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

Abstract 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.</jats:p

A RAPID IN VITRO METHOD FOR THE EVALUTION OF CANDIDATE REPELLENTS AGAINST LEPTOTROMBIDIUM CHIGGERS

Scrub typhus is an acute febrile zoonotic disease resulting from infection with the Gram-negative intracellular bacteria Orientia (formerly Rickettsia) tsutsugamushi (Hyachi) (Seong et al. 2001). The disease is endemic in much of south and central Asia, with approximately one million cases each year and more than a billion people at risk worldwide (Rosenberg 1997). Scrub typhus is transmitted by several species of larval trombiculid mites which are commonly known as chiggers (Tanskul et al. 1998). Repellents provide an effective method of protecting individuals from arthropods (Gupta and Rutledge 1994). In this study 6 essential oils were tested to evaluate their repellent activity against the chigger, Leptotrombidium imphalum Vercammen-Grandjean and Langston. A rapid and economic in vitro procedure which requires only 5 min and a small number of chiggers was used to determine the median effective doses. The results showed that clove oil was significantly more effective than others with ED50 of 53.2 µg followed by vetiver oil

A RAPID IN VITRO METHOD FOR THE EVALUATION OF CANDIDATE REPELLENTS AGAINST LEPTOTROMBIDIUM CHIGGERS

Scrub typhus is an acute febrile zoonotic disease resulting from infection with the gram-negative intracellular bacteria Orientia (formerly Rickettsia) tsutsugamushi (Hyachi) (Seong et al. 2001). The disease is endemic in much of south and central Asia, with approximately one million cases each year and more than a billion people at risk worldwide (Rosenberg 1997). Scrub typhus is transmitted by several species of larval trombiculid mites which are commonly known as chiggers (Tanskul et al. 1998). Repellents provide an effective method of protecting individuals from arthropods (Gupta and Rutledge 1994). In this study 6 essential oils were tested to evaluate their repellent activity against the chigger, Leptotrombidium imphalum Vercammen-Grandjean and Langston. A rapid and economic in vitro procedure which requires only 5 min and a small number of chiggers was used to determine the median effective doses. The results showed that clove oil was significantly more effective than others with ED50 of 53.2 µg followed by vetiver oil

A RAPID IN VITRO METHOD FOR THE EVALUTION OF CANDIDATE REPELLENTS AGAINST LEPTOTROMBIDIUM CHIGGERS

Scrub typhus is an acute febrile zoonotic disease resulting from infection with the Gram-negative intracellular bacteria Orientia (formerly Rickettsia) tsutsugamushi (Hyachi) (Seong et al. 2001). The disease is endemic in much of south and central Asia, with approximately one million cases each year and more than a billion people at risk worldwide (Rosenberg 1997). Scrub typhus is transmitted by several species of larval trombiculid mites which are commonly known as chiggers (Tanskul et al. 1998). Repellents provide an effective method of protecting individuals from arthropods (Gupta and Rutledge 1994). In this study 6 essential oils were tested to evaluate their repellent activity against the chigger, Leptotrombidium imphalum Vercammen-Grandjean and Langston. A rapid and economic in vitro procedure which requires only 5 min and a small number of chiggers was used to determine the median effective doses. The results showed that clove oil was significantly more effective than others with ED50 of 53.2 µg followed by vetiver oil

Development of multiplex PCR for neglected infectious diseases.

Scrub typhus, murine typhus, and leptospirosis are widely neglected infectious diseases caused by Orientia tsutsugamushi, Rickettsia typhi, and pathogenic Leptospira spp., respectively. Patients usually present with non-specific symptoms and therefore are commonly diagnosed with acute undifferentiated febrile illness. Consequently, patients face delayed treatment and increased mortality. Antibody-based serological test currently used as gold standard has limitations due to insufficient antibody titers, especially in the early phase of infection. In this study, we aimed to develop multiplex PCR to combine 3 primer pairs that target specific genes encoding 56-kDa TSA of O. tsutsugamushi, 17-kDa antigen of R. typhi, and LipL32 of L. Interrogans and evaluate its performance in comparison to the standard serological tests. Using EDTA blood samples of known patients, the sensitivity and specificity of our multiplex PCR was 100% and 70%, respectively. In addition, the assay was able to diagnose the co-infection of scrub typhus and leptospirosis. The assay may be useful in identifying causative agents during the early phase of these diseases, enabling prompt and appropriate treatment

Scrub Typhus Outbreak, Northern Thailand, 2006–2007

During a scrub typhus outbreak investigation in Thailand, 4 isolates of O. tsutsugamushi were obtained and established in culture. Phylogenetic analysis based on the 56-kDa type-specific antigen gene demonstrated that the isolates fell into 4 genetic clusters, 3 of which had been previously reported and 1 that represents a new genotype

Scrub Typhus Outbreak in Chonburi Province, Central Thailand, 2013

Investigation of a scrub typhus outbreak in Thailand during September 2013 found that 9.1% of Thai soldiers and 11.1% of residents living in areas surrounding training sites had antibodies against the causative agent, Orientia tsutsugamushi. Sequence analysis of O. tsutsugamushi from rodents and chiggers identified 7 genogroups and 3 genotypes