SFGR and other tick-borne pathogens. This paucity of baseline SFGR infection prevalence leaves vulnerable populations at risk of misdiagnosis. Further, tick-borne disease burdens in El Salvador are severely limited. To lay the foundation for tick-borne disease epidemiology in El Salvador, our team conducted two different enzyme-linked immunosorbent assays (ELISA) on banked human sera samples from a cohort of approximately 1000 pediatric participants from a high-risk vector-borne disease population. Eleven percent of all tested banked pediatric sera were positive for at least one ELISA assay at the time of enrollment: 10.7% were positive for only IgM antibodies (acute SFGR infection), and 2.5% were positive for IgG antibodies (a past SFGR infection). Older, male, children enrolled during the wet season, with a household history of infectious disease and higher maternal education level had higher odds of SFGR antibodies. Additionally, children from households with domestic poultry birds and previous knowledge of other vector-borne diseases had significantly reduced odds of SFGR antibodies. The large percentage of acute SFGR infections indicates that it continues to remain an underreported and undiagnosed issue in El Salvador and the Central American region. Much is still unknown regarding the complexity of the tick, animal host, and human host ecology transmission cycle of SFGR in El Salvador.

The Incidence of Orientia Tsutsugamushi Infection in Rural South India

Scrub typhus is a common bacterial infection in Asia caused by Orientia tsutsugamushi. This serological cohort study estimated the incidence of infection in a rural population in South India. Participants were enrolled through systematic sampling in 46 villages at baseline, and revisited the following year. Blood samples were tested for IgG antibodies using ELISA, followed by indirect immunofluorescence assays (IFA) in those positive for ELISA at both rounds. A case was defined as sero-conversion (ELISA), or at least a 4-fold titre increase (IFA), between the two time points. In addition to crude incidence rate estimates, we used piecewise linear rates across calendar months, with rates proportional to the monthly incidence of local hospital cases to address seasonality and unequal follow-up times. Of 402 participants, 61.7% were female. The mean age was 46.7 years, (range 13–88). 21 participants showed evidence for serological infection. The estimated incidence was 4.4 per 100 person-years (95% CI 2.8–6.7). The piecewise linear rates approach resulted in a similar estimate of 4.6 per 100 person years (95% CI 2.9–6.9). Considering previous estimates of symptomatic scrub typhus incidence in the same study population, only about 2–5% of infections may result in clinically relevant disease.

Scrub Typhus and COVID-19 Coinfection Unmasking Antiphospholipid Antibody Syndrome

Scrub typhus is an acute febrile disease caused by Orientia tsutsugamushi with a clinical course varying from mild to fatal. Vascular thrombosis is rare in scrub typhus. There is an increased risk of thrombotic events in Coronavirus disease 19 (COVID-19). We report a young diabetic female who presented with pulmonary embolism, followed by deep venous thrombosis (DVT) and was found to have coinfection with scrub typhus and COVID-19 with underlying antiphospholipid antibody syndrome.

Clinical characteristics of and antibody response to spotted fever group rickettsial infections in South India: Case series and serological cohort study

OBJECTIVE

The clinical and serological characteristics of spotted fever group rickettsial (SFGR) infections in South Asia are poorly understood. We studied the clinical presentation and the IgM/IgG response in cases enrolled at two health care centres in South India.

METHOD

We enrolled 77 patients. Fifty-seven of these patients were recruited at a tertiary care centre, the remaining 20 at a community hospital (secondary care level). Diagnostic tests included IgM and IgG enzyme-linked immunosorbent assay and polymerase chain reaction. Over a period of 1 year, 41 cases were followed up for repeated sero-analysis.

RESULTS

Median age was 9 years (range 1-79). A rash was present in 74% of cases (57/77). In cases aged <15 years, rash was present in 94% (44/47) vs. 43% (13/30) in cases aged ≥15 years. An eschar was found in two cases (3%). Severe infection or complications occurred in 10 cases (13%). These included central nervous system infection (6/77, 8%), kidney injury (3/77, 4%), shock (3/77, 4%), lung involvement (2/77, 3%) and peripheral gangrene (2/77, 3%). IgM antibody levels increased faster after fever onset than IgG antibodies, peaking at 50 and 60 days, respectively. After the peak, IgM and IgG levels showed a slow decline over one year with less than 50% of cases showing persistent IgG antibody levels.

CONCLUSION

Spotted fever group rickettsial infections in South India may be under-diagnosed, as many cases may not develop a rash. The proportion of cases developing severe infection seems lower than for scrub typhus in this region. IgG seroprevalence may substantially underestimate the proportion in a population with past SFGR infection.

Serological Detection and Epidemiological Factors Associated with Scrub Typhus among Undifferentiated Febrile Illness Patients in a Tertiary Care Hospital

Scrub typhus is an arthropod-borne zoonotic bacterial infection caused by Orientia tsutsugamushi. It presents clinically as a non-specific febrile illness that needs a high index of clinical suspicion for diagnosis. The mortality rate can be as high as 30% if not treated appropriately. Laboratory diagnosis is therefore important for confirming the cause of illness prior to initiating appropriate therapy. Hence we aimed to detect scrub typhus in serum samples of undifferentiated febrile illness patients and to correlate with the socioeconomic status of these individuals. We also aimed to study the seasonal variation associated with the disease. Serum samples from 143 febrile patients who were negative for other febrile illnesses were subjected to scrub typhus IgM ELISA. Scrub typhus IgM antibodies were found in 14 (9.8%) individuals of which 41-60 years being the most affected age group. Scrub typhus positivity was high during the months of October to December (P value 0.0056) with the individuals from the rural areas being the most affected (P value 0.027). To conclude, this study emphasises the importance of serological tests to detect scrub typhus and to include it as a differential diagnosis among undifferentiated febrile illnesses.

A spatio-temporal analysis of scrub typhus and murine typhus in Laos; implications from changing landscapes and climate

BACKGROUND

Scrub typhus (ST) and murine typhus (MT) are common but poorly understood causes of fever in Laos. We examined the spatial and temporal distribution of ST and MT, with the intent of informing interventions to prevent and control both diseases.

METHODOLOGY AND PRINCIPLE FINDINGS

This study included samples submitted from 2003 to 2017 to Mahosot Hospital, Vientiane, for ST and MT investigation. Serum samples were tested using IgM rapid diagnostic tests. Patient demographic data along with meteorological and environmental data from Laos were analysed. Approximately 17% of patients were positive for either ST (1,337/8,150 patients tested) or MT (1,283/7,552 patients tested). While both diseases occurred in inhabitants from Vientiane Capital, from the univariable analysis MT was positively and ST negatively associated with residence in Vientiane Capital. ST was highly seasonal, with cases two times more likely to occur during the wet season months of July-September compared to the dry season whilst MT peaked in the dry season. Multivariable regression analysis linked ST incidence to fluctuations in relative humidity whereas MT was linked to variation in temperature. Patients with ST infection were more likely to come from villages with higher levels of surface flooding and vegetation in the 16 days leading up to diagnosis.

CONCLUSIONS

The data suggest that as cities expand, high risk areas for MT will also expand. With global heating and risks of attendant higher precipitation, these data suggest that the incidence and spatial distribution of both MT and ST will increase.

Seroprevalence trends of Scrub typhus among the febrile patients of Northern India: A prospective cross-sectional study

BACKGROUND

Rickettsial infections remain one of the most neglected and underdiagnosed tropical diseases in the developing countries. Scrub typhus can prove to an important diagnosis in pyrexia of unknown origin (PUO) patients and is transmitted by a species of trombiculid mites ("chiggers"). The disease leads to a plethora of symptoms like fever, rash, headache, nausea, abdominal pain, thrombocytopenia, etc. The current study was aimed to assess the seroprevalence as well as other demographic parameters of scrub typhus among patients diagnosed with PUO in the northern part of India.

MATERIALS AND METHODS

This study was undertaken for a period of 3 years from September 2017 to September 2020. Serum samples of suspected cases were tested for IgM Scrub typhus along with other common febrile illnesses like Malaria, typhoid, dengue, leptospirosis, chikengunya, etc. Additional testing for COVID-19 was also planned for samples received after February 2020.

RESULTS

The overall seroprevalence of Scrub typhus during the 3 year study period was noted to be 18.6% in the PUO patients. Typhoid was noted in 39.5%, malaria in 9.2%, Dengue in 13.5%, leptospirosis in 4.8%, and chikungunya in 5.3% of the patients. No cause was identified in 9.1% of the PUO cases. 3.9% of the samples were positive by RT-PCR for COVID-19. No mortality was noted in the scrub typhus positive cases.

CONCLUSION

Scrub typhus is an emerging tropical rickettsial disease in the Indian subcontinent. The present study highlights the importance of screening of PUO cases for this important infection as timely institution of simple empirical treatment can prove to be life saving in such positive cases.

The burden of scrub typhus in India: A systematic review

BACKGROUND

Scrub typhus, a vector-borne zoonotic infection caused by the bacteria Orientia tsutsugamushi, is one of the most common and clinically important rickettsial infections worldwide. An estimated one million cases occur annually with a high case fatality rate. Although scrub typhus is a major public health threat in India, the burden and distribution remains unclear. We aimed to estimate the burden of scrub typhus in India.

METHODOLOGY

We performed a systematic review of published literature on scrub typhus from India to extract information on epidemiology, morbidity, and mortality. Important databases were searched using keywords and appropriate combinations. We identified observational, interventional, and population-based studies and extracted the data to evaluate the number of cases diagnosed using serology or PCR and the number of deaths due to scrub typhus. We conducted a systematic narrative synthesis to summarize included studies.

PRINCIPAL FINDINGS

In the last decade, there were 18,781 confirmed scrub typhus cases reported in 138 hospital-based studies and two community-based studies. IgM ELISA was used in 122 studies to confirm the cases in majority (89%). The proportion of scrub typhus among acute undifferentiated febrile illness (AUFI) studies was 25.3%, and community seroprevalence was 34.2%. Ninety studies had data published on multiple organ involvement out of which 17.4% of cases had multiple organ dysfunction syndromes, 20.4% patients required ICU admission, and 19.1% needed ventilation. The overall case-fatality rate was 6.3%, and the mortality among those with multi-organ dysfunction syndrome was as high as 38.9%.

CONCLUSION/SIGNIFICANCE

Scrub typhus, a common acute febrile illness in India causing severe morbidity, accounts for a large number of deaths. The burden of the disease has been underappreciated. Early diagnosis and prompt treatment can significantly reduce complications and mortality. Establishing good surveillance and instituting appropriate control measures are urgently needed.

High initial IgG antibody levels against Orientia tsutsugamushi are associated with an increased risk of severe scrub typhus infection

BACKGROUND

Scrub typhus is a dominant cause of febrile illness in many parts of Asia. Immunity is limited by the great strain diversity of Orientia tsutsugamushi. It is unclear whether previous infection protects from severe infection or enhances the risk.

METHODS/PRINCIPAL FINDINGS

We studied IgG antibody levels against O. tsutsugamushi at presentation in 636 scrub typhus patients using enzyme-linked immunosorbent assays (ELISA). The association between ELISA optical density (OD) and risk of severe infection was modelled using Poisson regression. OD was categorised as low (<1.0), intermediate (1.0 to 2.9), and high (≥3.0). OD was also modelled as a continuous variable (cubic spline). Median age of cases was 41 years (range 0-85), with 37% having severe infection. Compared to the low category, the age-adjusted risk of severe infection was 1.5 times higher in the intermediate category (95%CI 1.2, 1.9), and 1.3 times higher in the high category (95%CI 1.0, 1.7). The effect was stronger in cases <40 years, doubling the risk in the intermediate and high categories compared to the low category. The effect was more pronounced in cases tested within 7 days of fever onset when IgG ODs are more likely to reflect pre-infection levels.

CONCLUSIONS/SIGNIFICANCE

Intermediate and high IgG antibody levels at the time of diagnosis are associated with a higher risk of severe scrub typhus infection. The findings may be explained by severe infection eliciting an accelerated IgG response or by previous scrub typhus infection enhancing the severity of subsequent episodes.