Heartworm Disease in Dogs
Heartworm disease is a serious and potentially fatal condition caused by the parasite Dirofilaria immitis, which is transmitted by mosquitoes. Once inside the body, the larvae mature into adult worms that live in the heart, lungs, and blood vessels. Left untreated, the disease causes lung damage, heart failure, and organ dysfunction. It is not contagious from dog to dog.
Who’s Most Affected?
All dogs not on heartworm prevention are susceptible to infection—regardless of age, breed, or where they live. While the risk is higher in warm, humid climates with a large mosquito population, cases of heartworm disease have been diagnosed in all 50 U.S. states. Dogs that spend a lot of time outdoors, travel to endemic areas, or live in regions with year-round mosquito exposure are particularly vulnerable.
Signs
In early stages, dogs may not show any symptoms. As the disease progresses, you may notice:
Persistent cough
Exercise intolerance
Fatigue after activity
Decreased appetite
Weight loss
Difficulty breathing
Swollen abdomen (due to fluid buildup)
In severe cases, dogs may collapse suddenly due to a life-threatening complication called caval syndrome.
What You Can Do
If you notice the signs above, contact your veterinarian right away. Diagnosis is made through blood testing and may include imaging such as X-rays or ultrasound. Treatment should not be delayed once a diagnosis is confirmed.
Common Interventions
There are two main approaches to treating heartworm disease in dogs: the standard (fast-kill) protocol and the slow-kill method. Your veterinarian will recommend the most appropriate option based on your dog’s health, worm burden, and risk factors.
Standard Protocol (Fast-Kill Method)
This is the recommended approach by the American Heartworm Society. It uses an injectable medication called melarsomine to kill adult worms quickly and effectively.
Stabilization: Supportive care may be needed before treatment begins, especially for dogs with advanced disease.
Antibiotics and Preventives: Dogs are given antibiotics like doxycycline to reduce inflammation caused by the bacteria carried by heartworms (Wolbachia), along with monthly preventives to stop new infections.
Adulticide Treatment: Melarsomine injections are administered in a series of doses to safely kill adult worms.
Strict Rest: Dogs must be kept calm and have restricted activity to prevent complications like pulmonary embolism.
Follow-up Testing: Blood tests are done to confirm the worms have been cleared.
Slow-Kill Method
In some cases—such as when melarsomine is unavailable, cost-prohibitive, or the dog has other health risks—a slow-kill method may be used.
What it involves: The slow-kill method typically includes monthly administration of a heartworm preventive combined with a 4-week course of antibiotics. This can gradually reduce the worm burden over time.
Timeline: It may take 1 to 2 years or longer to eliminate adult heartworms using this approach.
Limitations: Dogs remain infected and at risk of continued lung and heart damage for the duration of treatment. There is also concern about promoting drug-resistant strains of heartworms.
When it’s used: This approach is generally reserved for dogs with serious health risks, extremely high worm burdens, or when access to melarsomine is limited.
Prevention
Heartworm prevention is safe, affordable, and highly effective. Monthly oral, topical, or injectable options are available. Dogs should receive preventive care year-round, even in colder climates. The American Heartworm Society recommends annual testing for all dogs over 7 months of age—even those on consistent prevention—to catch any missed doses or product failure.
Conclusion
Heartworm disease is dangerous but preventable. Keeping your dog on monthly prevention and scheduling annual testing are the most effective ways to protect them. If your dog is not currently on prevention or has not been tested recently, speak with your veterinarian today. Early action can save your dog’s life.
