Heartworm disease has expanded significantly beyond its previous reputation as a problem primarily in warmer states. The American Heartworm Society confirms that the parasite has now been diagnosed in all 50 states, with recent surveys indicating sharp increases in areas previously considered low-risk, including parts of the Midwest, Northeast, and West Coast. The American Veterinary Medical Association (AVMA) has reported similar findings, noting the presence of the disease in regions that are far from its traditional strongholds in the Gulf Coast and Southeast.
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Dr. Sandra Faeh, a veterinarian and past president of the AVMA, explained that several factors contribute to this geographic expansion of heartworm disease. She attributes it to environmental and climatic changes, which create sustained heat and humidity that foster mosquito vectors. The movement of domestic dogs following natural disasters and the territory expansion of wildlife that carry the disease, such as foxes and coyotes, are also contributing factors.
Mosquitoes play a crucial role in transmitting heartworm larvae, which require warm conditions to develop. Urban environments can create microclimates that extend mosquito activity. Dr. Faeh emphasized that while heartworm infections are widespread, they are entirely preventable. Pet owners are encouraged to ensure their pets receive timely testing and preventive measures.
The American Heartworm Society's 2025 Incidence Survey highlighted Texas as having the highest number of heartworm cases for the first time ever, with increasing infection rates also observed in the Florida Panhandle, central Carolinas, and southern Illinois.
Testing for heartworm involves a simple blood draw, and it is recommended that dogs be tested annually. Dr. Faeh noted that two types of blood tests can confirm the presence of heartworms: one detects antigens produced by adult female worms, and the other identifies their microscopic offspring. She advised that all dogs should be tested immediately before starting or restarting preventive medications to avoid severe reactions.
Heartworm affects pets differently: in dogs, the larvae migrate and settle in the pulmonary arteries, causing damage over time. Infected dogs may show no immediate symptoms, but Dr. Faeh warned that noticeable signs like coughing, fatigue, and breathing difficulties can indicate advanced heartworm disease. For cats, infections are harder to detect and can lead to serious health issues, although they are not the preferred host for heartworms. Consequently, prevention is crucial as there is currently no approved treatment for feline heartworm disease.
Dr. Faeh clarified the difference between being heartworm-positive and having heartworm disease. A positive result indicates an infection but does not necessarily mean the pet is ill yet. Treatment for dogs with confirmed infections is complex and requires strict activity limitations, as the dying worms can cause severe complications.
To avoid infections, Dr. Faeh highlighted that year-round heartworm prevention is necessary in all states, citing that climate changes can allow mosquitoes to survive in conditions previously considered too cold. Continuous prevention drastically reduces the risk of heartworm disease, making it a crucial aspect of pet care.