Heartworm: problems, treatment and prevention
Everything about heartworm: what it is, how it causes damage, treatment and especially prevention. Prevention is much easier than cure.
Heartworm (Dirofilaria immitis) is a serious parasite transmitted by mosquitoes. The worms lodge in the heart and pulmonary arteries and can be life-threatening. In warm regions—Southern Europe, Thailand, tropics—the risk is high. Prevention is essential.
⚠️ The problems: what is heartworm?
What is heartworm?
Heartworms are long, thin worms (up to ~30 cm) that live in the heart, pulmonary arteries and lungs. They cause inflammation, blocked blood vessels and eventually heart failure and lung damage. Without treatment the disease can be fatal.
Life cycle: how does your pet get infected?
Mosquitoes feed on an infected animal and pick up microscopic larvae (microfilariae). In the mosquito these develop into infective larvae in 10–14 days. On the next bite they are transmitted to a healthy animal. After about 6 months they grow into adult worms. Infection is via mosquitoes—not from animal to animal.
Dogs: natural host
Dogs are the natural host. Worms can mature, mate and reproduce. An untreated dog can harbour hundreds of worms. Symptoms: coughing, fatigue, weight loss, shortness of breath, swollen belly. In late stage: fainting, coughing up blood.
Cats: atypical host
Cats are an atypical host—most worms don't survive to adulthood. Cats typically have only 1–3 worms or none. Yet even immature worms cause damage (HARD: heartworm-associated respiratory disease). There is no safe treatment for cats like in dogs—prevention is the only option.
💊 Treatment
Treatment is complex, expensive and not without risk. Killing adult worms can cause temporary blockages. That's why prevention is so important.
Treatment in dogs
The standard is melarsomine injections (several times, with strict rest). Often a monthly macrocyclic lactone (ivermectin/moxidectin) is given first to kill larvae, plus doxycycline against Wolbachia bacteria. Activity must be severely restricted. Only under veterinary supervision.
Treatment in cats
There is no approved treatment for adult heartworm in cats. Drugs used in dogs are dangerous for cats. Supportive care (oxygen, anti-inflammatories) may relieve symptoms, but prevention is the only real protection.
🛡️ Prevention: the most important
Prevention is much simpler, cheaper and safer than treatment. A monthly preventive kills larvae before they become adults. In risk areas this is essential.
Monthly prevention: active ingredients
Macrocyclic lactones kill larvae at an early stage:
- • Ivermectine – In many products (e.g. some dewormers). Effective when used correctly. Caution in certain breeds (Collie, Sheltie) due to MDR1 gene.
- • Moxidectine – Longer duration, often safer in MDR1 breeds. In NexGard Spectra, Advocate etc. Also available as injection (6–12 months protection).
- • Selamectine – In Stronghold/Revolution—spot-on. Works against fleas, heartworm and some mites.
- • Milbemycine – In NexGard Spectra, Milbemax etc. Often combined with afoxolaner against fleas/ticks.
Products
NexGard Spectra, Stronghold/Advocate, Milbemax, Interceptor, Heartgard and others offer heartworm prevention. Some combine with fleas/ticks. Ask your vet which product and dosage suits your pet (weight, breed, medication).
When to start?
Puppies and kittens: according to your vet's schedule, often from 6–8 weeks. Adult animals in risk areas: year-round, monthly. In colder regions where mosquitoes are seasonal: during mosquito season, or year-round for maximum certainty.
Travelling to risk areas
Going to Southern Europe, Thailand or other tropical areas? Start prevention before departure (often 1–2 weeks earlier) and continue until after return. Discuss with your vet the right product and schedule.
🩺 This information is general. Contact your veterinarian for a consultation and tailored advice.
More about health
View our health guide for vaccinations, fleas, ticks and more.
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