A New Ecology for Scrub Typhus Associated with a Focus of Antibiotic Resistance in Rice Farmers in Thailand

Abstract

Following the documentation of chloramphenicol-resistant and doxycycline-resistant strains of Orientia tsutsugamushi (Hyashi) in northern Thailand, we conducted ecological and epidemiological studies near the houses of patients hospitalized with antibiotic-resistant infections. New associations between chiggers, rodents, and O. tsutsugamushi in active rice agriculture areas, an ecological habitat not described previously, are reported. Rattus rattus (L.) was the most common species (representing 85.8% of the 1,433 rodents processed), followed by Rattus losea (Swinhoe) (9.4%), Bandicota indica (Bechstein) (3.6%), and Rattus argentiventer (Robinson and Kloss) (1.3%). O. tsutsugamushi was isolated from 30% of the R. rattus and R. losea, 29% of the B. indica, and 33% of the R. argentiventer collected. Mean minimum infection rates were 0.03 in Leptotrombidium chiangraiensis Tanskul & Linthicum, a new species of chigger, and 0.002 in Leptotrombidium imphalum (Vercammen-Grandjean & Langston), a chigger species not previously associated with scrub typhus transmission. Efficient vertical and horizontal transmission of O. tsutsugamushi by L. chiangraiensis and L. imphalum was demonstrated. During a 19-mo period from October 1993 to April 1995, the overall prevalence of human IgM and IgG antibody to O. tsutsugamushi was 25.5 and 47.3%, respectively. L. chiangraiensis and L. imphalum are incriminated as vectors of O. tsutsugamushi in a rice field habitat associated with a focus of antibiotic resistance.

Occurrence of Orientia Tsutsugamushi in Small Mammals From Thailand

“…Historically, high O. tsutsugamushi infection rates have been found in chiggers, rodents, and humans from this area. 5,7,20,21 In this study, infected mammals were found in only four (Chiangrai, Bangkok, Nonthaburi, and Sukothai) of eight provinces; however, 99.6% (563 of 565) of infected mammals were found in Chiangrai Province, with only a single infected animal found in Bangkok, Nonthaburi, and Sukothai Provinces. Certain species of animals with low O. tsutsugamushi infection rates were rarely found in Chiangrai Province.…”

“…All procedures involving animals were conducted under animal use protocols approved by the Armed Forces Research Institute of Medical Sciences Institutional Animal Care and Use Committee. Small mammals were routinely collected during studies on the epidemiology of scrub typhus in Chiangrai, 7,20,21 Nonthaburi, 18 and Phitsanulok 21 Provinces of Thailand. Collections in additional provinces were made on an irregular basis.…”

“…Mice that did not develop clinical scrub typhus at the end of the 30-day observation period were humanely killed and their sera were assayed for antibody against O. tsutsugamushi using the indirect immunoperoxidase test. 7 This procedure was used to ascertain that mice did not develop subclinical infections.…”

“…6 Scrub typhus is transmitted by several species of larval trombiculid mites, which are commonly known as chiggers. 7 Chiggers are unique among vectors in that they are parasitic in only one stage (i.e., as larvae) and normally attach to and feed upon only a single vertebrate and therefore cannot acquire an infection from one host and subsequently transmit it to a second host. 12 Transovarial transmission is thought to be the only mechanism for maintenance of O. tsutsugamushi in the vector.…”

“…Traub and others 12 were the first to document horizontal transmission of O. tsustsugamushi, although even this report discussed the possibility that the observation was not representative of natural transmission. In Thailand, Leptotrombidium deliense is the primary vector of scrub typhus; 13 however, several other species have been implicated as vectors and include L. chiangraiensis, 7 L. imphalum, 7 and Blankaartia acuscutellaris.…”