Abstract
Transovarial passage of relapsing fever spirochetes (Borrelia species) by infected female argasid ticks to their progeny is a widespread phenomenon. Yet this form of vertical inheritance has been considered rare for the North American tick Ornithodoros hermsi infected with Borrelia hermsii. A laboratory colony of O. hermsi was established from a single infected female and two infected males that produced a population of ticks with a high prevalence of transovarial transmission based on infection assays of single and pooled ticks feeding on mice and immunofluorescence microscopy of eggs and larvae. Thirty-eight of forty-five (84.4%) larval cohorts (groups of larvae originating from the same egg clutch) transmitted B. hermsii to mice over four and a half years, and one hundred and three single and one hundred and fifty-three pooled nymphal and adult ticks transmitted spirochetes during two hundred and fourteen of two hundred and fifty-six (83.6%) feedings on mice over seven and a half years. The perpetuation of B. hermsii for many years by infected ticks only (without acquisition of spirochetes from vertebrate hosts) demonstrates the reservoir competence of O. hermsi. B. hermsii produced the variable tick protein in eggs and unfed larvae infected by transovarial transmission, leading to speculation of the possible steps in the evolution of borreliae from a tick-borne symbiont to a tick-transmitted parasite of vertebrates.
Highlights
Theobald Smith and Frederick Kilborne [1] were first to show that a blood-feeding arthropod was the biological vector of a pathogen
Included in Burgdorfer’s study were observations pertaining to transovarial transmission, in which 12 of 14 (86%) infected female O. moubata produced eggs infected with B. duttonii
The observations presented demonstrate that B. hermsii can be efficiently passed transovarially and transstadially by O. hermsi without amplification in vertebrate hosts
Summary
Theobald Smith and Frederick Kilborne [1] were first to show that a blood-feeding arthropod was the biological vector of a pathogen (in their case, the protozoan cause ofTexas cattle fever). An essential link in this transmission cycle was a new generation of larval ticks that acquired their parasites from the infected females that gave birth to them. This discovery was the first demonstration of vertical passage of a pathogen from infected female ticks to their progeny. In the first description of a tick-borne disease of humans (relapsing fever in Africa) the spirochetal cause Borrelia duttonii was shown to be passed to the generation of ticks by females of its vector Ornithodoros moubata [2] via infected eggs [3,4]. The most recent review of the Borrelia species associated with the relapsing fever group of spirochetes found that 11 of the 26 species discussed may be transmitted transovarially by their respective tick vectors [8], and the other species not yet studied may do so as well
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