How Dogs Get Heartworm Disease and How Prevention Works
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By Dr Duncan Houston
A dog does not have to meet another infected dog to acquire heartworm. Mosquitoes carry the parasite between susceptible animals, and a dog may look healthy for months after infection. Prevention and testing are designed to act before obvious illness develops.
Quick Answer
Dogs acquire heartworm when an infected mosquito deposits infective larvae near its bite, allowing them to enter through the bite wound. The parasites develop over months and adult worms primarily occupy the blood vessels of the lungs, sometimes involving the heart in severe disease. Use a veterinary prevention and testing plan appropriate to your dog and location, and contact your vet promptly if protection has lapsed.
The mosquito’s role
Heartworm disease is caused by Dirofilaria immitis. A mosquito picks up microscopic young parasites, called microfilariae, while feeding on an infected animal. The parasites develop inside the mosquito before it can transmit infective larvae during a later feed.
After entering a dog, larvae develop through several stages before adult worms establish in the pulmonary arteries. The full process takes months. The American Heartworm Society’s dog information explains why the mosquito is essential: heartworm does not spread directly through sharing bowls, playing or ordinary contact.
Why an indoor dog may still be exposed
Mosquitoes can enter homes, and brief outdoor toilet trips still create opportunities for bites. Local climate, mosquito populations, infected animal reservoirs and travel affect risk. A low-risk location is not the same as no risk, especially when a dog moves or holidays elsewhere.
Reducing standing water and using screens can reduce mosquito exposure but does not replace heartworm preventive medicine. Human insect repellents and unapproved essential-oil preparations may harm pets. Ask before applying any repellent to your dog.
Signs may be absent at first
| Possible pattern | What it means for the owner |
|---|---|
| No obvious signs | Infection can still be present; follow the testing plan |
| Persistent cough or tiring more easily | Arrange a veterinary assessment and mention prevention history |
| Reduced appetite, weight loss or reduced stamina | These signs have several causes and need investigation |
| Breathing difficulty, weakness or collapse | Seek emergency veterinary care |
Heartworm damages the lungs and their blood vessels and can lead to serious cardiovascular complications. Coughing is not specific to heartworm: airway disease, infections and other heart or lung conditions can look similar. Appearance alone cannot confirm or exclude infection.
Why testing remains important
Blood antigen tests primarily detect proteins associated with adult female worms. Testing for microfilariae provides additional information. Early infection may not yet be detectable, and a negative test soon after a lapse in prevention does not settle the question permanently.
The American Heartworm Society recommends annual testing, including for dogs receiving prevention. Young puppies and dogs with unknown or interrupted prevention histories need testing timed around the parasite’s development. Your vet will explain the initial and repeat tests suitable for the dog and local practice.
If a screening test is positive, further confirmation and assessment may be needed before treatment. Blood tests, chest imaging or other investigations help evaluate the infection and its effects rather than treating a test result in isolation.
How prevention works
Heartworm preventives target immature stages before they develop into adult worms. They are medicines, not a routine vaccine. Available forms and labelled intervals vary by country and product and can include oral, topical and long-acting injectable options.
- Start a puppy’s prevention plan early with your vet, using a product suitable for their actual age and weight.
- Check that the product specifically includes heartworm; a flea treatment or an “all-wormer” may not.
- Record administration and the next due date, including clinic-administered injections.
- Check whether food, bathing or other instructions affect correct use.
- Review the plan before travel and during regular health checks.
Do not assume all products work on the same monthly schedule. Growing puppies need weight checks, and long-acting products have specific eligibility and scheduling requirements.
What if a dose was missed?
Contact your vet with the product name, last confirmed dose, your dog’s age and weight, and any travel history. They can advise how to restore protection and when to test. Do not double a dose or use leftover livestock wormers. Delaying the conversation until the next annual visit creates further uncertainty.
Treatment and emergency care
Established heartworm infection needs a supervised treatment plan. The American Heartworm Society guidelines include treatment to address adult worms and associated infection, with strict activity restriction to reduce complications as worms die. The details depend on the dog’s condition and available medicines.
Do not attempt an unsupervised “slow-kill” approach with preventive medicine alone. Exercise restriction is a medical part of treatment, not an optional precaution. Breathing difficulty, collapse, severe weakness or dark urine in a dog with suspected or confirmed heartworm needs emergency assessment.
Frequently asked questions
Can my dog catch heartworm directly from another dog?
No. Mosquitoes are required for the usual transmission cycle.
Does a negative test mean prevention is unnecessary?
No. It describes what the test can detect at that time; it does not prevent a future infection.
Is the long-acting heartworm injection a vaccine?
No. It delivers preventive medicine over a labelled period.
Can an infected dog recover?
Many dogs can be treated successfully, but severity and complications affect the outlook. Some damage may persist, making prevention preferable.
A dependable prevention schedule and appropriately timed testing are the most useful habits you can control.
For help organising your dog’s prevention history and questions for their vet, visit ASK A VET™. Breathing difficulty or collapse needs immediate veterinary care.
