Transstadial transfer of West Nile virus by three species of ixodid ticks (Acari: Ixodidae).

Larvae and/or nymphs of four species of ixodid ticks, Ixodes scapularis Say, Amblyomma americanum (L.), Dermacentor andersoni Stiles, and Dermacentor variabilis Say, were fed to completion on laboratory hamsters or mice which had been inoculated with a West Nile (WN) virus isolate from Culex pipiens L. captured in Connecticut USA. Maximum titers in mice and hamsters were approximately 5 and two logs, respectively, lower than recorded (10 logs) in a naturally infected American crow, Corvus brachyrhynchos Brehm. WN virus was isolated in Vero cell culture from ticks and detected by TaqMan RT-polymerase chain reaction (PCR) in ticks that had completed their feeding as larvae or nymphs, and in I. scapularis, D. andersoni, and D. variabilis that had molted into the next stage of development. Naive hosts, fed upon by nymphs that as larvae had fed on viremic hosts, did not become infected. WN virus was isolated in Vero cell culture from one female I. scapularis and was detected by TaqMan RT-PCR in 24 adult I. scapularis, one D. andersoni, and two D. variabilis adults that had fed to completion as larvae on viremic hosts and as nymphs on naive mice or hamsters. Three species of ixodid ticks acquired WN virus from viremic hosts and transstadially passed the virus, but vector competency was not demonstrated.