Heartworm in dogs: symptoms, prevention and treatment

Heartworm sounds like something that only happens to dogs on the other side of the world, and for most dogs living in Britain, that's broadly true. But as more of us travel abroad with our dogs, and as more dogs are rescued from or imported through mainland Europe, this parasite has quietly become a real, if still uncommon, risk for UK pets.

The good news is that heartworm is almost entirely preventable with the right medication, given at the right time. The bad news is that by the time symptoms appear, the worms have often been established for months, and treatment is long, expensive and not without risk to the dog. Understanding how the parasite spreads, who's actually at risk, and what prevention looks like in practice is the best way to make sure your dog never needs treating for it at all.

This guide covers what heartworm is, how it's transmitted, the current picture for dogs in the UK, the signs to watch for, and what prevention and treatment involve if your dog has travelled, been imported, or is planning a trip abroad.

What is heartworm?

Heartworm is caused by a parasitic nematode called Dirofilaria immitis. The adult worms are long and thread-like, reaching up to around 30cm, and they live in the right side of the heart and the major blood vessels leading to the lungs (the pulmonary arteries). A dog can carry a surprising number of worms without showing obvious symptoms early on, because the damage builds up gradually as the worms mature, move, and eventually die inside the blood vessels.

Heartworm is quite different from the more familiar intestinal worms (like roundworms and tapeworms) that most UK dog owners already worm for routinely. It's also a different parasite from lungworm (Angiostrongylus vasorum), which is a genuine and growing concern within the UK. Heartworm and lungworm are sometimes confused because both can affect the heart and lungs and are spread by other animals, but heartworm is spread exclusively by mosquitoes, not by slugs and snails, and (for now) it isn't established in the UK the way lungworm is.

How dogs catch heartworm

Heartworm has an unusually complex lifecycle that depends entirely on mosquitoes. When a mosquito bites an infected dog, it picks up microscopic larval-stage worms (microfilariae) circulating in the dog's blood. Those larvae develop further inside the mosquito over one to two weeks, reaching an infective stage. When that mosquito then bites another dog, it deposits the infective larvae onto the skin, and they migrate into the body through the bite wound.

Once inside a new dog, the larvae take several months to migrate through the tissues and mature into adult worms living in the heart and pulmonary arteries. Adult heartworms can live for five to seven years, and during that time female worms produce their own microfilariae, ready to be picked up by the next mosquito that bites. This means an infected dog effectively becomes a reservoir the disease can spread from, which is one reason heartworm can become endemic in warmer regions with the right mosquito populations.

Critically, a dog cannot catch heartworm directly from another dog. It always requires a mosquito as the intermediate host, and the mosquito itself needs a sufficiently warm environment for long enough for the larvae to mature to the infective stage.

Is heartworm a risk in the UK?

The UK is currently considered non-endemic for heartworm, meaning there is no established, self-sustaining transmission cycle here. A 2010 UK-wide survey testing over 1,000 dogs found no heartworm antibodies at all, and more recent European surveillance work covering 2016 to 2020 has found only very low seroprevalence (around 5–7%) in some parts of central and southern England, with no positive dogs detected elsewhere in the country, according to peer-reviewed research published via the National Library of Medicine.

A handful of locally acquired cases have been documented in England and Scotland, but on close investigation these have almost always traced back to a dog that was imported from, or had travelled to, an endemic country such as Romania. One Scottish case involved a dog imported from a Romanian shelter that was diagnosed with the infection roughly two years after arriving in the UK.

The main reason heartworm hasn't become established here is climate. The parasite needs a sustained period of warmth (broadly, a minimum daily average significantly above 14°C, maintained for enough cumulative "heat units") for the larvae to fully develop inside a mosquito before the mosquito dies. UK summers, even warm ones, generally don't provide that consistently. That said, researchers have flagged this as a moving target: warming summers and the detection of invasive mosquito species such as Aedes albopictus in parts of the UK in recent years mean the risk picture could shift over time, and it's one vets are watching.

For now, the practical takeaway is straightforward: a dog that has never left the UK is not considered to be at meaningful risk of heartworm. The real risk groups are dogs that travel to or through mainland Europe, and dogs imported or rescued from abroad, particularly from southern and eastern Europe where the disease is well established.

Signs and symptoms

Heartworm symptoms tend to build slowly and can be easy to miss early on, especially in dogs that aren't especially active anyway. Severity depends heavily on how many worms are present and how long the infection has gone untreated.

• Mild or early disease: a soft, persistent cough, and reluctance to exercise or noticeable tiredness after activity that wouldn't normally bother the dog.
• Moderate disease: the cough becomes more frequent, exercise intolerance worsens, appetite drops and weight loss can follow.
• Severe disease: laboured or rapid breathing, a swollen abdomen from fluid build-up (a sign the heart is struggling), and in advanced cases, heart failure.
• Caval syndrome: a rare but life-threatening emergency where a heavy worm burden physically blocks blood flow through the heart, causing sudden collapse, pale gums, and dark or "coffee-coloured" urine. This requires emergency veterinary treatment.

Because early signs are subtle and easily mistaken for a dog simply "slowing down," heartworm is often picked up on a routine blood test in an at-risk dog rather than because of obvious symptoms. This is exactly why prevention and screening matter more than watching for symptoms after the fact.

How vets diagnose heartworm

If your vet suspects heartworm, based on travel history, import history, or symptoms, diagnosis usually starts with a blood test that detects a protein produced by adult female worms. This antigen test can be run in many veterinary practices or sent to an external laboratory. Because false positives can occur, a positive result is usually confirmed with a second, different type of test before treatment begins.

For dogs that travel regularly or have been imported from an endemic country, vets may recommend heartworm testing as part of a broader travel health check, alongside screening for other mosquito- and tick-borne diseases common in mainland Europe, such as leishmaniasis, ehrlichiosis and babesiosis. None of these are UK-endemic either, but all are realistic risks for dogs that travel with their owners or arrive here from affected countries.

Prevention: what to do before travelling

Heartworm prevention is highly effective and considerably simpler than treatment. If you're planning to travel with your dog to mainland Europe, or anywhere heartworm is established, veterinary advice is consistent on timing and product choice:

• Book a pre-travel consultation with your vet four to six weeks before departure to allow time to start (or check) an appropriate preventative and get any documentation sorted.
• Preventative medication should be started at least one month before travel, continued every four weeks while abroad, and continued for at least one month after returning to the UK, because the medication clears larvae that were picked up recently rather than protecting retrospectively.
• Licensed UK products for heartworm prevention include spot-on treatments (such as those containing selamectin or moxidectin) and oral tablets (such as milbemycin-based combination wormers), some of which also cover other parasites like tapeworm.
• Missing a single dose can leave a genuine window of risk, so consistency matters more than which specific product you use — your vet can advise on the best fit for your dog's travel plans and any other conditions they may have.

Even one missed dose of preventative medication can be enough to put a travelling dog at risk of heartworm.

It's also worth asking your vet about combined protection, since dogs travelling to affected regions are often exposed to ticks and sandflies carrying other diseases at the same time, and a single conversation can cover flea, tick and heartworm prevention together rather than as separate afterthoughts.

Treatment if a dog is diagnosed

Treating an existing heartworm infection is a much bigger undertaking than preventing one, and it's not without risk to the dog, which is why prevention is so strongly emphasised by vets.

Treatment protocols typically involve an initial period of stabilisation, sometimes including a course of an antibiotic and an anti-inflammatory medication to help reduce the dog's immune reaction and target bacteria that live inside the worms themselves. The adult worms are then killed using a specific injectable medication given in a veterinary hospital, usually across a series of injections spread over several weeks.

Throughout treatment, strict exercise restriction is essential. As the worms die, fragments can break loose and travel to the lungs, and physical exertion significantly increases the risk of a serious, sometimes fatal, reaction. Dogs undergoing treatment are typically kept on lead-only, low-key activity for many weeks, sometimes months, with the exercise restriction continuing for several weeks after the final injection to allow the body to safely clear the dead worms.

Follow-up testing, usually around three to four months after treatment finishes, confirms whether the treatment has been successful. Prognosis is generally good for dogs treated before significant heart or lung damage has occurred, but it becomes more guarded the more advanced the disease is at diagnosis — another reason early detection through routine screening in at-risk dogs is so valuable.

Imported and rescue dogs

A significant proportion of the very small number of heartworm cases identified in the UK have been in dogs imported or rescued from abroad rather than dogs that travelled on holiday with their owners. Relaxed pet travel and import routes in recent years have meant large numbers of dogs have entered the UK from countries such as Romania and Spain, some of which have well-established heartworm populations.

If you're adopting a rescue dog that has come from overseas, it's worth asking the rescue organisation directly what parasite testing and preventative treatment the dog has already had, and flagging the dog's origin to your own vet at the first check-up. Because heartworm can take months to show up on a test after infection, and longer still to cause visible symptoms, a dog can appear perfectly healthy on arrival and still be incubating an infection. A simple blood test soon after adoption, repeated a few months later if the first was too early to be reliable, gives real peace of mind.

When to see your vet

Contact your vet promptly if a dog that has travelled abroad or been imported develops a new, persistent cough, seems to tire much more easily than usual, is breathing harder than normal, or develops a swollen abdomen. These can have several causes, not just heartworm, but they all warrant proper investigation rather than waiting to see if things settle.

If you're planning travel abroad with your dog, speak to your vet well ahead of time about heartworm and other travel-specific risks, rather than leaving it until the week before you go. And if you're bringing a rescue dog into your home from overseas, make heartworm and other parasite screening part of that dog's first vet visit, even if they seem completely well.

*This is general guidance, not a substitute for advice from your vet, who can assess your individual pet.*

Sources

• National Library of Medicine (PMC) — peer-reviewed research on Dirofilaria immitis prevalence and risk factors in the UK, "The long-distance relationship between Dirofilaria and the UK" (pmc.ncbi.nlm.nih.gov).
• Ash Croft Vets — European travel disease advice for dogs, including heartworm prevention timing and licensed products (ashcroftvet.co.uk).
• Companion Care Vets — heartworm in dogs: transmission, UK risk status, symptoms and treatment overview (companioncare.co.uk).
• American Heartworm Society — heartworm basics: symptoms by severity, diagnosis and treatment protocol (heartwormsociety.org).