Powassan virus multiplication in Dermacentor andersoni ticks

All stages of Dermacentor andersoni (Stiles) ticks were infected by feeding on rabbits injected intravenously with large doses of Powassan virus 48 to 72 hours after the ticks became attached. Viremia titers at least 10²˙⁵ mouse LD₅₀ per ml were required to establish infection in 1 to 5% of ticks ingesting viremic blood. The "infection-threshold" of D. andersoni adults for Powassan virus was elevated when viremia was induced towards the end of engorgement. When each stage in the tick's life cycle was infected a viral-eclipse phase was demonstrated in gut cells. Other organs such as salivary glands, Gene's organ glands and accessory glands supported virus multiplication. This was demonstrated by infectivity titrations and observation of fluorescent foci when the tissues were stained with Powassan virus antiserum conjugated with fluorescein isothiocyanate (FITC). Transstadial transfer of Powassan virus was observed through ecdysis of larvae to nymphs and from nymphs to adults. This occurred both after feeding on hamsters (which produced a substantial viremia) and guineapigs (which did not become viremic). Virus-was detected only in larval gut and invaded nymphal salivary glands during the molt. Infection localized in gut and salivary glands of engorged nymphs and flat adults, and was not detected in adult female accessory and Gene's glands until repletion. Adult males maintained high virus titers in salivary glands only. Transovarial transfer was not detected although infectivity was present on the surface of eggs laid by the mediation of an infected Gene's organ gland. Powassan virus was detected in salivary gland secretions but not in rectal excretions in adults or nymphs. Transmission of Powassan virus to hamsters, guinea-pigs, and rabbits was demonstrated by the bite of D. andersoni nymphs and adults which were infected by ingestion of blood by their antecedent larvae.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat