THE ACQUISITION OF RICKETTSIA TSUTSUGAMUSHI BY CHIGGERS (TROMBICULID MITES) DURING THE FEEDING PROCESS*

Abstract

INTRODUCTIONA fundamental concept in the ecology of ectoparasite-borne disease is that of the reservoir, or the wellspring of the infection in nature during interepidemic periods. In the case of chigger-borne rickettsiosis (scrub typhus), it has often been argued that chiggers (larval trombiculid mites) are not only the vectors but also the true reservoirs of the infection and that the small mammals (theraphions) that serve as hosts of the chiggers are of no importance as a source of rickettsiae for the chiggers.' This point of view has been based largely upon several factors, namely, the known transovarian transmission of the causative agent, Rickettsia tsutsugamushi. from mother to progeny in certain species of chiggers,' the demonstration of the efficiency of the mechanism of transovarian transmission by Rapmund and his colleagues,+-" the difficulty or impossibility to show that chiggers can acquire R . tsutsugamushi while feeding on infected hosts in the laboratory,'. c -l l the fact that chiggers are unique among vectors in that in their lifetime, they are parasitic only in one stage (i.e., as larvae) and normally attach to, and feed upon, only one vertebrate and therefore could not acquire an infection from one such host and transmit it later directly to a second, and, finally, the widespread belief that chiggers do not imbibe blood but, instead, feed solely on serum exudate when in the parasitic stage and thus are unlikely to come in contact with pathogens that circulate in the blood of the host.', In the present paper, it is shown that, contrary to general belief, chiggers can acquire R . tsutsugarnushi while feeding on rickettsemic mice and that, when tested by pools of chiggers, this acquisition is relatively frequent. Persistence of acquired rickettsiae for at least 1-2 weeks in the chiggers is demonstrated, and a single case of presumed transovarian transmission to the next generation is reported. The preliminary data presented suggest, somewhat surprisingly, that in nature, under certain conditions, a small but perhaps significant proportion of chiggers may partially feed on one host and then later feed to repletion on a second one, raising the possibility that such "reattached" chiggers may * 91 92 Annals New York Academy of Sciences transmit acquired rickettsiae. These points, coupled with the observation in our laboratory that chiggers may, indeed, imbibe blood on o c~a s i o n ,~~ 5 , l2 indicate that theraphions may perhaps truly be of significance as a source of R. tsutsugumushi infection in chiggers in nature, even though the trombiculids themselves apparently constitute the prime reservoir of chigger-borne rickettsiosis.The approach in this study has primarily been concerned with the acquisition of R. tsrrtsugamushi by chiggers that were presumably free of natural infection; we utilized colonies of six species of chiggers that are known or suspected vectors of chigger-borne rickettsiosis and fed the chiggers on mice that had been inoculated with R. tsutsugumushi. The absence of natural infect...

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.…”

Epidemiological Studies on the Background of the Endemic Occurrence of Tsutsugamushi Disease in Toyama Prefecture

“…It has been reported by Rapmund's group that Rt is maintained by transovarial transmission in Rtcarrying colonies of L. fletcheri and L. arenicola that are known to be Rt-vector mites in Malaysia, and that most of the Rt-carrying mites were female (1,(11)(12)(13)(14)19). These and other authors have also reported that when non-Rt-carrying larvae were fed on infected laboratory mice, only a few of them acquired Rt through feeding, and no vertical transmission to the next generation through the egg was observed (10,18), though the number of mites examined was small. It is not yet clear whether oi-not Rt is maintained in L. pallidum by vertical transmission.…”

Occurrence of Orientia Tsutsugamushi in Small Mammals From Thailand

“…Although studies by Traub and others, 12 Walker and others, 8 Takahashi and others, 11 and Frances and others 10 have demonstrated that chiggers can acquire O. tsutsugamushi during the feeding process, to date mammals have not been shown to play a conclusive role in the cyclical transmission of this pathogen. 13 In spite of this, rodents play a key role in the epidemiology of scrub typhus, as rodents serve as maintenance hosts for the vector mites.…”

“…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. 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.…”

“…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. 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.…”