How Do Dogs Get Heartworms From Mosquitoes?

Heartworms reach dogs in exactly one way: through the bite of an infected mosquito. A mosquito that has fed on an infected dog, fox or coyote picks up microscopic larvae, those larvae develop inside the insect for around 10 to 14 days, and the next blood meal delivers them into your dog's skin. Your dog cannot catch heartworm from another dog, from soil, from water, or from a shared food bowl.

That single route explains almost everything else about the disease, including why it is rare in Britain. Here is how the cycle works, which UK dogs are actually at risk, and which parasite owners here confuse for heartworm far more often than they should.

The Transmission Cycle, Step by Step

Heartworm disease is caused by Dirofilaria immitis, a thread-like roundworm that lives as an adult in the pulmonary arteries and the right side of the heart. Parasitology references list several mosquito genera that can carry it, including Aedes, Anopheles and Culex, and all of them are ordinary biting mosquitoes. No special insect is involved, and the worm cannot skip its insect stage.

When a mosquito bites an infected dog it takes up microfilariae, the microscopic offspring that adult female worms release into the bloodstream. Those microfilariae cannot infect a new dog on their own. Inside the mosquito they develop through larval stages, which veterinary references put at roughly 10 to 14 days in favourable conditions, and temperature decides whether that happens at all. The larvae need sustained warmth, with veterinary sources describing a threshold in the region of 14C.

Infective larvae are then deposited on a dog's skin during the mosquito's next blood meal. They enter through the tiny wound left by the bite, migrate through tissue for several months, and settle around the heart and lungs as adults that grow to roughly 25 to 30cm. A dog can carry a dozen or more of them, and they live for years.

Your Dog Cannot Catch Heartworm From Another Dog

An infected dog carries microfilariae in its blood, but that blood is not infectious on its own, because the parasite must pass through a mosquito first. Saliva, urine, faeces, shared bowls and mating are not routes of infection, and a dog cannot pick heartworm up from grass, from a puddle, or from the bedding of an infected dog. The only question that matters is whether an infected mosquito bites your dog.

Do UK Dogs Actually Get Heartworm?

In practical terms, no, or at least not from a cycle running in this country. UK veterinary bodies such as BSAVA describe heartworm as not established in the UK, and confirmed cases here are almost always dogs that have travelled to an endemic area or arrived from one. A dog that lives its whole life in Britain and never goes abroad is at very low risk, which is why blanket year-round heartworm medicine is not standard advice here.

The warning attached to that advice is worth knowing. BSAVA has noted that rising temperatures and a growing number of imported dogs could increase the risk of heartworm becoming established over time, so this is a picture vets are watching rather than one that has already changed.

Which UK Dogs Are at Real Risk

If your dog travels, the risk profile changes completely. Heartworm is common in southern Europe, including Spain, Portugal, Italy, Greece and southern France, and it is also found across parts of eastern Europe and much of the United States. Rescue dogs imported from endemic countries are the other group vets flag, because they can arrive already carrying adult worms.

Under the UK Pet Travel Scheme, dogs returning from certain countries need tapeworm treatment before they travel, and that requirement is about Echinococcus rather than heartworm. Heartworm prevention is a separate conversation with your vet, usually started before you leave rather than after you come home.

The UK Parasite Owners Confuse With Heartworm

Ask a UK vet about worms affecting the heart and lungs and they are far more likely to talk about lungworm, Angiostrongylus vasorum, which is genuinely common in parts of Britain. It is sometimes nicknamed French heartworm, which is where much of the confusion starts, but it has nothing to do with mosquitoes.

Lungworm spreads through slugs and snails. Dogs pick it up by eating an infected slug or snail, and also from the slime those molluscs leave on grass, toys, bowls and outdoor water containers. The signs stay vague at first: a cough that will not settle, tiring on walks that used to be easy, weight loss, or in serious cases bruising and nosebleeds. Because this one really does circulate here, it belongs on the list of things to raise at your dog's next check-up. Our flea and tick prevention guide explains how the pieces of that routine fit together.

Signs Owners Notice, and Why They Come Late

Heartworm is quiet for a long time. Adult worms take around five to seven months to mature after infection, and many dogs show nothing obvious for months or years after that. When signs do appear they tend to be exercise-related: a cough, tiring earlier than usual, weight loss despite a normal appetite, and in advanced cases fluid collecting in the abdomen or fainting. Cats can be infected too, and because they carry far fewer worms the disease often shows up as breathing difficulty rather than a gradual decline.

None of those signs are specific to heartworm, which is why diagnosis is a blood test rather than guesswork. A vet will typically test for adult worm antigen and may also check for microfilariae in the blood.

How Heartworm Is Prevented and Tested

Prevention in the UK is prescription-only. Licensed heartworm preventives for dogs, whether monthly tablets, chewables, spot-on treatments or a long-acting injection, are veterinary medicines rather than shop shelf products. That matters, because no spray, collar or herbal supplement sold on a general retail site can claim to prevent heartworm infection.

For a dog heading to an endemic area, the usual approach is a blood test to confirm it is not already infected, then a preventive timed to the trip on a schedule your vet sets. Treating an established infection is far harder than preventing one, typically a course of injections from your vet to kill the adult worms plus strict exercise restriction while they break down.

What You Can Do at Home

Reducing mosquito bites is the only thing within an owner's control, and most of it is unglamorous garden maintenance. Mosquitoes breed in standing water, so empty plant saucers, buckets, watering cans and folded tarpaulins regularly, and check that water butts and gutters are not holding stagnant pools through the warm months. Mosquitoes are busiest around dawn and dusk, so walks at those times mean more bites for both of you.

For dogs that get bitten heavily, a deterrent spray can cut down the nuisance. The Be:Loved Be:Gone natural bug reducing spray is a UK-made cedarwood grooming mist sold for coats, around £11.61 on Amazon UK with a 4.5 star rating from 12 ratings at the time of writing. Treat it as a comfort measure on a summer evening rather than protection against a parasite. The same goes for a herbal deterrent such as the Pets Calm Down natural repellent spray, which is marketed for fleas and bedding rather than for heartworm.

The protection work that matters for a UK dog is the routine you keep all year, because fleas and ticks are active here every month and ticks carry disease. 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 if a monthly pipette is the part that slips, the Seresto collar for dogs over 8kg releases its active ingredients over about eight months.

Two small additions make the practical side easier. If you are heading abroad with your dog, a pet first aid kit in the boot covers the basics until you find a vet, and a tick remover tool is worth packing even for walks at home, because ticks are active here all year and prompt removal is what reduces the risk of disease.

One last habit: if your dog came from abroad or spent time in an endemic country, mention it at the vet visit even if the dog seems perfectly well. A history of travel is the most useful thing a vet can know when deciding whether heartworm testing is worth doing.

Frequently Asked Questions

Final Verdict

Heartworm is a mosquito-borne parasite and nothing else. The worm cannot jump from dog to dog, it cannot survive in soil or water, and the larvae it leaves in an infected animal's blood only become dangerous after a mosquito has carried them for a week or two in warm weather. Understand that cycle and the geography makes sense: it is a disease of warm regions with long mosquito seasons.

For the overwhelming majority of UK owners, the honest answer is that heartworm is not a risk worth medicating against. Your dog's real parasite exposure here is fleas, ticks and lungworm, and that routine is worth keeping tight all year, whether that means a monthly FRONTLINE Spot On, a size-appropriate spot-on for a smaller dog, or an eight-month Seresto collar for owners who would rather not think about it monthly.

If your dog leaves the UK, the calculation changes and it changes quickly. Ask your vet about testing and a licensed preventive before the trip rather than after, and be sceptical of anything sold as an off-the-shelf heartworm treatment, because in the UK that product does not exist. A mosquito bite is the only way in, and a conversation at the vet is the simplest way to keep it out.