Many RNA viruses can have multiple hosts, but for most a common host is evident. Multiple hosts for an RNA virus are not as common as one might guess by reading the local paper and watching the world news. There are a number of details excluded from the headlines that show viruses like West Nile Virus do not cross species barriers with regularity. Rather, it is the nature of these viruses that proves they are evolutionary species with host specificity (1,6). Information to support this idea lies in the ecological, evolutionary and molecular genetics analysis of the virus.
To better understand the evolutionary and molecular arguments an ecological perspective will help. West Nile Virus is a good example of an RNA virus associated with several hosts because of the research that has been done to track this virus. WNV is an Arbovirus (Arthropod born) that is carried in mosquitoes. It is a ss(+)RNA virus consisting of 11kb and a nucleocapsid. In the USA the Culex mosquito is the most common carrier of this virus (1,2). Female mosquitoes are the blood feeders in their species, as it is needed for their progeny. The most investigated infectious cycle starts with a Culex feeding on the blood of an infected bird. The female comes back to her nest and produces infected progeny. These progeny then can infect more birds, which in turn infect any biting mosquito. The birds are considered an introductory host and research has shown that Crows and Jays are the most susceptible to WNV in the United States. Other vertebrates like horses and humans are infected incidentally by WNV via mosquito vectors as well, but this does not indicate the virus has a wide host range (3,4). These hosts are incidental hosts and are not the targeted host of the virus. If not for the involvement of the mosquito vector, infection of humans and horses might prove to be less likely. Mammal hosts are not good targets for WNV.
