Failed Vertical Transmission of Rickettsia tsutsugamushi (Rickettsiales: Rickettsiaceae) Acquired from Rickettsemic Mice by Leptotrombidium pallidum (Acari: Trombiculidae)

Abstract

Larvae of Leptotrombidium pallidum (Nagayo, Miyagawa, Mitamura & Tamiya) from uninfected laboratory colonies were fed on mice infected with Rickettsia tsutsugamushi (Hayashi) Ogata. Infection of the chiggers with R. tsutsugamushi was determined by passage of chigger exudates into ddY mice. The passage method was modified so that an inoculum was considered to be positive when R. tsutsugamushi or anti-R. tsutsugamushi antibody, or both, were detected in mice up to the third blind passage. R. tsutsugamushi was detected in six of 18 larvae (33.3%) and in all developmental stages. In adults, five of 18 males and 10 of 46 females were infected with R. tsutsugamushi. In L. fuji (Kuwata, Berge & Philip), R. tsutsugamushi was not found in 57 engorged larvae fed on rickettsemic mice but was found in a very low percentage of deutonymphs and adults. Female L. pallidum from larvae fed on infected mice were paired individually, and F1 larvae were collected. Although eight females were found to be positive for R. tsutsugamushi, the rickettsia was not detected in 12 pools (249 larvae) of F1 larvae from these infected females. We concluded that uninfected mites became infected by feeding on rickettsemic mice at comparatively high rates depending on the species and transmitted this infection transstadially to succeeding life stages, but not vertically to larvae in the following F1 generation.

Characterization Based on the 56-Kda Type-Specific Antigen Gene of Orientia tsutsugamushi Genotypes Isolated from Leptotrombidium Mites and the Rodent Host Post-Infection

“…[1][2][3][4][5] The chigger infects a rodent or human host when feeding on tissue fluid. [6][7][8] With no vaccine, scrub typhus prevention is a major challenge and infections may be fatal if untreated. 9 Orientia tsutsugamushi-infected mites are widely distributed throughout many Asian countries, northern Australia, and the western Pacific islands thereby placing an estimated one billion people at risk for scrub typhus, which averages one million cases annually.…”

Mite Vectors (Acari: Trombiculidae) of Scrub Typhus in a New Endemic Area in Northern Kyoto, Japan

“…Orientia tsutsugamushi is considered to be transmitted vertically from infected female parents to the next generation via transovarian transmission (Rapmund et al 1969;Roberts and Robinson 1977;Rapmund 1984;Urakami et al 1994a, b;Takahashi et al, 1988;Takahashi 1990;Takahashi and Tanaka 1995;Frances et al 2001;Lerdthusnee et al 2002). In the case of O. tsutsugamushi-positive testing in enlarged chiggers recovered from rodents, a small doubt always remains as to which O. tsutsugamushi-negative unfed larvae acquired rickettsiae while feeding on the rickettsemic Þeld rodents (Traub and Wisseman 1968a, b;Traub et al 1975;Walker et al 1973Walker et al , 1975Takahashi et al1990bTakahashi et al , 1994. In fact, uninfected chiggers became infected by feeding on rickettsemic mice at comparatively high rates, depending on the species (Fukuzumi 1953, Ito and Obata 1961, Takahashi et al 1994.…”

“…In the case of O. tsutsugamushi-positive testing in enlarged chiggers recovered from rodents, a small doubt always remains as to which O. tsutsugamushi-negative unfed larvae acquired rickettsiae while feeding on the rickettsemic Þeld rodents (Traub and Wisseman 1968a, b;Traub et al 1975;Walker et al 1973Walker et al , 1975Takahashi et al1990bTakahashi et al , 1994. In fact, uninfected chiggers became infected by feeding on rickettsemic mice at comparatively high rates, depending on the species (Fukuzumi 1953, Ito and Obata 1961, Takahashi et al 1994. The chiggers then transmitted this infection transstadially to succeeding life stages but not vertically to larvae in the following generation (Takahashi et al 1994).…”

Occurrence of Orientia Tsutsugamushi in Small Mammals From Thailand

“…12 Transovarial transmission is thought to be the only mechanism for maintenance of O. tsutsugamushi in the vector. 8,9 Infected chiggers can therefore be considered the true hosts of O. tsutsugamushi, and even commonly infected mammals are dead-end hosts rather than true reservoirs. Frances and others 10 demonstrated that O. tsutsugamushi could be transmitted to co-feeding mites, and Takahashi and others 11 were able to infect chiggers fed on wild rodents; however, neither study determined if infected mites transmitted the rickettsiae to their eggs.…”