Ixodid and Argasid Tick Species and West Nile Virus Charles Henderson Lawrie,* Nathalie Yumari Uzcátegui,* Ernest Andrew Gould,* and Patricia Anne Nuttall* Control of West Nile virus (WNV) can only be effective if the vectors and reservoirs of the virus are identified and controlled. Although mosquitoes are the primary vectors, WNV has repeatedly been isolated from ticks. Therefore, tick-borne transmission studies were performed with an ixodid (Ixodes ricinus) and an argasid tick species (Ornithodoros moubata). Both species became infected after feeding upon viremic hosts, but I. ricinus ticks were unable to maintain the virus. In contrast, O. moubata ticks were infected for at least 132 days, and the infection was maintained through molting and a second bloodmeal. Infected O. moubata ticks transmitted the virus to rodent hosts, albeit at a low level. Moreover, the virus was nonsys- temically transmitted between infected and uninfected O. moubata ticks co-fed upon uninfected hosts. Although ticks are unlikely to play a major role in WNV transmission, our findings suggest that some species have the potential to act as reservoirs for the virus. Twithin the Western Hemisphere occurred in 1999 in he first report of a West Nile virus (WNV) outbreak New York City and resulted in human, equine, and avian deaths (1). Since 1999, WNV has been found in an addi- tional 44 states of the United States as well as in parts of Canada, the Caribbean, and South America (2,3). During 2002 more than 4,000 people diagnosed with WNV and 284 deaths were reported in the United States (latest records available from: http://www.cdc.gov/ncidod/dvbid/ westnile/index.htm). WNV is a member of the genus Flavivirus that contains over 70 identified viruses. Most of these viruses are vec- tored by mosquitoes or ticks, although a few have no known vectors (4). WNV has been isolated from 43 species of mosquito in the United States (5), the most important of which is Culex pipiens (6). It has also been isolated from hard (ixodid) and soft (argasid) tick species in regions of Europe, Africa, and Asia (7–13) where WNV is endemic. Ticks rank second only to mosquitoes in their importance as vectors of human pathogens and transmit a greater variety of infectious agents than any other arthro- *Centre for Ecology and Hydrology, Oxford, United Kingdom pod group (14). However, whether or not ticks are major vectors of WNV has not been adequately investigated. Current strategies to control WNV in the United States are largely based on measures to avoid exposure and to control vector species, but at present only mosquito species are targeted by government surveillance and pre- ventive control programs (15). Resident U.S. tick popula- tions could also play a role in the WNV transmission cycle in the current outbreak. We investigated an argasid tick species and an ixodid tick species for their competence as vectors and reservoirs of the New York strain (NY99) of WNV. Materials and Methods Ticks We tested a hard tick species, Ixodes ricinus, and a soft tick species, Ornithodoros moubata, for their vector com- petence with WNV (NY99 strain). These species are not native to the United States and were chosen mainly for their availability. O. moubata ticks were considered poten- tial vectors for the Eg101 strain of WNV in a study by Whitman and Aitken in 1960 (16). I. ricinus ticks are the primary vectors of Borrelia burgdorferi, the agent causing Lyme disease in Europe and are important vectors of the flaviviruses tick-borne encephalitis virus (TBEV) and Louping ill virus (LIV) (17). Ticks were taken from colonies reared and maintained for many generations at the Centre for Ecology and Hydrology, Oxford, according to standard methods (18). Colony ticks were WNV negative by reverse transcrip- tase–polymerase chain reaction (RT-PCR) testing (15 members of each species tested). Virus and Viral Assays The WNV strain used (NY99) was supplied by Robert Shope, University of Texas. High-titer mouse brain sus- pension stocks of WNV (2.9 x 107 PFU/mL-1) were dilut- ed in phosphate-buffered saline (PBS) to a concentration of 105 PFU/mL-1 before use. Viral stocks and the serum samples from infected mice were tested for infectious virus Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 4, April 2004 653