Prevalence studies indicate that cats throughout the world residing in areas enzootic for heartworm disease in dogs are prone to infection. The prevalence in cats seems to parallel the prevalence in dogs in the same area but at a much lower level with cats having approximately one tenth (1/10) the prevalence seen in the unprotected dog population. This relative prevalence varies dramatically by region. This variation may reflect different vector populations with relative prevalence in cats being higher in regions with vectors which are indescriminant feeders. Indescriminant feeding vectors are those which will take a blood meal from any source as opposed to many mosquito species which have very specific host preferences. Aberrant migration of larvae seems to be much more common in the cat than in the dog.