Heartworm disease is most prevalent in regions with warmer climates due to the higher population and activity of mosquitoes, which transmit the disease. These areas primarily include the southeastern United States but also extend to other parts of the country during the warmer months. Coastal regions and places with standing water also see higher incidences of heartworm infections, as these environments support mosquito breeding. Regular preventive treatments and monitoring are crucial in these areas to protect dogs and other susceptible animals from heartworm disease.