Heartworm is one of the few parasitic diseases where the treatment is nearly as worrying as the disease. That single fact is why vets are so insistent about prevention, and why the recommendation in endemic areas is year-round rather than seasonal.
The life cycle
A mosquito bites an infected animal and picks up microscopic immature worms called microfilariae. Inside the mosquito they develop for a couple of weeks into infective larvae. The mosquito then bites your dog and deposits those larvae on the skin, where they enter through the bite wound.
From there it is a slow journey. The larvae migrate through tissue for months, maturing as they go, and eventually reach the pulmonary arteries and the right side of the heart. Around six to seven months after the bite, they are adult worms up to thirty centimetres long, and they begin producing microfilariae of their own.
A dog can carry dozens of them. They damage the blood vessels, strain the heart and inflame the lungs.
Where the risk is
Heartworm is well established across much of the United States, particularly the south east and the Mississippi valley, and is found in parts of southern Europe. It is not currently endemic in the UK, though dogs imported from or travelling to endemic regions can bring it back.
Risk is not neatly seasonal any more. Mosquito activity extends further into the year with warmer winters, dogs travel, and a single missed month is enough. That is why year-round prevention is the standard recommendation in endemic areas rather than a summer-only approach.
Symptoms, and why they come late
For the first several months there is nothing to see. By the time signs appear, adult worms are already present.
Early: a soft persistent cough, tiring on walks, reluctance to exercise.
Later: weight loss, a swollen abdomen from fluid, laboured breathing, fainting.
Severe: caval syndrome, where a mass of worms blocks blood flow through the heart. It causes sudden collapse, dark bloody urine and is usually fatal without emergency surgical removal.
Testing
An annual blood test detects proteins produced by adult female worms. Because it takes around six to seven months for worms to mature, a test cannot detect a very recent infection — which is why vets test yearly and why a dog starting prevention after six months of age is tested first.
Giving a preventive to a dog that already carries microfilariae can cause a severe reaction, so the test comes before the first dose.
Prevention
Preventives do not stop mosquito bites. They kill the immature larvae acquired in the previous month, before those larvae can mature. That is why the timing matters so much: they work backwards, clearing what has already arrived.
Options include:
- Monthly oral chews, often combined with intestinal wormers
- Monthly topical spot-ons, some of which also cover fleas
- Injectable products lasting six or twelve months, given at the practice
All are prescription. Your vet will choose based on your dog’s other parasite needs, lifestyle and any sensitivities.
Some dogs spit out chews or vomit them shortly after. Watch that the dose actually goes down, and ring the practice if it comes back up within an hour.
Reducing mosquito exposure
Prevention medication is the main defence, but reducing bites helps:
- Remove standing water: buckets, plant saucers, blocked gutters, paddling pools
- Keep dogs in at dawn and dusk during peak mosquito season where practical
- Use screens on windows
- Only use repellents licensed for dogs, never human products containing DEET
Treatment, and why you want to avoid it
Treating an established infection is a months-long, expensive and genuinely risky process. It typically involves:
- Stabilising the dog, often with steroids and an antibiotic that weakens the worms
- A series of deep intramuscular injections of a drug that kills adult worms
- Strict exercise restriction for weeks to months
That last part is the hard bit, and it is not optional. As the worms die they break apart and travel into the lungs. Exercise increases blood flow and dramatically raises the chance of a fatal blockage. Owners are asked to keep an otherwise well-feeling dog on lead-only toilet breaks and crate rest for a long time.
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If crate rest is ever needed — after heartworm treatment, surgery or injury — having a dog already comfortable in a crate makes the difference between rest and a fight. Train it before you need it.
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Even with treatment, damage to the heart and lung vessels may be permanent.
The comparison
A year of prevention costs a fraction of a course of treatment, carries almost no risk, and takes about four seconds a month. Treatment costs a great deal, takes months, requires strict confinement and can go wrong.
That is the entire argument, and it is a strong one.
Practical checklist
- Test annually, even on continuous prevention
- Give preventives on the same day every month, year round in endemic areas
- Test before starting or restarting after any gap
- Ask your vet about heartworm before travelling to an endemic region
- If you adopt an imported or rescued dog, ask whether it has been tested
Speak to your own vet about local risk and the right product. This is one area where following the standard recommendation, without improvising, is straightforwardly the right call.
