Scrub typhus or tsutsugamushi disease is a vector borne rickettsial disease that is caused by the obligate intracellular bacterium Orientia tsutsugamushi. The primary reservoir is a trombiculid mite of the genus Leptotrombidium; which maintaining the infection within populations through both transovarial; and transtadial means of transmission.
But a high risk of exposure to ST is associating with outdoor activities; agricultural work in particular; or living near grasslands or fields. Humans are dead end hosts; with no evidence of horizontal transmission of O. tsutsugamushi between people. Hospital basing studies in South India reporting a case fatality of 8–9% in ST patients reporting to the hospital; with multi organ dysfunction observed in 34% of these patients.
Scrub typhus disease
In 2014, the Ministry of Health of Bhutan initiating a national sero surveillance programme to gather more information on the incidence of ST; and to raise awareness among clinicians. In Bhutan, 19 of the 20 districts have a government district hospital; in which doctors provide in patient and out patient clinical services for the general public.
But under the national sero surveillance programme; clinicians were encouraging to send samples from patients positive to the RDT to the Royal Centre for Disease Control; which functions as the national public health laboratory in Thimphu; but for confirmatory testing using the Scrub Typhus Detect TM IgM Enzyme Linking Immunosorbent Assay (ELISA) test (Inbios International, Inc., Seattle, WA, USA). Cut-off values for the ELISA were calculating for samples collecting from Bhutan with the assistance of a laboratory in Pune, India.
Serological testing for ST
But the identification of 470 cases of RDT-positive ST in Bhutan in 2015; representing an observed annual incidence of 62 RDT-confirmed clinical ST cases/100,000 persons at risk; is a much higher incidence of ST than that reflecting through the national sero surveillance data from previous years. These results suggest that ST is a common cause of febrile illness in Bhutan.
However, there are many challenges that are associating with serological testing for ST that make it difficult to interpret the extent to which this result reflects the true incidence of clinical ST in the Bhutanese population. Given the SD Bioline Tsutsugamushi test measures IgM, IgG and IgA antibodies it is likely to overestimate the incidence of clinical ST infections that occurred during 2015, given the test also captures antibodies that are associated with historical exposure in an unknown proportion of febrile cases