Abstract. The seroprevalence of Orientia tsutsugamushi, Rickettsia typhi, and TT118 spotted fever group (SFG) rickettsiae in 300 rubber estate workers in Slim River, Malaysia was determined in December 1996 and March 1997. In December, which was the wet season, 23.3%, 3.0%, and 57.3% of the population had antibodies detected against the three rickettsiae, respectively. The highest seropositive rate of 40% was detected for single infection with SFG rickettsiae, followed by a rate of 15.3% for both O. tsutsugamushi and SFG rickettsiae among the rubber estate workers. Subjects less than 21 years old had a lower seroprevalence of SFG rickettsiae compared with the other age groups. Indians had a higher seroprevalence of O. tsutsugamushi compared with other ethnic groups. Rubber tappers had a higher seroprevalence of SFG rickettsiae compared with other occupational groups. During the dry season in March 1997, there was a significant increase in the seroprevalence of R. typhi. The seroconversion rates for IgM against O. tsutsugamushi, R. typhi, and SFG rickettsiae were 5.7%, 12.3%, and 15.1%, respectively, during the fourmonth period. Significant variations of antibody titers towards the three rickettsiae was noted among subjects who were bled twice. This suggests a significant and continual exposure of rubber estate workers to the three rickettsiae.Rickettsial infections are major causes of febrile illnesses throughout the Asia-Pacific region. 1 The gram-negative, obligate intracellular bacteria that cause these infections are transmitted by arthropods and infect vertebrates. Rickettsiae can be grouped into three antigenically defined groups: scrub typhus, typhus, and spotted fever. Scrub typhus infection, which is caused by Orientia tsutsugamushi (previously known as Rickettsia tsutsugamushi), 2 is endemic in the western Pacific region and has been reported as the most frequent infection (19.3%) among febrile hospitalized patients in rural areas of Malaysia. 3 Murine typhus is caused by infection with a typhus group rickettsiae, Rickettsia typhi, and was once considered a sporadic and uncommon cause of illness in Malaysia. 4,5 However, in 1977, a high prevalence of this infection in the human and rodent population was reported. 6 The seroprevalence of spotted fever group (SFG) rickettsiae TT118 among febrile patients was 0.3% in Mentakab Hospital 3 and 16% in a study conducted in Sabah. 5 There is no information on the etiologic agent of tick typhus in Malaysia.In a serologic survey conducted in 1994-1995, it was shown that sera from a large proportion of febrile outpatients at a health center in Slim River, a small town located on the middle-northern west coast of Peninsular Malaysia, were reactive to antigens of O. tsutsugamushi, R. typhi, and SFG rickettsiae. 7 The majority of these patients were agricultural workers: mostly rubber tappers, oil palm plantation workers, and farmers. The objectives of the present study were to assess in more detail the seroprevalence of O. tsutsugamushi, R. typhi, and SFG rickettsia...

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