Lungworm in Dogs: A Growing Threat from Slugs and Snails

Lungworm — specifically Angiostrongylus vasorum, also known as French heartworm — is a parasitic worm that lives in the heart and pulmonary arteries of infected dogs. Unlike the intestinal worms most owners are familiar with, lungworm resides in the cardiovascular system, making it a uniquely dangerous parasite with potentially fatal consequences. Over the past two decades, A. vasorum has spread dramatically across the UK, Ireland, and much of Europe, transforming from a relatively obscure regional problem into one of the most significant parasitic threats facing dogs today.

"Lungworm is the parasite I worry about most as a vet, because it breaks all the rules owners have come to rely on. It won't show up on a standard faecal float, it isn't killed by most over-the-counter wormers, and by the time obvious symptoms appear, the infection may already be causing serious damage to the heart and lungs. The single most important thing dog owners can learn is that not all worming products are equal — and lungworm protection must be specifically chosen." — Dr. Sarah Chen, DVM

The life cycle of Angiostrongylus vasorum is fascinatingly complex. Adult worms live in the right side of the heart and the pulmonary arteries, where they produce eggs. These eggs are carried to the lungs, where they hatch into first-stage larvae (L1). The larvae are coughed up, swallowed, and passed out in the dog's faeces. In the environment, slugs and snails serve as intermediate hosts — they ingest the larvae, which develop into the infective third-stage larvae (L3) within the mollusc. When a dog eats an infected slug or snail, the cycle begins again as the L3 larvae migrate through the body to the heart and pulmonary arteries, maturing into adults over approximately six to eight weeks.

The increasing prevalence of lungworm is believed to be driven by several factors, including climate change creating milder, wetter conditions that favour slug and snail populations, the spread of foxes (a major wildlife reservoir) into urban areas, and increased movement of dogs between regions. In the UK, what was once primarily a problem in southern England and South Wales has now been reported across the entire country, including Scotland and Northern Ireland. For any dog owner, understanding lungworm is no longer optional — it is essential to keeping your pet safe, and it starts with knowing how dogs become infected. Learn more about common parasites in our guide to parasites in dogs and cats.

The most obvious route of infection is a dog deliberately eating a slug or snail — and some dogs do exactly that, particularly curious puppies and young dogs who investigate the world with their mouths. But deliberate consumption is far from the only way dogs become infected, and this is where many owners are caught off guard. Accidental ingestion is actually the more common route, and it can happen in ways that are nearly impossible to prevent without medication.

Tiny slugs and snails — some barely a few millimetres long — can be present on grass blades that dogs eat, in water bowls left outside overnight, on toys left in the garden, and in puddles or outdoor water sources that dogs drink from. Slug and snail slime trails alone may contain lungworm larvae, meaning a dog doesn't even need to eat the mollusc itself — simply licking a surface where a slug has crawled could potentially result in exposure. Dogs that eat grass regularly, which is a common and usually harmless behaviour, are at particular risk because they may inadvertently consume tiny slugs hidden among the blades.

Frogs can also serve as paratenic (transport) hosts for Angiostrongylus vasorum larvae, meaning dogs that catch and mouth or eat frogs face an additional risk of infection. This is particularly relevant for breeds with high prey drive or dogs in rural areas with abundant frog populations. Some dogs are habitual 'frog catchers' and represent a higher-risk group that should always be on a lungworm preventative.

Seasonal patterns play a role too. Slugs and snails are most active in damp, mild conditions — spring and autumn are peak seasons in most regions, but in areas with wet summers, the risk persists almost year-round. Evening and overnight hours are when molluscs are most active, meaning water bowls left out, toys not brought in, and late-night garden visits all carry elevated risk. While these precautions can help reduce exposure — bringing in water bowls, removing toys, avoiding walks in long wet grass at dawn and dusk — they cannot eliminate the risk entirely, which is why preventative medication is considered essential by veterinary parasitologists.

Lungworm symptoms can be subtle in the early stages and are frequently mistaken for other conditions, which is part of what makes this parasite so dangerous. The severity of symptoms depends on the worm burden — the number of adult worms in the heart and pulmonary arteries — and how long the infection has been established. Because the worms take six to eight weeks to mature after ingestion, there is a significant silent period during which the dog shows no signs at all.

The most common early symptom is a persistent cough, which occurs as larvae migrate through the lung tissue and adult worms cause inflammation in the pulmonary arteries. The cough may be dry or productive and is often worse after exercise or excitement. Owners frequently describe it as a soft, intermittent cough that comes and goes over weeks — quite different from the hacking kennel cough or the gagging associated with a collapsed trachea. As the disease progresses, dogs develop exercise intolerance — they tire more quickly on walks, may pant excessively, and can show visible breathing difficulty during activities that previously posed no problem.

Perhaps the most distinctive and alarming feature of lungworm infection is its effect on blood clotting. Angiostrongylus vasorum causes a condition called coagulopathy — a disruption of the normal clotting process — that can manifest as unexplained bruising on the skin, gums, or inner ears; prolonged or excessive bleeding from minor wounds or after surgery; nosebleeds; blood in the urine or faeces; or bleeding into the eyes causing visible redness. This bleeding tendency is one of the hallmarks that should prompt immediate suspicion of lungworm, especially in young dogs in endemic areas.

In severe or advanced cases, lungworm can cause seizures and other neurological signs due to aberrant larval migration to the brain or bleeding within the central nervous system. Collapse, severe respiratory distress, and sudden death can occur, particularly in dogs with heavy worm burdens that have gone undetected for months. Weight loss, poor coat condition, and general malaise are also common in chronic infections. If your dog shows any combination of coughing, exercise intolerance, and unexplained bleeding — especially if you live in an area where lungworm is known to be present — seek veterinary attention urgently. This is not a condition that can wait for a routine appointment.

This is the single most important fact about lungworm that every dog owner needs to understand: most standard worming products do not kill Angiostrongylus vasorum. The majority of over-the-counter dewormers available in pet shops and supermarkets contain active ingredients such as fenbendazole, pyrantel, or praziquantel, which are effective against common intestinal worms — roundworms, hookworms, whipworms, and tapeworms — but have no activity against lungworm when used at standard dosing intervals. This means that a dog on a seemingly 'complete' worming programme may have absolutely no protection against this potentially fatal parasite.

The reason is biological: Angiostrongylus vasorum is not an intestinal worm. It resides in the cardiovascular system, and its larvae circulate through the blood and lungs. The drugs that reach therapeutic concentrations in the gut to kill intestinal worms do not necessarily reach the pulmonary arteries and heart at levels sufficient to kill lungworm. Only specific active ingredients administered at appropriate doses and frequencies have been proven effective against A. vasorum.

Similarly, lungworm is not detected by the standard faecal flotation test that most veterinary practices use to screen for intestinal parasites. Standard faecal floats are designed to detect the eggs of roundworms, hookworms, whipworms, and tapeworms — and Angiostrongylus does not produce eggs in the faeces. Instead, first-stage larvae (L1) are passed in the stool, and these require a different diagnostic technique entirely. This means a dog can have a 'clean' faecal test result and still be heavily infected with lungworm. Understanding this distinction is critical — a negative worm test does not rule out lungworm.

This double gap — in both prevention and detection — is what makes lungworm such a hidden threat. Many well-meaning owners believe their dog is fully protected because they worm regularly and get annual faecal tests, when in reality their pet has no lungworm coverage at all. The same is true of heartworm prevention — while heartworm products containing milbemycin or moxidectin do cover lungworm, not all heartworm preventatives include these specific ingredients. Always check with your vet to confirm your dog's parasite prevention programme includes lungworm.

Because standard faecal floats miss lungworm, specific diagnostic tests are required when infection is suspected. The most widely used screening tool is the Angio Detect SNAP test — a rapid, in-clinic blood test that detects antigens (proteins) produced by adult Angiostrongylus vasorum worms circulating in the dog's blood. The test provides results within approximately 15 minutes and has good sensitivity for detecting moderate to heavy infections. Many veterinary practices in endemic areas now include this as part of routine wellness screening, particularly for dogs with any respiratory or bleeding symptoms.

The Baermann faecal test is the specific faecal technique designed to detect lungworm larvae. Unlike a standard faecal float, which uses solutions to make eggs rise to the surface, the Baermann method exploits the fact that living L1 larvae actively migrate out of the faecal sample into warm water, where they can then be collected and identified under the microscope. This test is more labour-intensive than a standard float and is not available at all practices, but it is highly specific — if larvae are found, infection is confirmed. However, larvae are shed intermittently, so a single negative Baermann test does not definitively rule out infection.

Chest X-rays (radiographs) are valuable both for diagnosis and for assessing the severity of established infections. Lungworm causes characteristic changes visible on radiographs, including a diffuse interstitial or alveolar pattern in the lungs (often described as a hazy or 'ground glass' appearance), enlarged pulmonary arteries, and in advanced cases, signs of right-sided heart enlargement. These changes are not unique to lungworm — they can be seen with other respiratory conditions — but in a young dog with coughing and bleeding tendencies, radiographic findings can strongly support the diagnosis.

Complete blood work, including a clotting profile, is also important. Dogs with lungworm often show prolonged clotting times, low platelet counts, and sometimes elevated liver enzymes. Eosinophilia — an increase in a specific type of white blood cell associated with parasitic infections — may be present but is not always reliable. In some cases, bronchoalveolar lavage (collecting fluid from the airways under anaesthesia) may be performed to look for larvae directly in the lungs. Your vet will determine the most appropriate diagnostic approach based on your dog's symptoms and risk factors. For a comprehensive overview of common parasitic threats, see our guide on preventative care schedules for dogs.

If your dog is diagnosed with lungworm, treatment typically involves a course of fenbendazole (Panacur) administered daily for a minimum of seven to twenty-one days, depending on the severity of infection. Fenbendazole at the higher treatment doses (as opposed to the lower standard worming doses) is effective at killing both adult worms and larvae. During treatment, your vet will monitor your dog closely, because as the worms die, they can cause an inflammatory reaction in the pulmonary arteries and lungs — paradoxically, some dogs may temporarily worsen before they improve. Anti-inflammatory medication and supportive care, including oxygen therapy in severe cases, may be necessary during the treatment phase.

For dogs with significant bleeding disorders caused by lungworm, emergency treatment may include blood transfusions, plasma transfusions to replace clotting factors, and vitamin K supplementation, alongside the antiparasitic medication. Dogs with seizures require anti-convulsant therapy and intensive monitoring. The prognosis for lungworm depends heavily on how early the infection is caught — dogs treated in the early stages typically make a full recovery, while those with advanced cardiac or neurological involvement may have lasting damage or a guarded prognosis.

Prevention is far better than cure, and fortunately, effective preventative products exist. The key active ingredients proven to prevent lungworm are milbemycin oxime (found in Milbemax, NexGard Spectra) and moxidectin (found in Advocate/Advantage Multi). These must be administered monthly to maintain protection — unlike some intestinal wormers that are given every three months, lungworm prevention requires strict monthly dosing because of the worm's life cycle timing. Missing even a single monthly dose can leave your dog unprotected long enough for ingested larvae to mature beyond the point where preventative doses can kill them.

It is absolutely essential to understand that not all worming products cover lungworm. Products containing only praziquantel, pyrantel, or febantel at standard doses will not prevent lungworm. Even some popular combination products lack lungworm coverage. Before purchasing any worming product, check the label specifically for claims against Angiostrongylus vasorum, or better yet, discuss your dog's complete parasite prevention plan with your vet to ensure every threat is covered. If you live in a lungworm-endemic area or your dog has any exposure to slugs, snails, or foxes, monthly lungworm prevention is not optional — it is a necessary part of responsible pet ownership that could save your dog's life.