Heartworm disease is a serious, potentially fatal parasitic infection caused by Dirofilaria immitis, a worm transmitted through mosquito bites. Adult heartworms live in the heart, lungs, and associated blood vessels of infected animals. Dogs are the primary host; cats are atypical hosts but can still be affected. Prevention is far more effective than treatment.
Heartworms are transmitted when a mosquito bites an infected animal, picks up microscopic heartworm larvae (microfilariae), and then bites another animal, depositing the larvae. The larvae migrate through the tissue, eventually reaching the heart and pulmonary arteries, where they mature into adult worms over approximately six months. Adult worms can reach 12 inches in length, and dogs can carry dozens to hundreds of worms simultaneously.
In dogs, symptoms develop as the worm burden increases and the heart and lungs sustain damage: mild persistent cough, fatigue after moderate exercise, reduced appetite, and weight loss in early stages. Advanced disease causes a distended abdomen (from fluid), difficulty breathing, pale gums, and collapse. Treatment (melarsomine injections) is available but expensive, involves weeks of strict activity restriction, and carries its own risks — prevention is far preferable.
In cats, heartworm disease is different and more unpredictable. Cats are resistant hosts — most larvae die before becoming adults — but the immune response to dying larvae and adult worms causes a syndrome called Heartworm Associated Respiratory Disease (HARD), which mimics feline asthma. There is no approved treatment for heartworm in cats; only prevention and supportive care.
📌 Key Facts
Transmitted by: Mosquito bite
Time from infection to adult worms: ~6 months
Average adult worm length: 6–12 inches
Treatment available for dogs: Yes (melarsomine) | Treatment for cats: No approved treatment
Frequently Asked Questions about Heartworm Disease
Q: Does my indoor cat need heartworm prevention?
Yes. Mosquitoes enter homes, and cats are susceptible regardless of whether they go outdoors. Additionally, since there is no treatment for cats, prevention is the only protection. Many feline heartworm preventives are combined with flea and intestinal parasite prevention.
Q: Can heartworm disease be completely cured in dogs?
Treatment eliminates the adult worms, but damage to the heart and lungs may be permanent depending on how advanced the disease was at diagnosis. Dogs treated early with a low worm burden typically have a good prognosis. Severe cases may have lasting cardiac changes.
Q: Why is heartworm prevention given monthly if the transmission cycle takes 6 months?
Monthly preventives work by eliminating the larvae that were transmitted in the previous 30 days — they work backwards, not forwards. This is why missing even one dose creates a window of risk, and why a test before starting prevention is needed if the dog has not been consistently protected.
⚠ Medical Disclaimer: This glossary entry is for educational purposes only
and does not constitute veterinary advice. Always consult a licensed veterinarian for diagnosis
and treatment of your pet's health conditions.