While heartworm disease is not as prevalent in San Diego County compared to some other parts of the United States, it still exists. The risk of heartworm infection depends on various factors, including the local mosquito population, climate, and the number of infected animals in the area. Therefore, it is more prevalent in areas with a high mosquito population and warmer climates, but it can occur in any geographic location. Dogs are the primary host for heartworms, but cats can also become infected.