health
Heartworm
The life cycle in plain terms
- An infected mosquito takes a blood meal, injecting infective L3 larvae into the dog's skin.
- Larvae migrate through tissues for ~50–70 days, molting to L4 and then to juvenile adults.
- Juveniles enter the bloodstream and travel to the pulmonary arteries, maturing into adult worms over ~6 months total from infection.
- Adults can reach 30 cm long and live 5–7 years; females release microfilariae into the bloodstream, completing the cycle when another mosquito feeds.
Prevention is straightforward; treatment is brutal
Monthly oral or topical preventives (ivermectin, milbemycin, selamectin, moxidectin) and a twice-yearly injectable (moxidectin) kill the early larval stages before they reach the heart. The American Heartworm Society recommends year-round prevention for every dog in every US state, heartworm has been confirmed in all 50, and seasonality is no longer a reliable boundary.
Treating an established adult infection requires melarsomine injections, strict exercise restriction for months, and carries real risk of pulmonary thromboembolism as worms die. Prevention is cheap; treatment is not.
Heartworm in cats
Cats are atypical hosts, most never develop circulating microfilariae and many die from a single immature worm reaching the lungs (HARD, heartworm-associated respiratory disease). There is no approved adulticide treatment for cats; prevention is the only option. CAPC and AHS recommend year-round prevention in cats, indoor or outdoor, in endemic regions.
Why it matters
Heartworm is one of the most preventable serious diseases in veterinary medicine, yet incidence remains high because owners stop preventives in winter, miss doses, or assume indoor cats are safe. The annual test plus monthly prevention pair is the lowest-effort high-impact decision in canine and feline care.
Frequently asked questions
- Do I still need an annual heartworm test if my dog is on prevention?
- Yes. No preventive is 100% effective, missed doses happen, and giving an adulticide-class preventive to a dog with an undiagnosed infection can be dangerous. Annual antigen testing is the standard of care.
- Is heartworm contagious?
- Not directly between dogs. Transmission requires a mosquito vector, a dog cannot give heartworm to another dog or to a human through contact.