Systematic review of the scrub typhus treatment landscape: Assessing the feasibility of an individual participant-level data (IPD) platform

Background

Scrub typhus is an acute febrile illness caused by intracellular bacteria from the genus Orientia. It is estimated that one billion people are at risk, with one million cases annually mainly affecting rural areas in Asia-Oceania. Relative to its burden, scrub typhus is understudied, and treatment recommendations vary with poor evidence base. These knowledge gaps could be addressed by establishing an individual participant-level data (IPD) platform, which would enable pooled, more detailed and statistically powered analyses to be conducted. This study aims to assess the characteristics of scrub typhus treatment studies and explore the feasibility and potential value of developing a scrub typhus IPD platform to address unanswered research questions.

Methodology/principal findings

We conducted a systematic literature review looking for prospective scrub typhus clinical treatment studies published from 1998 to 2020. Six electronic databases (Ovid Embase, Ovid Medline, Ovid Global Health, Cochrane Library, Scopus, Global Index Medicus), ClinicalTrials.gov, and WHO ICTRP were searched. We extracted data on study design, treatment tested, patient characteristics, diagnostic methods, geographical location, outcome measures, and statistical methodology.

Among 3,100 articles screened, 127 were included in the analysis. 12,079 participants from 12 countries were enrolled in the identified studies. ELISA, PCR, and eschar presence were the most commonly used diagnostic methods. Doxycycline, azithromycin, and chloramphenicol were the most commonly administered antibiotics. Mortality, complications, adverse events, and clinical response were assessed in most studies. There was substantial heterogeneity in the diagnostic methods used, treatment administered (including dosing and duration), and outcome assessed across studies. There were few interventional studies and limited data collected on specific groups such as children and pregnant women.

Conclusions/significance

There were a limited number of interventional trials, highlighting that scrub typhus remains a neglected disease. The heterogeneous nature of the available data reflects the absence of consensus in treatment and research methodologies and poses a significant barrier to aggregating information across available published data without access to the underlying IPD. There is likely to be a substantial amount of data available to address knowledge gaps. Therefore, there is value for an IPD platform that will facilitate pooling and harmonisation of currently scattered data and enable in-depth investigation of priority research questions that can, ultimately, inform clinical practice and improve health outcomes for scrub typhus patients.

Scrub typhus is a febrile illness most commonly found in rural tropical areas. It is caused by a Gram-negative bacteria belonging to the family Rickettsiaceae and transmitted by mites when they feed on vertebrates. There is an estimate of one million cases annually, with an estimated one billion people at risk, mostly in Asia-Oceania. But relative to the scale of the problem, scrub typhus is largely understudied. Evidence-based treatment recommendations by policymakers vary or are non-existent. We searched databases and registries for prospective scrub typhus clinical treatment studies published from 1998 to 2020 and reviewed them. Data from clinical trials and particularly for specific groups, such as pregnant women and children, were minimal. The methods used to measure treatment efficacy were heterogeneous, making it difficult to directly compare or conduct a meta-analysis based on aggregated data. One way to improve the current level of evidence would be by pooling and analysing individual participant-level data (IPD), i.e. the raw data from individual participants in completed studies. This review demonstrated that there is scope for developing a database for individual participant data to enable more detailed analyses. IPD meta-analyses could be a way to address knowledge gaps such as optimum dosing for children and pregnant women.

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.

Secreted Wnt antagonists in scrub typhus

BACKGROUND

The mechanisms that control local and systemic inflammation in scrub typhus have only been partially elucidated. The wingless (Wnt) signaling pathways are emerging as important regulators of inflammation and infection, but have not been investigated in scrub typhus.

METHODOLOGY/PRINCIPAL FINDINGS

Plasma levels of secreted Wnt antagonists (i.e. DKK-1, sFRP-3, WIF-1 and SOST) were analyzed in patients with scrub typhus (n = 129), patients with similar febrile illness without O. tsutsugamushi infection (n = 31), febrile infectious disease controls, and in healthy controls (n = 31) from the same area of South India, and were correlated to markers of inflammation, immune and endothelial cell activation as well as for their association with organ specific dysfunction and mortality in these patients. We found i) Levels of SOST and in particular sFRP-3 and WIF-1 were markedly increased and DKK-1 decreased in scrub typhus patients at admission to the hospital compared to healthy controls. ii) In recovering scrub typhus patients, SOST, sFRP-3 and WIF-1 decreased and DKK-1 increased. iii) SOST was positively correlated with markers of monocyte/macrophage and endothelial/vascular activation as well as with renal dysfunction and poor outcome iv) Finally, regulation of Wnt pathways by O. tsutsugamushi in vitro in monocytes and ex vivo in mononuclear cells isolated from patients with scrub typhus, as evaluated by gene expression studies available in public repositories, revealed markedly attenuated canonical Wnt signaling.

CONCLUSIONS/SIGNIFICANCE

Our findings suggest that scrub typhus is characterized by attenuated Wnt signaling possibly involving dysregulated levels of several secreted pathway antagonists. The secreted Wnt antagonist SOST was strongly associated with renal dysfunction and poor prognosis in these patients.

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.

Invasive plants facilitated by socioeconomic change harbor vectors of scrub typhus and spotted fever

BACKGROUND

Ecological determinants of most emerging vector-borne diseases are understudied, particularly for neglected tropical disease. Moreover, although socioeconomic impacts can have significant downstream effects on human risks to vector-borne diseases via a change in land cover, particularly facilitating the invasion of exotic plants, related studies remains very scarce. Scrub typhus and spotted fever are neglected diseases emerging around the globe and are transmitted by chigger mites and ticks infective of Orientia tsutsugamushi and Rickettsia spp., respectively, with small mammals as the primary hosts of both vectors.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated how invasions of the plant Leucaena leucocephala caused by widespread abandonment of farmlands driven by industrialization affected abundance of chiggers and ticks in Penghu Island, Taiwan. We determined ectoparasite abundance by trapping small mammals in three types of habitats (invasion site, agricultural field, human residential) every two months for a year. Based on ectoparasite burdens, invasion sites harbored more chiggers and ticks than the other two habitats. Furthermore, hosts maintained higher burdens of both vectors in early winter and burdens of chiggers were more stable across seasons in invasion sites, suggesting that sites with invasive plants could be a temporary refuge for both vectors and might help mitigate the negative influence of unfavorable climate. Infective rates of O. tsutsugamushi in chiggers and Rickettsia in ticks were also consistently not lower in invasion sites. Top soil temperature and relative humidity were similar across the three habitats, but invasion sites contained more of the rat Rattus losea, on which chiggers and ticks were more engorged than those from the most commonly trapped species (Suncus murinus shrew), indicating that abundance of the host R. losea instead of microclimate might better determine the abundance of both vectors.

CONCLUSIONS/SIGNIFICANCE

This study highlights an important but largely neglected issue that socioeconomic change can have unexpected consequences for human health induced particularly by invasive plants, which could become a hotspot for emerging infectious diseases but usually are very hard to be eradicated. In the future, a more comprehensive approach that integrates socio-economics, land use, exotic species, and human health should be considered to fully understand potential emergence of vector-borne diseases.

Scrub typhus ecology: a systematic review of Orientia in vectors and hosts

Scrub typhus, caused by Orientia tsutsugamushi, is an important and neglected vector-borne zoonotic disease with an expanding known distribution. The ecology of the disease is complex and poorly understood, impairing discussion of public health interventions. To highlight what we know and the themes of our ignorance, we conducted a systematic review of all studies investigating the pathogen in vectors and non-human hosts. A total of 276 articles in 7 languages were included, with 793 study sites across 30 countries. There was no time restriction for article inclusion, with the oldest published in 1924. Seventy-six potential vector species and 234 vertebrate host species were tested, accounting for over one million trombiculid mites ('chiggers') and 83,000 vertebrates. The proportion of O. tsutsugamushi positivity was recorded for different categories of laboratory test and host species. Vector and host collection sites were geocoded and mapped. Ecological data associated with these sites were summarised. A further 145 articles encompassing general themes of scrub typhus ecology were reviewed. These topics range from the life-cycle to transmission, habitats, seasonality and human risks. Important gaps in our understanding are highlighted together with possible tools to begin to unravel these. Many of the data reported are highly variable and inconsistent and minimum data reporting standards are proposed. With more recent reports of human Orientia sp. infection in the Middle East and South America and enormous advances in research technology over recent decades, this comprehensive review provides a detailed summary of work investigating this pathogen in vectors and non-human hosts and updates current understanding of the complex ecology of scrub typhus. A better understanding of scrub typhus ecology has important relevance to ongoing research into improving diagnostics, developing vaccines and identifying useful public health interventions to reduce the burden of the disease.

Risk factors for acquisition of scrub typhus in children admitted to a tertiary centre and its surrounding districts in South India: a case control study

BACKGROUND

Scrub typhus is a mite borne zoonosis common in the tropics with no good preventive strategy. Children are also affected leading to considerable morbidity and mortality. We conducted a case control study and a vector survey to determine the risk factors for acquisition of scrub typhus.

METHODS

A case control study with a 1:2 case control ratio was conducted over a 2 year period at a tertiary care centre and its surrounding districts in South India. Cases were children < 15 years with confirmed scrub typhus. Controls were age and locality matched community controls without fever. Demographic, environmental and behavioural risk factors were obtained in cases and controls by an interview and an environmental survey. A vector survey was also undertaken in the immediate vicinity of the cases.

RESULTS

Case Control study: 101 cases and 167 controls were analysed. On multivariate analysis, significant association was observed with environmental factors such as the presence of a water body within 100 m of the house (OR 3.56(1.36,9.75); p 0.011), cooking outside the house (OR 5.61 (1.51,23.01); p 0.011), owning pets (OR 3.33(1.16,9.09); p 0.031), and the presence of bushes within 5 m of the house (OR 2.78 (1.11,7.69); p 0.033). Of the behavioural factors, the child going to school by a vehicle (OR 3.12 (2.29,8.37); p 0.006) was associated with an increased risk. Drying clothes on a clothesline showed a trend towards protection from acquiring scrub typhus (OR 0.31 (0.08, 1.08); p 0.077). Vector survey:26 rodents were trapped in as many houses. Trombiculid mites were isolated in 24 houses with 9(34.6%) being able to transmit scrub typhus. 254 trombiculid mites belonging to four species and two genera were collected. Leptotrombidium deliense, (33.5%). Schoengastiella ligula, (11.0%) of the total mite specimens collected. S. ligula always co-existed with L. deliense. The estimated Chigger index for Leptotrombidium deliense and Schoengastiella ligula was 3.27and 1.08 per animal respectively.

CONCLUSIONS

Our study highlights risk factors for scrub typhus, some of which may be modifiable. A clean peri-domestic environment free of vegetation, drying clothes on a clothesline and cooking indoors may decrease the risk of scrub typhus.

Transcriptional profiles of cytokines and chemokines reveal important pro-inflammatory response from endothelial cells during Orientia tsutsugamushi infection

Endothelial cells (EC) are key targets during Orientia tsutsugamushi infection. Knowledge of the pro-inflammatory response against O. tsutsugamushi by ECs is limited. The aim of the present study was to characterize the pro-inflammatory transcriptional response during the first 24 h of infection of the human dermal microvascular endothelial cell line with O. tsutsugamushi Karp by examining five-time points. The transcriptional profiles of 84 genes including cytokines, chemokines, growth factors, and TNF receptor superfamily genes were studied using a RT-PCR array. We identified 40 of the 84 genes that were up or down modulated during the early O. tsutsugamushi infection that differed remarkably from genes of non-infected cells. The modulated genes included: the interleukins (IL-1α/β, IL-4, IL-6, IL-7, IL-10, IL-11, IL-18, and IL-24), chemokines (CXCL8, CCL2/MCP1, CCL5/RANTES, and CCL17), growth factors (NODAL, CNTF, and CSF2/GM-CSF), and TNFSF13B. IL-1β, IL-4, and IL-11 were highly induced at one hour post infection, whereas, CCL17 was profoundly up-regulated and IFNα2 was greatly down-regulated during the entire 24-hour time course. These results provide insight into the early pro-inflammatory response of endothelial cells to O. tsutsugamushi infection and indicate their potential role in the pathophysiology of the host's initial response to O. tsutsugamushi infection.

Survival and Growth of Orientia tsutsugamushi in Conventional Hemocultures

Orientia tsutsugamushi, which requires specialized facilities for culture, is a substantial cause of disease in Asia. We demonstrate that O. tsutsugamushi numbers increased for up to 5 days in conventional hemocultures. Performing such a culture step before molecular testing could increase the sensitivity of O. tsutsugamushi molecular diagnosis.

In Vitro Activity of Tigecycline Against Orientia tsutsugamushi

Scrub typhus is a zoonosis caused by Orientia tsutsugamushi (O. tsutsugamushi) occurring mainly in autumn in Korea. The need of new antibiotics has arisen with a report on strains resistant to antibiotics and chronic infection. This study aims to identify susceptibility of tigecycline in-vitro as a new therapeutic option for O. tsutsugamushi. Antibacterial activity of tigecycline against the O. tsutsugamushi was compared with doxycycline using flow cytometry assay. The inhibitory concentration 50 (IC₅₀) was 3.59×10⁻³ μg/mL in doxycycline-treated group. Whereas in 0.71×10⁻³ μg/mL tigecycline-treated group. These findings indicate that tigecycline may be a therapeutic option for the treatment of scrub typhus.

Comparative morphology and ultrastructure of the prosomal salivary glands in the unfed larvae Leptotrombidium orientale (Acariformes, Trombiculidae), a possible vector of tsutsugamushi disease agent

The prosomal salivary glands of the unfed larvae Leptotrombidium orientale (Schluger) were investigated using transmission electron microscopy. In total, four pairs of the prosomal glands were identified--three pairs, the lateral, the medial and the anterior, belong to the podocephalic system, and one pair, the posterior, is separate having an own excretory duct. All glands are simple alveolar/acinous with prismatic cells arranged around a relatively small intra-alveolar lumen with the duct base. The cells of all glands besides the lateral ones contain practically mature electron-dense secretory granules ready to be discharged from the cells. The secretory granules in the lateral glands undergo formation and maturation due to the Golgi body activity. The cells of all gland types contain a large basally located nucleus and variously expressed rough endoplasmic reticulum. Specialized duct-forming cells filled with numerous freely scattered microtubules are situated in the middle zone of each gland's acinus and form the intra-alveolar lumen and the duct base. Both the acinar (secretory) and the duct-forming cells contact each other via gap junctions and septate desmosomes. Axons of nerve cells come close to the basal extensions of the duct-forming cells where they form the bulb-shaped synaptic terminations. The process of secretion is under the control of the nerve system that provides contraction of the duct-forming cells and discharge of secretion from the secretory cells into the intra-alveolar lumen and further to the exterior. Unfed larvae of L. orientale, the potential vector of tsutsugamushi disease agents, contain the most simply organized salivary secretory granules among known trombiculid larvae, and this secretion, besides the lateral glands, does not undergo significant additional maturation. Thus, the larvae are apparently ready to feed on the appropriate host just nearly after hatching.

Establishment of Orientia tsutsugamushi Lc-1 (Rickettsiales: Rickettsiaceae) infection in ICR outbred mice (Rodentia: Muridae) by needle challenge

Orientia tsutsugamushi is a pathogen transmitted by Leptotrombidium that causes scrub typhus. To develop an infection mouse model, a mite-derived isolate of O. tsutsugamushi was established from a laboratory-maintained colony of Leptotrombidium chiangraiensis (O. tsutsugamushi Lc-1). This Lc-1 isolate was initially presented to ICR (CD-1) mice by feeding an infected Lc chigger on the ear of a mouse. Once the Lc-1 was adapted to the ICR mice, quantitative real-time polymerase chain reaction was used to investigate O. tsutsugamushi genomic equivalent copies in tissues and sera. Furthermore, times to onset of the signs of infection are reported in this study. This study provides information useful for future research on this host-pathogen interaction and the associated vaccine efficacy trials.

Repellency of cassia bark, eucalyptus, and star anise oils and their major constituents to Leptotrombidium pallidum (Acari: Trombiculidae)

Leptotrombidium pallidum (Nagoya, Miyagawa, Mitamura & Tamiya) is a primary vector of Orientia tsutsugamushi (Hyashi), the causative agent of scrub typhus. An assessment is made of the repellency to L. pallidum larvae (chiggers) of cassia bark, eucalyptus, and star anise oils and major constituents (E)-cinnamaldehyde, 1,8-cineole, and (E)-anethole of the corresponding oils. Results were compared with those of conventional repellents DEET (N,N-diethyl-3-methylbenzamide), IR3535 [(ethyl 3-[acetyl(butyl)amino]propanoate)], and permethrin. Based on the median repellent concentration (RC50) values, (E)-cinnamaldehyde, (E)-anethole, cassia bark oil, and star anise oil (RC50, 0.95-1.52 mg/cm2) exhibited significantly more potent repellency than DEET (3.85 mg/cm2). (E)-cinnamaldehyde, (E)-anethole, cassiabark oil, 1,8-cineole, and star anise oil were approximately 43, 16, 11, 8, and 4 times more effective than IR3535 (CC5, 6.51%) as judged by the median climbing distance-disturbing concentration (CC50) values. The median residual duration time of repellency (RT50) was significantly more pronounced in DEET (RT50, 323 min) than in all essential oils and constituents (108-167 min). In the light of global efforts to reduce the level of highly toxic synthetic repellents, the three essential oils and their major constituents described merit further study as potential biorepellents for the control of L. pallidum populations.

Orientia tsutsugamushi subverts dendritic cell functions by escaping from autophagy and impairing their migration

Background

Dendritic cells (DCs) are the most potent antigen-presenting cells that link innate and adaptive immune responses, playing a pivotal role in triggering antigen-specific immunity. Antigen uptake by DCs induces maturational changes that include increased surface expression of major histocompatibility complex (MHC) and costimulatory molecules. In addition, DCs actively migrate to regional lymph nodes and activate antigen-specific naive T cells after capturing antigens. We characterize the functional changes of DCs infected with Orientia tsutsugamushi, the causative agent of scrub typhus, since there is limited knowledge of the role played by DCs in O. tsutsugamushi infection.

Methodology/Principal Finding

O. tsutsugamushi efficiently infected bone marrow-derived DCs and induced surface expression of MHC II and costimulatory molecules. In addition, O. tsutsugamushi induced autophagy activation, but actively escaped from this innate defense system. Infected DCs also secreted cytokines and chemokines such as IL-6, IL-12, MCP5, MIP-1α, and RANTES. Furthermore, in vitro migration of DCs in the presence of a CCL19 gradient within a 3D collagen matrix was drastically impaired when infected with O. tsutsugamushi. The infected cells migrated much less efficiently into lymphatic vessels of ear dermis ex vivo when compared to LPS-stimulated DCs. In vivo migration of O. tsutsugamushi-infected DCs to regional lymph nodes was significantly impaired and similar to that of immature DCs. Finally, we found that MAP kinases involved in chemotactic signaling were differentially activated in O. tsutsugamushi-infected DCs.

Conclusion/Significance

These results suggest that O. tsutsugamushi can target DCs to exploit these sentinel cells as replication reservoirs and delay or impair the functional maturation of DCs during the bacterial infection in mammals.

Scrub typhus is an acute febrile illness caused by Orientia tsutsugamushi infection and is one of the main causes of febrile illness in the Asia-Pacific region. If not properly treated with antibiotics, patients often develop severe vasculitis that affects multiple organs, and the mortality rate of untreated patients reaches up to 30%. To understand the pathogenic mechanisms of the infectious disease, we characterized the functional changes of O. tsutsugamushi–infected dendritic cells (DCs), which play a pivotal role in orchestrating innate and adaptive immune responses. The obligate intracellular bacteria efficiently infected bone marrow-derived DCs and activated the cells as measured by induced surface expression of MHC II and costimulatory molecules, secretion of cytokines and chemokines, and autophagy induction. However, the live bacteria actively escaped from host autophagosomes and the migration of infected cells was severely impaired in vitro, ex vivo, and in vivo infection models. Finally, we found that MAP kinases involved in chemotactic signaling were differentially activated in O. tsutsugamushi-infected DCs. These results suggest that O. tsutsugamushi can target DCs to exploit these sentinel cells as replication reservoirs and delay or impair the functional maturation of DCs during the bacterial infection in mammals.

Influence of Orientia tsutsugamushi infection on the developmental biology of Leptotrombidium imphalum and Leptotrombidium chiangraiensis (Acari: Trombiculidae)

Leptotrombidium chiangraiensis Tanskul & Linthicum, and Leptotrombidium imphalum Vercammen-Grandjean are important vectors of scrub typhus in rice field habitats in northern Thailand. The developmental biology of all stages of the life cycle of two generations of these species of mites infected with Orientia tsutsugamushi (Hayashi) and uninfected mites is reported. The development of the infected lines of both F1 and F2 L. chiangraiensis were significantly longer than their respective uninfected lines (P < 0.05). The developmental times of uninfected and infected F1 lines of L. imphalum were not significantly different; however, F2 infected lines took significantly longer to develop (P < 0.05). Both F1 and F2 generations of infected L. imphalum and L. chiangraiensis oviposited on average >150 fewer eggs than uninfected mites.

Variable clinical responses of a scrub typhus outbred mouse model to feeding by Orientia tsutsugamushi infected mites

Rodents are the natural hosts for Leptotrombidium mites that transmit Orientia tsutsugamushi, the causative agent of scrub typhus, a potentially fatal febrile human disease. Utilizing mite lines that included O. tsutsugamushi infected and non-infected Leptotrombidium species we investigated the varied infection response of outbred mice (ICR) exposed to L. chiangraiensis (Lc), L. imphalum (Li) and L. deliense (Ld). Each of six mite lines (Lc1, Lc5, Li3, Li4, Li7 and Ld) was separately placed in the inner ears of ICR mice either as a single individual (individual feeding, IF) or as a group of 2-4 individuals (pool feeding, PF). The species of infected chigger feeding on mice significantly affected mortality rates of the mice, with mite lines of Lc causing higher mean (±SE) mortality (90.7 ± 3.6 %) than mite lines of Li (62.9 ± 5.6 %) or Ld (53.6 ± 5.8 %). Mouse responses which included time to death, food consumption and total mice weight change depended on mite species and their O. tsutsugamushi genotype, more than on feeding procedure (IF vs. PF) except for mite lines within the Lc. Infected mite lines of Lc were the most virulent infected mites assessed whereas the infected Ld species was the least virulent for the ICR. Mice killed by various mite lines showed enlarged spleens and produced ascites. The results of this investigation of the clinical responses of ICR mice to feeding by various infected mite lines indicated that the different species of infected mites and their O. tsutsugamushi genotype produced different clinical presentations in ICR mice, a scrub typhus mouse model which mimics the natural transmission of O. tsutsugamushi that is critical for understanding scrub typhus disease in terms of natural transmission, host-pathogen-vector interaction and vaccine development.

Host cell/Orientia tsutsugamushi interactions: evolution and expression of syndecan-4 in Asian rodents (Rodentia, Muridae)

Scrub typhus is an acute febrile zoonotic disease and worldwide more than a billion people may be at risk for infection. Orientia tsutsugamushi, the causative agent of scrub typhus, is an obligate intracellular bacterium. Rodents are reported to be the primary reservoir hosts of the disease and according to the most recent surveys, all species within the Rattus sensu lato complex of the tribe Rattini are carriers of scrub typhus. There is no evidence that any of mouse (Mus) species serves as the primary reservoir of the bacterium even when occurring in sympatry with wild infected rats. This contrast in the host/syndecan-4 interactions between Rattini and Asian Murini may be due to intrinsic (i.e., genetic) differences. Herein we compare the sequence and expression levels of syndecan-4 (the putative cell receptor of O. tsutsugamushi) between Rattini species that are known to be natural reservoirs for the typhus agents, and Murini species that are not. Although it was not possible to conclusively link the structural variations detected in syndecan-4 with carrier status in either Rattini and Murini, our findings indicate the absence of a strong Orientia-mediated selective regime acting on gene structure. In contrast, variable spleen-specific syndecan-4 expression levels show a strong correlation between under-expression of syndecan-4 in Murini and seropositive Rattini, compared to seronegative Rattini rodents. We postulate that two divergent responses may be at work in Murini and Rattini, both linked with differential expression of syndecan-4: (i) reduced syndecan-4 transcription in Murini decreases the likelihood that the host cells will become infected by the Orientia bacterium, while (ii) reduced syndecan-4 expression in seropositive Rattini limits the pathogenicity of Orientia and consequently improves the longevity of the rat hosts. These patterns may underpin the poor carrier status of wild mice on the one hand, and the effective role of wild rats as reservoir hosts on the other.

Exploitation of the endocytic pathway by Orientia tsutsugamushi in nonprofessional phagocytes

Orientia tsutsugamushi, a causative agent of scrub typhus, is an obligate intracellular bacterium that requires the exploitation of the endocytic pathway in the host cell. We observed the localization of O. tsutsugamushi with clathrin or adaptor protein 2 within 30 min after the infection of nonprofessional phagocytes. We have further confirmed that the infectivity of O. tsutsugamushi is significantly reduced by drugs that block clathrin-mediated endocytosis but not by filipin III, an inhibitor that blocks caveola-mediated endocytosis. In the present study, with a confocal microscope, O. tsutsugamushi was sequentially colocalized with the early and late endosomal markers EEA1 and LAMP2, respectively, within 1 h after infection. The colocalization of O. tsutsugamushi organisms with EEA1 and LAMP2 gradually disappeared until 2 h postinfection, and then free O. tsutsugamushi organisms were found in the cytoplasm. When the acidification of endocytic vesicles was blocked by treating the cells with NH(4)Cl or bafilomycin A, the escape of O. tsutsugamushi organisms from the endocytic pathway was severely impaired, and the infectivity of O. tsutsugamushi was drastically reduced. To our knowledge, this is the first report that the invasion of O. tsutsugamushi is dependent on the clathrin-dependent endocytic pathway and the acidification process of the endocytic vesicles in nonprofessional phagocytes.

Electron microscopic observations of Orientia tsutsugamushi in salivary gland cells of naturally Infected Leptotrombidium pallidum larvae during feeding

We performed a detailed electron microscopic observation on the escaping process of Orientia tsutsugamushi from the salivary gland cells of naturally infected trombiculid larvae into the acinar lumen of the gland during feeding on mice. In unfed larvae, many O. tsutsugamushi were intermingled with secretory granules in the cytoplasm of the salivary gland cell. O. tsutsugamushi was neither found in the acinar lumen nor observed escaping from the apical surface of the gland cell. In contrast, in the larvae fed on mice, many O. tsutsugamushi were observable in the acinar lumen. They were enveloped with the host glandular cell membrane. In salivary gland cells, secretory granules changed the distribution and accumulated in the apical region. In such cells, the majority of O. tsutsugamushi were found at the base of the cell. Some O. tsutsugamushi were pushing the glandular cell membrane outward in various degrees, showing different stages of escape. These findings suggest that larval feeding induced O. tsutsugamushi escape from salivary gland cells, that the escape was by budding, during which O. tsutsugamushi were enveloped in the host cell membrane, and that O. tsutsugamushi would be injected into the mouse skin as a mixture with mite saliva. The study also revealed the presence of many small vesicles that had the same cell wall structure as O. tsutsugamushi in the cytoplasm of the salivary gland cell. Most of them seemed to be products from degenerated Orientia.

In vitro reactivation of human immunodeficiency virus-1 upon stimulation with scrub typhus rickettsial infection

While a number of microbial infections induce a transient burst in viral load in individuals infected with human immunodeficiency virus-1 (HIV-1), a recent study has suggested that scrub typhus may suppress HIV-1 infection. We investigated the effects of Orientia tsutsugamushi on HIV-1 infection. In vitro HIV-1 infection experiments were conducted using peripheral blood mononuclear cells (PBMC) acutely infected with R5 and X4 HIV-1 or PBMC derived from patients receiving highly active antiretroviral therapy (HAART) whose plasma viral load was undetectable. Stimulation of PBMC with O. tsutsugamushi induced production of proinflammatory cytokines and beta-chemokines, and markedly down-regulated expression of CCR5. Although pretreatment with O. tsutsugamushi rendered PBMC resistant to R5 HIV-1, it otherwise enhanced HIV-1 replication. Stimulation by O. tsutsugamushi induced HIV-1 replication in PBMC from patients receiving HAART. These findings suggest that scrub typhus does not necessarily suppress HIV-1 infection and does have potential to enhance HIV-1 replication.

Orientia tsutsugamushi inhibits apoptosis of macrophages by retarding intracellular calcium release

Orientia tsutsugamushi shows both pro- and antiapoptotic activities in infected vertebrate cells. Apoptosis of THP-1 cells induced by beauvericin was inhibited by O. tsutsugamushi infection. Beauvericin-induced calcium redistribution was significantly reduced and retarded in cells infected with O. tsutsugamushi. Antiapoptotic activities of O. tsutsugamushi in infected cells are most probably due to inhibition of the increase in the cytosolic calcium concentration.

[HIV-suppressive factors]

A number of host and microbial factors have been shown to modulate HIV-1 infection. Their inhibitory effects are either HIV-specific or non-specific, and involve many different kinds of mechanisms. Among anti-HIV host factors are natural ligands or natural antibodies to HIV coreceptors, anti-inflammatory cytokines, interferons and several body fluid components (such as lactoferrin and prostaglandins). Microbial pathogens/factors that may suppress HIV-1 infection include lipopolysaccharide, scrub-typhus rickettsia, human herpesviruses-6 or -7, and GB virus C. While simple application of these HIV-suppressive factors for HIV-infected individuals is not realistic, investigation of mechanisms involved may lead to better understanding of HIV pathogenesis and help establish novel anti-HIV strategy.

Orientia tsutsugamushi suppresses the production of inflammatory cytokines induced by its own heat-stable component in murine macrophages

Orientia tsutsugamushi is a Gram-negative obligate intracellular bacterium, which causes scrub typhus. To understand the pathogenesis of scrub typhus, we have investigated the induction of tumor necrosis alpha (TNF-alpha) and interleukin-6 (IL-6) by O. tsutsugamushi in two murine macrophage cell lines. Both live and heat-killed orientia stimulated the production of cytokines in J774A.1 cells. Polymyxin B does not affect the secretion of cytokines. These together with the fact that the immature macrophage cell line, P388D1, did not produce TNF-alpha when induced by either live or heat-killed O. tsutsugamushi strongly argue against any roles of lipopolysaccharide (LPS) in cytokine production. Furthermore, the result that the cytokine responses were more brisk when macrophage cell lines had been induced by heat-killed O. tsutsugamushi than by live organisms strongly suggest that a heat-stable molecule might be responsible for the induction of cytokine production and O. tsutsugamushi might have mechanisms suppressing the production of inflammatory cytokines induced by its own heat-stable molecule.

Microtubule- and dynein-mediated movement of Orientia tsutsugamushi to the microtubule organizing center

The host cell microfilaments and microtubules (MTs) are known to play a critical role in the life cycles of several pathogenic intracellular microbes by providing for successful invasion and promoting movement of the pathogen once inside the host cell cytoplasm. Orientia tsutsugamushi, an obligate intracellular bacterium, enters host cells by induced phagocytosis, escapes to the cytosol, and then replicates in the cytosol. ECV304 cells infected with O. tsutsugamushi revealed the colocalization of the MT organizing center (MTOC) and cytosolic orientiae by indirect immunofluorescence assay. Using immunofluorescence microscopy in the presence and absence of MT-depolymerizing agents (colchicine and nocodazole), it was shown that the cytosolic oriential movement was mediated by MTs. By transfection study (overexpression of dynamitin [also called p50], which is known to associate with dynein-dependent movement), the movement of O. tsutsugamushi to the MTOC was also mediated by dynein, the minus-end-directed MT-related motor. Although the significance of this movement in the life cycle of O. tsutsugamushi was not proven, we propose that the cytosolic O. tsutsugamushi bacteria use MTs and dyneins to propel themselves from the cell periphery to the MTOC.

Apoptosis of endothelial cell line ECV304 persistently infected with Orientia tsutsugamushi

Endothelial cells are major targets of Orientia tsutsugamushi. To examine the consequences of the infection of endothelial cells with O. tsutsugamushi, we used human endothelial cell line ECV304. Persistent infection was established and infected cultures could be maintained for over seven months without the addition of normal cells. The heavily infected cells became round and floated in the culture medium, harboring large numbers of organisms inside them. Some of the infected ECV304 cells showed features of apoptotic cells, as determined by the terminal deoxytransferase-mediated dUTP nick end-labeling reaction and DNA fragmentation. We also found that O. tsutsugamushi increased transcription of the mRNAs of proinflammatory cytokines such as IL-6 and IL-8. These results show the first evidence of in vitro-persistent infection by O. tsutsugamushi, which may be related to in vivo persistence reported previously.

Recovery and viability of Orientia tsutsugamushi from packed red cells and the danger of acquiring scrub typhus from blood transfusion

BACKGROUND

The purpose of this study was to determine whether infective Orientia tsutsugamushi, the etiologic agent of scrub typhus, could survive normal blood banking processing and storage procedures.

STUDY DESIGN AND METHODS

Mononuclear cells isolated from whole blood by density gradient centrifugation were inoculated with O. tsutsugamushi, Karp strain. Infection of the mononuclear cells was confirmed by Giemsa stain, direct fluorescent antibody assay, and polymerase chain reaction using primers specific for the groESL operon of O. tsutsugamushi. The quantity of rickettsial particles in each preparation was determined by direct counts from the Giemsa-stained preparations. Infected mononuclear cells were returned to their respective aliquots of packed red blood cells, which were then either stored at 4 degrees C or glycerolized and frozen at -70 degrees C.

RESULTS

Rickettsiae survived up to 10 days (but not 30 days) of refrigerated storage and 45 days of frozen storage, as determined by inoculation of mice with 0.5-mL aliquots of the blood components. Infection of the mice was determined by illness, death, direct fluorescent antibody assay of peritoneal smears, polymerase chain reaction of blood, and enzyme-linked immunosorbent assay detection of antibodies in plasma.

CONCLUSION

Because the quantity of rickettsiae injected into the mice was comparable to the quantity reported in the literature for human blood during natural infections, scrub typhus could present a risk in blood collected from donors in endemic areas. This may especially be true, because people can be rickettsemic before illness, after successful antibiotic treatment, and chronically after resolution of disease.

An ultrastructural study of vertical transmission of Rickettsia tsutsugamushi during oogenesis and spermatogenesis in Leptotrombidium pallidum

Rickettsia tsutsugamushi in Leptotromibidium pallidum was observed by electron microscopy and rickettsiae were found in the various tissues and organs of both larvae and adults. Budding of rickettsiae, a manner of release from the host cells, was observed only in the rudiments of the reproductive organs in larvae. Oogonia and maturing oocytes in adult females and eggs after oviposition contained the microorganisms. In adult males, rickettsiae were also found in the spermatogonia, spermatocytes, and spermatids in the early stage of spermatogenesis, but were eliminated from these cells during maturation. Only the maturing spermatids, but not the eliminated rickettsiae, migrated to another rickettsia-free area of the testis, resulting in the separation of spermatids from rickettsiae and in the production of rickettsia-free spermatophores. Based on these observations, the mechanism of vertical transmission of the rickettsiae to the progeny occurs only in the female parents. Most rickettsiae in the somatic cells of larvae and adults were coccoid, but some rickettsiae in the ovary and the testis of adult mites showed bacillary forms and were enveloped by a membrane of unknown origin.

Rickettsial diseases of the Far East: new perspectives

The rickettsial disease of man found only in Asia is mite-borne (scrub) typhus, caused by Rickettsia tsutsugamushi. Unique to southern Japan is a little-known human mononucleosis-like disease caused by Rickettsia sennetsu. In 1981 and 1982, there was a remarkable resurgence in the number of reported cases of mite-borne typhus in Japan after some years of virtual absence. Recent studies of R sennetsu have resulted in its reclassification to the genus Ehrlichia, members of which until now have been exclusively pathogens of animals. The historical background of ecologic investigations, in Malaysia and elsewhere, of these two developments suggest directions for future research.

Effect of immune serum on infectivity of Rickettsia tsutsugamushi

Hyperimmune antirickettsial serum was shown to prevent the attachment/penetration stage of Rickettsia tsutsugamushi infection of suspended chicken cells. The extent of the inhibition depended on the serum concentration but not on the presence of complement. The neutralizing activity was reduced by prior adsorption of immune serum with staphylococcal protein A or with intact rickettsiae but was not affected by adsorption with target cells. In the neutralization tests, there was no cross-reactivity between the Karp and Gilliam strains of R. tsutsugamushi. Incubation of rickettsiae with immune serum did not alter their capacity to metabolize glutamate nor grossly damage the permeability barrier function of their cytoplasmic membranes. Although the assay method had the capacity to detect some aggregated infectious organisms, none were found in immune serum-treated suspensions. It was concluded that immune serum may inhibit rickettsial infection by blocking a surface component(s) whose function is necessary for attachment to and/or penetration of target cells.

Penetration of Rickettsia tsutsugamushi into cultured mouse fibroblasts (L cells): an electron microscopic observation

The mechanism of penetration of purified Rickettsia tsutsugamushi (Gilliam strain) into cultured mouse fibroblasts (L cells) was examined by electron microscopy. After 10-40 min of infection, rickettsiae in the process of being phagocytized were often seen on the cell surface. These were restricted to the rickettsiae which seemed to be intact in morphology, while heavy plasmolyzed ones were never phagocytized. Additionally, rickettsiae were taken up individually into a phagosome, and phagocytosis of several rickettsiae together was rarely observed, except in the case of heat-inactivated microorganisms. In the cells, phagosomes whose membranes enclosed rickettsiae either tightly or loosely were seen. Rickettsiae in the loose phagosomes often showed signs of plasmolysis and were rarely released into the cell cytoplasm. Partial disintegration of phagosomal membranes and the escape of rickettsiae from the phagosomes were seen only in tight phagosomes. Large phagosomes containing a clump of several rickettsiae were observed occasionally, in which case the microorganisms were deformed and seemed to be denatured. From the above observations and the frequency of appearance of these different penetration stages in the specimens 10, 20, and 40 min after infection, it was concluded that the rickettsiae enter initially into a tight phagosome by phagocytosis and are then released into the cell cytoplasm by disruption of the phagosomal membrane. No other mechanisms of penetration were found. On the other hand, rickettsiae inactivated by trypsin did not attach to host cells. Inactivation by heat or UV irradiation resulted in reduction of phagocytosis, and rickettsiae treated with rifamycin could penetrate into the host cell cytoplasm to the same extent as in the case of infection with intact rickettsiae.

Effect of antibody on entry of Rickettsia tsutsugamushi into polymorphonuclear leukocyte cytoplasm

The effects of rickettsial antibodies on the entry of Rickettsia tsutsugamushi, Gilliam strain, into guinea pig polymorphonuclear leukocyte cytoplasm were studied by electron microscopy. Immunoglobulin G fractions were obtained from four rabbit antisera against: yolk sac-propagated rickettsiae; baby hamster kidney cell (BHK-21)-propagated rickettsiae; formaldehyde-fixed, BHK-21 cell-propagated rickettsiae; and glutaraldehyde-fixed, BHK-21 cell-propagated rickettsiae. A fuzzy coating was observed on the rickettsiae after reaction with each of the antibodies. All of the antibodies increased the uptake of rickettsiae by polymorphonuclear leukocytes. The opsonization effect was higher with an antibody against BHK-21 cell-propagated rickettsiae than with an antibody against yolk sac-propagated rickettsiae, and an antibody to live rickettsiae had a higher opsonization effect than did antibodies to chemically fixed rickettsiae. Rickettsiae were released from phagosomes into the cytoplasm with the four antibodies. The highest number of rickettsiae were released into the cytoplasm with antibody against live rickettsiae propagated in BHK-21 cells. The four antibodies inhibited the translocation of the cytoplasmic rickettsiae from the filamentous area to glycogen-rich zones. Almost 100% inhibition of translocation was observed with antibodies against live rickettsiae. These results indicate that rabbit antibodies against rickettsiae, when used alone, were unable to completely prevent rickettsial entry into polymorphonuclear leukocyte cytoplasm in vitro.

Entry of Rickettsia tsutsugamushi into polymorphonuclear leukocytes

Factors involved in the phagocytosis and entry into polymorphonuclear leukocytes (PMNs) of Rickettsia tsutsugamushi were studied by electron microscopy. R. tsutsugamushi propagated in baby hamster kidney cell cultures was incubated with guinea pig peritoneal PMNs in vitro at 35 degrees C. Structurally intact and degenerating rickettsiae were found in phagosomes, but only intact rickettsiae escaped phagosomes and specifically entered the glycogen-rich cytoplasm. The extraphagosomal cytoplasmic rickettsiae were found within 30 min after incubation; continued incubation for 4 h increased the rickettsial entry about fourfold as seen in ultrathin sections. Most rickettsiae in phagosomes were degenerating after 4 h of incubation. When incubated at 25 degrees C, no entry and very few phagocytized rickettsiae were observed. At 40 degrees C, rickettsial entry was greatly reduced, but more rickettsiae were found in phagosomes than at 35 degrees C. Preincubation of rickettsiae at 56 degrees C for 20 min with trypsin or with 2,4-dinitrophenol inhibited entry, but many rickettsiae were in phagosomes. Glutaraldehyde or formaldehyde fixation of rickettsiae and addition of 2-deoxyglucose, iodoacetamide, cytochalasin B, colchicine, or vinblastine inhibited all rickettsial uptake by PMNs. Acid phosphatase cytochemistry of infected PMNs revealed the enzyme activity only in phagosomes with degenerated rickettsiae and not in those with intact rickettsiae. These observations indicated that rickettsiae are passively phagocytized by PMNs, and only those that are intact actively escape from phagosomes, which selectively inhibits lysosomal fusion.