Scrub Typhus Outbreak, Northern Thailand, 2006–2007

Wuttikon Rodkvamtook, Jariyanart Gaywee

, Suparat Kanjanavanit, Toon Ruangareerate, Allen L. Richards, Noppadon Sangjun, Pimmada Jeamwattanalert, and Narongrid Sirisopana

Author affiliations: Armed Forces Research Institute of Medical Science, Bangkok, Thailand (W. Rodkvamtook, J. Gaywee, T. Ruangareerate, N. Sangjun, P. Jeamwattanalort, N. Sirisopana); Nakhornping Hospital, Chiang Mai, Thailand (S. Kanjanavanit); Naval Medical Research Center, Silver Spring, Maryland, USA (A. L. Richards).

Abstract

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 is a febrile disease endemic to the Asia–Australia–Pacific region, where ≈1 million cases occur annually (1). The causative agent of scrub typhus in this region is the gram-negative obligate intracellular bacterium Orientia tsutsugamushi (2). The bacterium maintains itself in trombiculid mites, and small mammals serve as reservoir hosts in the natural life cycle of the mites. Chiggers, the larval stage of mites, act as the transmission vector for O. tsutsugamushi (1). Humans and small animals become infected following the bite of chiggers harboring O. tsutsugamushi. After an incubation period of 7–14 days, high fever, chills, headache, rash, and an eschar usually develop in infected persons (3).
Scrub typhus is endemic to northern Thailand, especially Chiang Mai Province, where >200 cases are reported each year (4). During June 2006–May 2007, a total of 142 febrile children with clinically suspected scrub typhus were admitted to Nakornping Hospital in the city of Chiang Mai. Serologic and molecular laboratory test results showed that 65 of the children were positive for O. tsutsugamushi. Among the 142 hospitalized children, 30 were Hmong hill tribe people living in Ban Pongyeang, a village in the mountain area located north of the Chiang Mai. Laboratory testing also confirmed that 26 of the 30 Hmong children had scrub typhus.
To better characterize the specific strain(s) of O. tsutsugamushi present in the area and to determine how the agent(s) is transmitted to humans, we genetically typed O. tsutsugamushi obtained from these 26 children and small mammals. The Royal Thai Army Medical Department Ethical Committee approved all procedures (protocol S014q/45). Small mammals were handled according to guidelines in the Guide for the Care and Use of Laboratory Animals (National Institutes of Health publication no. 85–23, revised 1985).