Feline calicivirus (FCV) is a very common infection of cats, especially in multiple cat environments. Cats often shed virus for months to weeks after recovery. Although FCV is not usually fatal, since 1998 at least five local outbreaks have been reported of a highly virulent strain, causing death in up to 50% of cases and affecting both vaccinated and unvaccinated cats. Analysis of tissue samples has only been reported for one of the outbreaks. There has never been follow-up of survivors or exposed cats. A new outbreak was reported in July 2002 and is currently being investigated. Case reports have been collected, the spread of the disease has been tracked, and virus has been isolated and characterized from affected cats. Surviving cases have been identified, and specimens have been obtained from cats that died. The proposed research will follow up on the initial investigation in two areas: follow up of survivors and exposed cats to assess length of time after recovery cats remain infectious, and whether sub-clinical or mild infection is possible; and detailed analyses of tissues from cats that died of the disease, including electron microscopy and special staining to see where virus is located in affected organs. Tissue examination will provide the most detailed information to date regarding what types of organ damage can be seen with this strain of calicivirus, and how the virus causes the observed symptoms.