What Is Lungworm in Dogs? (Symptoms and How It Spreads)

Lungworm in dogs is an infection caused by parasitic worms that live in the airways, the lung tissue, or the blood vessels running between the heart and the lungs, and dogs catch it by swallowing slugs, snails or their slime. If your dog is coughing, breathing hard, bleeding unusually or suddenly exhausted, book a veterinary appointment today, and treat collapse, laboured breathing or bleeding that will not stop as an emergency that needs immediate care.

The word lungworm is an umbrella term rather than the name of a single parasite. Different species behave differently, cause different signs, and live in different parts of the respiratory system. That is why two dogs with lungworm can look nothing alike, and why this is a diagnosis made with laboratory testing rather than by eye.

The worms behind the name

The species vets most often discuss under this heading are all nematodes, or roundworms, with life cycles that pass through slugs and snails.

Angiostrongylus vasorum, sometimes called French heartworm, is the one that worries vets most. The adult worms live in the pulmonary arteries and the right side of the heart. Their eggs hatch in the lungs, the larvae are coughed up, swallowed and passed out in faeces, which is how the cycle continues. Because this worm sits in the bloodstream, it can interfere with normal blood clotting and cause problems well beyond the chest.

Crenosoma vulpis, the fox lungworm, lives in the airways themselves and tends to produce a persistent cough. Oslerus osleri forms nodules where the windpipe divides into the two main bronchi and is classically seen as a harsh, dry cough in young dogs. Eucoleus aerophilus also inhabits the airways and is frequently picked up incidentally on faecal testing.

Searches for lungworm in cats and dogs often assume one parasite crosses between them. In practice cats have their own species, chiefly Aelurostrongylus abstrusus, and a cat does not catch dog lungworm by living alongside an infected dog. The shared factor is the environment, not the household pet.

Species	Where it lives in the dog	Signs commonly associated with it
Angiostrongylus vasorum	Pulmonary arteries and right side of the heart	Cough, exercise intolerance, breathing difficulty, abnormal bleeding or bruising, collapse
Crenosoma vulpis	Bronchi and smaller airways	Persistent cough, sometimes worse after exercise
Oslerus osleri	Nodules at the base of the windpipe	Dry harsh cough, often in young dogs, noisy breathing
Eucoleus aerophilus	Airways and trachea	Often silent, sometimes mild cough, often found on faecal testing
Aelurostrongylus abstrusus (cats)	Lung tissue	Coughing, wheezing, increased breathing effort

How dogs catch it: slugs, snails and slime trails

This is what causes lungworm in dogs at a practical level. An infected dog or wild canid passes larvae in its faeces. Slugs and snails ingest those larvae while feeding, and inside the mollusc the larvae develop into the stage that can infect a dog. When a dog swallows that slug or snail, the cycle closes. Frogs can also carry infective larvae for Angiostrongylus vasorum and act as a stepping stone.

Owners picture a dog deliberately eating a large garden snail, and some dogs do exactly that. Far more often the exposure is accidental and invisible:

• Tiny slugs hidden in wet grass that a dog mouths while playing
• Chew toys, balls and bones left outside overnight
• Water bowls left in the garden, where molluscs travel across the rim
• Drinking from puddles, bird baths or pots of standing water
• Grazing on grass after rain, when slug activity is highest

Lungworm is not passed directly from dog to dog. A dog cannot catch it by greeting an infected dog at the park. What does matter locally is wildlife: research suggests red foxes act as an important reservoir for Angiostrongylus vasorum, which is one reason urban gardens visited by foxes are part of the conversation.

Can dogs get lungworm at any age or size? Yes. Any breed and any age can be infected. Young dogs and inveterate mouthers simply get more opportunities, and dogs that live with damp, shaded, slug-friendly gardens get more exposure than dogs that do not.

What lungworm looks like in a dog

There is no single picture, and you will never see the parasite itself. Signs fall into three broad groups.

Respiratory signs come first for most owners: a cough that will not settle, coughing after exercise or excitement, reduced stamina, fast or effortful breathing, and in severe cases obvious respiratory distress.

Bleeding and clotting signs are the hallmark of Angiostrongylus vasorum and the reason this parasite is taken so seriously. They include nosebleeds, bruising on the belly or gums, bleeding from the gums, blood in the white of the eye, prolonged bleeding from a small cut or after a routine procedure, and pale gums from blood loss. Any of these in a dog with a cough deserves a same-day veterinary call.

General and neurological signs are easier to miss: lethargy, reluctance to exercise, poor appetite, weight loss, vomiting or diarrhoea, and occasionally wobbliness, seizures or a change in behaviour when bleeding occurs in or around the nervous system.

Some infected dogs show very little for a long stretch, then deteriorate quickly. Others cough for weeks with nothing else wrong. That inconsistency is exactly why owners should not try to rule lungworm in or out at home.

Can you see lungworm in dog poop?

No. Adult lungworms are not shed in faeces, and the larvae that are shed are microscopic. Normal-looking stool tells you nothing either way, and a dog with lungworm usually passes normal-looking stool.

Diagnosis is done in the clinic. A vet may use a Baermann technique on faecal samples collected over several consecutive days, because larval shedding is intermittent and a single sample can easily miss an infection. A blood test that detects Angiostrongylus vasorum antigen is widely used. Depending on the dog, the workup may also include chest imaging, airway sampling, clotting tests and full blood work, which also helps rule out the other causes of a chronic cough.

Lungworm is a diagnosis your veterinarian makes with testing, not one you can confirm or dismiss by looking at your dog or at their faeces.

How long lungworm takes to develop and clear

This is a weeks-long story, not a days-long one. After a dog swallows infective larvae, the worms must migrate and mature before any larvae reappear in the faeces, and that interval spans several weeks. Two consequences follow. First, signs can appear well after the slug was eaten, so there is often no obvious event to point at. Second, faecal testing performed very early can be negative in a dog that is genuinely infected, which is why vets may repeat testing or combine methods.

Treatment is prescription-only and is directed by a veterinarian, usually with antiparasitic medication given over a defined course rather than as a single dose. Dogs with significant lung damage, bleeding or heart strain may also need hospitalisation, oxygen support, clotting support or a transfusion. Recovery time depends on how long the infection has been present and how much damage it has done, and results vary from dog to dog. Rapid die-off of worms can itself provoke a reaction, which is another reason this is never a do-it-yourself situation. Talk to your vet before giving anything, including over-the-counter wormers.

How common it is, and where it turns up

Lungworm is not a rare curiosity, but it is also not evenly distributed, and honest answers about frequency are local answers.

In the UK, Angiostrongylus vasorum was historically associated with pockets of southern England and south Wales and has since been reported across a much wider area, a spread generally linked to fox populations and to mild, damp conditions that suit slugs and snails. Veterinary awareness campaigns there mean most practices test for it readily.

Interest in lungworm in dogs in South Africa reflects the same underlying biology. Lungworm species are recognised in dogs there, and the risk factors do not change with the map: damp microclimates, abundant molluscs, wildlife reservoirs and dogs that forage outdoors. Because distribution shifts over time and differs sharply between neighbourhoods, the only reliable read on your dog's risk comes from a vet practising in your area, who knows what they are actually diagnosing.

Seasonality matters too. Slug and snail activity climbs in wet, mild weather, so exposure risk tends to rise in damp spells rather than staying flat year round.

Lowering the risk around your home and garden

Prevention is partly environmental and partly medical.

Bring toys, chews and water bowls indoors overnight rather than leaving them on the lawn. Refresh outdoor water rather than topping it up. Pick up faeces promptly, which breaks the cycle for every dog using that space. Discourage puddle drinking and slug hunting, and supervise dogs that mouth everything, especially after rain. Avoid slug pellets, which carry their own poisoning risk.

On the medical side, it is worth knowing that not every routine wormer covers lungworm. Preventive products that do are prescription items, and the right choice depends on your dog's local risk, age, weight and health history. That is a conversation to have at your next appointment rather than a decision to make from a shelf.

Key Takeaways

• Lungworm in dogs is infection with parasitic nematodes that live in the airways, lung tissue or pulmonary blood vessels, with Angiostrongylus vasorum the species of greatest concern.
• Dogs get it by swallowing slugs, snails, frogs or slime, often accidentally from grass, toys, puddles or outdoor water bowls, not from other dogs.
• Signs range from a stubborn cough and poor stamina to unusual bleeding, bruising, pale gums, collapse or neurological changes.
• You cannot see lungworm in dog poop; larvae are microscopic and shedding is intermittent, so diagnosis relies on faecal and blood testing done by a vet.
• Weeks usually pass between exposure and detectable infection, so early testing can be falsely reassuring.
• Distribution varies by region and season, including within the UK and South Africa, so local veterinary advice beats general prevalence figures.
• Treatment and prevention are prescription-led, and over-the-counter wormers do not all cover lungworm.

Your veterinarian owns the diagnosis and the treatment plan here. They decide which tests to run, which medication is appropriate, how closely your dog needs monitoring and what prevention makes sense where you live. Everything an owner does at home, tidying the garden, managing water bowls, picking up faeces, watching for early signs, works best as support for that plan rather than as a substitute for it. If anything about your dog's breathing, energy or bleeding changes, get them seen rather than waiting to see what happens.

For a plain-language guide to the signs that mean a dog needs veterinary care immediately, read Dog Emergency Signs.