In regions where Dirofilaria immitis is endemic, dogs without proper heartworm protection are almost 100 percent likely to suffer from heartworm infestation. The heartworm is mainly endemic in geographic areas with tropical and subtropical climates, and is also commonly found along the Atlantic and Gulf Coasts, and the Ohio and Mississippi river basins. The presence of Dirofilaria immitis is not limited to these areas, however, it is found worldwide. Dogs have been diagnosed with heartworm disease in all 50 U.S. states.