How Do Vets Check for Heartworm in Dogs?
- A heartworm test is a blood test. Your vet takes a small sample in the consulting room, and an in-practice assay gives a result in about ten minutes.
- The main test detects antigen from adult female worms, so it cannot turn positive until those worms have matured, which takes roughly six months after infection.
- A second test, the modified Knott's, concentrates and counts microfilariae. It sharpens sensitivity and shows how heavy an infection is before treatment.
- UK government guidance describes heartworm as not endemic here, so screening is aimed at imported dogs, travelling dogs and dogs with unexplained signs.
- Most British dogs need flea, tick and lungworm cover instead. A monthly FRONTLINE Spot On handles fleas and ticks for most dogs, or an eight-month Seresto collar if you would rather not count weeks.
Suggested reading: How Dogs Catch Heartworm, How Often Dogs Need Heartworm Medicine, and Dog Health Warning Signs Answered.
A heartworm test is a blood test. Your vet draws a small sample during an ordinary consultation, runs it through an in-practice assay, and has an answer in about ten minutes, with no fasting or sedation.
The awkward part is timing. The test looks for antigen released by adult female heartworms, and those worms need roughly six months inside a dog before there is anything to find. A dog bitten last week will read negative. Here is what each test does, which UK dogs get screened, and what a result really tells you.
The Antigen Test Vets Reach For First
UK government guidance on imported disease describes an ELISA test that detects heartworm antigen, and notes it is highly sensitive and specific in dogs carrying adult worm infestations. That phrase carries the limitation. The test hunts for a protein shed by mature female worms, so it only reports what has already grown inside the dog.
Most practices use an in-clinic cartridge that returns a result in about ten minutes, and some screen for several things at once: heartworm plus the tick-borne infections dogs pick up abroad, including ehrlichia, anaplasma and Lyme disease.
Veterinary references list three situations where antigen testing can miss an infected dog: a low burden of female worms, an infection made up of male worms only, and a dog already given a preventive from the macrocyclic lactone family, which includes moxidectin, selamectin and milbemycin. A negative result is strong evidence rather than proof.
The Second Test Counts Microfilariae
Adult female worms release microscopic offspring called microfilariae into the bloodstream. The modified Knott's test, sometimes called a filtration test, concentrates them out of a blood sample so a laboratory can see them and count them. Run alongside the antigen test, it sharpens the sensitivity of screening.
The count matters for treatment more than diagnosis. A dog carrying a heavy microfilarial load needs a more careful approach, because killing those larvae in one go can trigger a serious reaction. UK guidance is explicit that microfilarial numbers should be established before adult worms are treated.
Which UK Dogs Get Screened, and When
Heartworm is not established in Britain. UK government guidance states it plainly: heartworm is not endemic in the UK, and the sporadic cases that appear mostly involve imported dogs or dogs with a travel history. Mosquitoes able to carry the parasite do live across northern Europe, but the climate here is generally too cold for the worm to complete its life cycle.
Two groups are worth screening: dogs that have spent time in an endemic region, which includes southern and eastern Europe, much of the United States, and parts of Africa and Asia, and rescue dogs arriving from those places, because they can land here already carrying adult worms.
Veterinary guidance recommends testing those dogs on arrival and repeating the test six to nine months later. That gap is not paperwork: adult worms need that long to mature to the point where antigen testing can find them, so a dog that tested negative on arrival can still be infected. Dogs with signs are screened as well, though the signs arrive late: a cough that will not settle, tiring on walks, or weight loss with a normal appetite. None of it points at heartworm on its own, which is why a blood test beats guesswork.
What Age Do Dogs Get Heartworm Tested?
Age matters far less than the clock that starts at infection. Antigen testing cannot detect a worm that is still growing, so a puppy infected a few weeks ago will read negative however often you test. For a puppy imported from an endemic area, the workable approach is a test on arrival and a second six to nine months later, once any worms from that trip are old enough to find.
A dog born in the UK that never travels does not need heartworm testing at any age. What puppies do need is the routine that suits Britain, licensed from the age and weight printed on each pack.
Do Dogs Need a Heartworm Test Every Year?
Where heartworm circulates, annual testing is standard advice even for dogs on year-round prevention, which is the position of the American Heartworm Society and the Companion Animal Parasite Council. Prevention mostly works, but a missed dose leaves a gap, and yearly testing finds it before the dog is unwell.
The UK works differently, because there is no home-grown transmission cycle to break. Routine annual testing is not standard here and a dog that stays in Britain does not need it. A dog that travels to an endemic region often, or spends part of the year there, is usually retested on a schedule your vet sets. Ask what a negative result covers, because the honest answer is about six to seven months of history rather than a lifetime all-clear.
If the Test Comes Back Positive
A positive antigen test is a starting point rather than a full diagnosis. Vets confirm it with microfilaria testing and often imaging of the pulmonary arteries, then treat with injections that kill the adult worms, usually alongside doxycycline. That treatment carries real risk, because dying worms can lodge in the lungs and block blood vessels, which is why strict rest is part of the plan.
That gap is the argument for screening first: prevention is a routine, while treating an established infection is long and carries genuine risk.
What UK Dogs Actually Need Instead
For almost every British dog, the routine that matters is fleas, ticks and lungworm, all active here all year. Flea and tick cover runs monthly. The FRONTLINE Spot On for medium dogs covers the 10 to 20kg range that fits most spaniels, terriers and collies, the small dog version handles 2 to 10kg, and owners who would rather not count weeks can use the Seresto collar for dogs over 8kg, which releases its active ingredients over about eight months.
If your dog travels, two small items earn their place in the bag. A tick removal tool matters because prompt removal is what reduces the risk of tick-borne disease, and a pet first aid kit covers the basics until you reach a vet abroad. On warm evenings in Europe, a cedarwood bug reducing spray can cut down on biting, though treat it as a comfort measure rather than parasite protection.
Heartworm preventives are prescription medicines in the UK, so they come from your vet rather than a shelf. Anything marketed as an off-the-shelf heartworm treatment is not one.
Frequently Asked Questions
Final Verdict
The test itself is simple: one blood sample, an antigen assay, and a microfilaria check if the result is positive or unclear. What decides whether your dog needs any of it is history rather than postcode. A dog that has been to southern Europe, arrived from a rescue abroad, or developed a cough that will not settle is a dog worth raising with your vet.
For everyone else in the UK, heartworm testing is not part of routine care, because the parasite has no established cycle here. The cover that does matter runs monthly against fleas and ticks, whether that means FRONTLINE Spot On, a size-appropriate spot-on for a smaller dog, or an eight-month Seresto collar, with a tick remover in the walking bag. Keep that routine tight, and mention any travel at the vet visit.
