Heartworm disease, once considered primarily a concern in warmer states, is now prevalent across all 50 states. According to the American Heartworm Society, recent surveys indicate an increase in cases in areas where pet owners historically did not have to worry about this parasite, particularly in parts of the Midwest, Northeast, and West Coast. The American Veterinary Medical Association (AVMA) has also reported that heartworm is now appearing in cooler, drier regions far from its traditional Gulf Coast and Southeast strongholds.
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Dr. Sandra Faeh, a veterinarian and past president of the AVMA, explained that a combination of environmental changes and animal movement are contributing to the spread of heartworm disease. Mosquitoes are the only carriers of heartworm larvae, and extended warm seasons are allowing them to thrive in new regions. She noted, "Heartworm is expanding due to environmental and climatic changes, both natural and human-made, which are creating sustained heat and humidity that support mosquito vectors. Additionally, relocating infected domestic dogs and the territorial expansion of wild dogs continue to spread the parasite."
Urban heat islands and climate change increase the warm conditions necessary for heartworm larvae to develop within mosquitoes. While heartworm transmission decreases during colder months, some microenvironments in cities prolong mosquito activity.
Dr. Faeh reassured pet owners that heartworm infection is preventable. The 2025 American Heartworm Society Incidence Survey revealed substantial clusters of heartworm cases extending beyond traditional high-risk areas. She emphasized that pet owners should ensure their animals receive necessary testing and preventive measures.
Annual blood tests are recommended to detect heartworm infections, especially before starting preventive medication. Dr. Faeh explained that two blood tests can determine if a dog has heartworms, one detecting proteins produced by adult female worms and the other identifying larval offspring.
For dogs starting heartworm prevention after missed doses, testing is critical to prevent severe reactions if the dog is already infected. Cats face different challenges as heartworm infections are harder to detect in them. Dr. Faeh noted that while cats are not preferred hosts for heartworm, they are still susceptible, and even a few adult worms can make them ill due to a condition called heartworm-associated respiratory disease (HARD).
A heartworm-positive dog may show no symptoms if the infection is mild, while a dog with active heartworm disease could display symptoms like coughing, fatigue, and even collapse in severe cases. Treatment of infected dogs involves a complex, multi-month process to eliminate the parasites and may require strict limits on physical activity.
Dr. Faeh reiterated the importance of year-round prevention, as conditions can allow mosquito vectors to survive in unexpected months. She concluded, "The risk of heartworm disease drops to almost zero when pets receive continuous, year-round heartworm protection."
This article is meant for informational purposes only; pet owners should consult with their veterinarian for specific advice regarding heartworm testing and prevention.