West Nile virus (WNV)is a virus that causes encephalitis (brain inflammation). WNV is usually transmitted to dogs and cats through the bite of an infected mosquito. Some birds, including crows, jays, sparrows, finches, grackles, and robins, are competent reservoirs for the virus (meaning they are able to infect mosquitoes). Some infected birds can shed WNV in their feces and other body fluids. In theory, cats and dogs can become infected through ingestion of (or contact with) an infected bird, but mosquito bites remain the primary route of infection.