Experimental transmission of the relapsing fever spirochete Borrelia persica in its tick vector Ornithodoros tholozani by transstadial, transovarial, and hyperparasitism routes with description of dynamics within the tick host

Background: Borrelia persica causes tick-borne relapsing fever, a potentially fatal human and animal disease, in the Middle East and Central Asia. The transmission of B. persica in Ornithodoros tholozani ticks has been reported to be transovarial, but little is known about the pathogen’s lifecycle in its vector and other pathways of transmission. Methods: To further understand the transmission of B. persica, colony-bred O. tholozani ticks were fed on blood in an animal-free system, followed during three lifecycles, and infected with cultured B. persica. Spirochetes were detected, and bacterial load was quantified by polymerase chain reaction (PCR). Results: B. persica colonization of the tick salivary glands was observed at week 4 post-infection of third-stage nymphs, a timeframe compatible with the duration required for the tick to become infective. Experimental B. persica transmission showed a decreasing rate of transstadial infection from 100% infection in larvae, to 55%, 20% and 25% in first-, second-, and third-stage nymphs, respectively, and 20% in adults. Five of the eight female ticks were PCR-positive for B. persica, and transovarial transmission was detected in the progeny of 3/5 infected females. Infection due to transovarial transmission was detected in 4% of the eggs laid by positive females, and in 2% of the larvae that originated from positive females. Transmission of B. persica between O. tholozani ticks was also experimentally shown to occur through hyperparasitism, when Borrelia-negative male ticks fed on infected third-stage nymphs. Conclusions: This study shows that B. persica can be maintained transstadially and transovarially in a tick colony, although transovarial transmission occurred at low rates. Additionally, transmission of B. persica was experimentally shown to occur through hyperparasitism between O. tholozani ticks, a transmission pathway that is likely to occur also under natural conditions. Finally, we determined that B. persica colonized the salivary glands within a period of 4 weeks, and thereafter the tick may be considered as ready to infect a host. Improved understanding of tick colonization by spirochetes and pathways of transmission described here will help to form concepts to disrupt the infectious cycle of tick-borne relapsing fever.