A NEW ECOLOGY FOR SCRUB TYPHUS ASSOCIATED WITH A FOCUS OF ANTIBIOTIC-RESISTANCE IN RICE FARMERS IN THAILAND

Following the documentation of chloramphenicol-resistant and doxycycli ne-resistant strains of Orientia tsutsugamushi (Hyashi) in northern Th ailand, we conducted ecological and epidemiological studies near the h ouses of patients hospitalized with antibiotic-resistant infections. N ew associations between chiggers, rodents, and O. tsutsugamushi in act ive rice agriculture areas, an ecological habitat not described previo usly, are reported. Rattus rattus (L.) was the most common species (re presenting 85.8% of the 1,433 rodents processed), followed by Rattus l osea (Swinhoe) (9. 4%), Bandicota indica (Bechstein) (3.6%), and Rattu s argentiventer (Robinson and Kloss) (1.3%). O. tsutsugamushi was isol ated 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 w ere 0.03 in Leptotrombidium chiangraiensis Tanskul & Linthicum, a new species of chigger, and 0.002 in Leptotrombidium imphalum (Vercammen-G randjean & Langston), a chigger species not previously associated with scrub typhus transmission. Efficient vertical and horizontal transmis sion of O. tsutsumushi by L. chiangraiensis and L. imphalum was demons trated. During a 19-mo period from October 1993 to April 1995, the ove rall prevalence of human IgM and IgG antibody to O. tsutsugamushi was 25.5 and 47.3%, respectively. L. chiangraiensis and L. imphalum are in criminated as vectors of O, tsutsugamushi in a rice field habitat asso ciated with a focus of antibiotic resistance.