answer Heartworms have been found in all 50 states. Certain areas have a higher risk of heartworm. Some very high risk areas cover large regions, such as near the Atlantic and Gulf coasts and along river tributaries. Most states have "hot spots" where the heartworm infection rate is very high compared to other areas in the same state. Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas and presence of animal reservoirs (such as infected dogs or coyotes).