Potential for horizontal transmission ofOrientia tsutsugamushiby chigger mites (Acari: Trombiculidae)

Experiments to study the potential for horizontal transmission of the rickettsia, Orientia tsutsugamushi (Hayashi) (Rickettsiaceae: Rickettsiales) using laboratory rats and mice are reported. Uninfected larvae of Leptotrombidium deliense Walch (Acari: Trombiculidae) were allowed to engorge on mice that had been infected with O. tsutsugamushi either by inoculation of cultured rickettsiae or from the bite of naturally infected L. deliense larvae. When mice were infected by inoculation with cultured rickettsiae it was shown that uninfected larvae acquired O. tsutsugamushi and transtadial transmission of rickettsiae to tritonymphs occurred, but none of the resultant adults passed infection to their progeny. When uninfected L. deliense larvae were placed on mice ten days after infection by larvae, it was not possible to detect O. tsutsugamushi in uninfected larvae, showing that rickettsiae were not acquired during feeding. The progeny of 37 adults, who as larvae fed on these mice, did not transmit rickettsiae to their progeny. In a subsequent experiment, Rattus rattus L. were infected by attachment of a single infected L. deliense. Batches of uninfected L. deliense and Blankaartia acuscutellaris Walch (Acari: Trombiculidae) were placed on infected rats two and six weeks after infection. O. tsutsugamushi was not detected in engorged larvae of either species, suggesting that rickettsiae were not acquired during feeding. The progeny of one of ten B. acuscutellaris were found to be infected with O. tsutsugamushi, but it was not possible to determine if these mites could transmit rickettsiae vertically. It is possible that rickettsiae persisted at an undetectable level in this batch of mites when they developed to adults. These data support previous studies that show horizontal transmission rarely occurs, and would not be epidemiologically important. However the occurrence of horizontal transmission in B. acuscutellaris is noteworthy, and further studies concerning its role as a vector of O. tsutsugamushi are warranted.