Evidence of natural infection of Orientia tsutsugamushi in vectors and animal hosts – Risk of scrub typhus transmission to humans in Puducherry, South India

Background: Scrub typhus infection is endemic in India and reported to be the major cause for acute encephalitis syndrome (AES) in humans. Periodic occurrence of scrub typhus cases and presence of pathogen in rodents were also reported in areas with human cases of scrub typhus in Puducherry. Objectives: This study was carried out to screen Orientia tsutsugamushi in rodent/shrew reservoirs and vectors in villages of Puducherry with no reports of human scrub typhus cases. Methods: This study was conducted during October 2017 to January 2018 in ten randomly selected villages in Puducherry. Rodents/shrews in the peridomestic area were trapped using Sherman traps. Screening of O. tsutsugamushi in rodents/shrews and mite vectors was done by polymerase chain reaction (PCR). Weil-Felix test was done to screen antibodies against O. tsutsugamushi in rodent serum samples. Results: Among the 54 rodents trapped, Suncus murinus was the major small animal and Leptotrombidium deliense was the major mite species retrieved. PCR screening revealed pathogen positivity in 8 rodent blood and 3 pooled mite samples. Phylogenetic analysis has shown that Kato was the circulating serotype of O. tsutsugamushi. None of the rodent serum samples was tested positive for antibodies against O. tsutsugamushi by Weil-Felix test. Conclusions: The presence of pathogen in both vectors and reservoir animal hosts imposes a risk for scrub typhus transmission to the inhabitants; hence, initiation of vector control measures before the start of winter is recommended in the study area. It is also recommended to screen scrub typhus in patients with undifferentiated acute febrile illness and AES.