Vector Competence of Geographical Populations of Ornithodoros turicata for the Tick-Borne Relapsing Fever Spirochete Borrelia turicatae

Abstract

Vector competence refers to the ability of an arthropod to acquire, maintain, and successfully transmit a microbial pathogen. Tick-borne relapsing fever (TBRF) spirochetes are globally distributed pathogens, and most species are transmitted by argasid ticks of the genus <i>Ornithodoros.</i> A defining characteristic in vector competence is an apparent specificity of a species of TBRF spirochete to a given tick species. In arid regions of the southern United States, <i>Borrelia turicatae</i> is the primary cause of TBRF. Interestingly, there are two populations of the tick vector distributed throughout this region. <i>Ornithodoros turicata</i> is a western population that ranges from California to Texas. There is a gap through Louisiana, Mississippi, and Alabama where the tick has not been identified. An isolated eastern population exists in Florida and was designated a subspecies, <i>O. turicata americanus.</i> A knowledge gap that exists is the poor understanding of vector competence between western and eastern populations of ticks for <i>B. turicatae.</i> In this study, we generated uninfected colonies of <i>O. turicata</i> that originated in Texas and Kansas and of <i>O. turicata</i><i>americanus. B. turicatae</i> acquisition, maintenance through the molt, and subsequent transmission were evaluated. Our findings revealed significant differences in murine infection after feeding infected <i>O. turicata</i> and <i>O. turicata</i><i>americanus</i> ticks on the animals. Interestingly, the salivary glands of both tick populations were colonized with <i>B. turicatae</i> to similar densities. Our results suggest that the salivary glands of the tick colonies assessed in this study impact vector competence of the evaluated <i>B. turicatae</i> isolates.<b>IMPORTANCE</b> Several knowledge gaps exist in the vector competence of various geographical populations of <i>O. turicata</i> that transmit <i>B. turicatae</i> A western population of this tick is distributed from California to Texas, and an eastern population exists in Florida. Utilizing western and eastern populations of the vector, we studied acquisition and transmission of two <i>B. turicatae</i> isolates. Regardless of the isolate used, infection frequencies were poor in mice after the eastern population feeding on them. Since salivary gland colonization is essential for <i>B. turicatae</i> transmission, these tissues were further evaluated. Interestingly, the salivary glands from the two populations were similarly colonized with <i>B. turicatae.</i> These findings suggest the role of tick saliva in the establishment of infection and that the salivary glands may be a bottleneck for successful transmission.