Lungworm in Dogs and Cats: Signs, Risks, Prevention

Most owners have a mental list of the worms their pet needs protecting from, and it usually stops at the intestines. Roundworms, tapeworms, hookworms — these live in the gut, they are covered by routine worming, and the whole subject feels reasonably well handled.

Lungworms break that pattern in almost every respect. They live in the lungs and, in some species, in the blood vessels around the heart. They are not passed on by ordinary contact with another pet or by contaminated ground in the usual way; they are picked up by eating slugs, snails, or infected prey — with one exception, since kittens can be infected by their mother. And critically, many routine worming products do not cover them, which means an owner can be entirely diligent about worming and still have an unprotected pet.

This guide explains what lungworms are, how dogs and cats actually pick them up, the signs that are worth taking seriously — including some that have nothing to do with breathing at all — how diagnosis works, and what genuinely effective prevention looks like.

Important: This article shares general educational information for pet owners and is not a substitute for an in-person veterinary examination, diagnosis, or treatment. For any medical concern or emergency, contact your veterinarian promptly.

What Lungworms Are

Lungworm is a general term covering several different parasites that affect the respiratory system. They are not one organism, and the species that infect dogs are different from those that infect cats.

In dogs, the parasite of greatest concern in many regions is a worm whose adults live in the blood vessels between the heart and the lungs. Its larvae migrate through lung tissue, and beyond causing respiratory signs it can interfere with normal blood clotting — which produces some of the most alarming and least expected presentations of the disease.

In cats, the relevant parasite typically lives within the lung tissue itself, causing inflammation and airway signs that can look very much like feline asthma. It is more commonly a problem for cats who hunt, and many infected cats show mild or no signs at all.

Distribution varies enormously by region, and lungworm is much more of a concern in some areas than others. Your veterinarian is the right source of information about local prevalence, because a parasite that is a genuine worry in one country may be almost unheard of in another.

• Not spread by ordinary contact: infection is picked up by eating an intermediate or prey host rather than directly from another pet, with the exception of transmission from queen to kitten described in cats.
• Not covered by every wormer: many routine products treat only intestinal worms.
• Species-specific parasites: dogs and cats are affected by different lungworms.
• Regionally variable: prevalence differs sharply between areas.
• Treatable: prescribed treatment is effective when the diagnosis is made in time.

How Pets Become Infected

The life cycle explains everything about prevention, and it is worth understanding properly.

Larvae passed in the faeces of an infected animal are taken up by slugs and snails, which act as intermediate hosts. Inside them the larvae develop to a stage capable of infecting a dog or cat. When a pet eats an infected slug or snail — deliberately or accidentally — the larvae are released, migrate through the body, and mature in the lungs or associated blood vessels. The cycle then repeats through the faeces.

• Eating slugs or snails: the classic route, and puppies who investigate everything are particularly at risk.
• Accidental ingestion: slug slime trails on grass, toys left outside overnight, or water bowls in the garden.
• Eating grass: picking up tiny slugs or snails hidden in it.
• Hunting: in cats, eating infected prey such as rodents or birds carrying larvae.
• Drinking from outdoor sources: puddles or containers where slugs have been.
• Queen to kitten: in cats, transmission through the mother's milk has been described.

Two practical points fall out of this. Dogs do not need to be seen eating a slug to become infected — accidental ingestion of very small slugs, or contact with slime on a toy, is enough. And bringing toys and water bowls indoors overnight is a genuinely useful precaution rather than an over-cautious one.

Wet weather increases slug and snail activity, so risk periods tend to follow rainfall rather than being spread evenly through the year.

Signs in Dogs

The presentation in dogs is unusually varied, and the non-respiratory signs are the ones most likely to be missed.

• Coughing: often persistent, sometimes worse after exercise.
• Exercise intolerance: tiring quickly, or reluctance to complete a normal walk.
• Laboured or rapid breathing: increased effort at rest.
• Unexplained bruising: patches on the skin or gums with no known injury.
• Prolonged bleeding: from minor cuts, after nail trimming, or after routine procedures.
• Nosebleeds: particularly recurrent ones.
• Blood in urine or faeces: or pale gums suggesting blood loss.
• Vomiting or diarrhoea: non-specific but common.
• Weight loss and poor condition: in longer-standing cases.
• Neurological signs: including seizures or sudden weakness, if bleeding occurs in the brain or spinal cord.
• Collapse: an emergency presentation.

The bleeding tendency is the feature that catches owners and sometimes clinicians by surprise. A young dog who bruises easily, bleeds excessively from a small wound, or has a nosebleed for no apparent reason is a dog in whom lungworm belongs on the list of possibilities, particularly in an area where the parasite occurs.

Young dogs and puppies are over-represented, likely because they are the ones most inclined to pick things up and eat them. But dogs of any age can be affected, and outdoor lifestyle matters more than age alone.

Signs in Cats

Feline lungworm tends to be a quieter disease, and many affected cats never show anything obvious. When signs do occur, they mimic other respiratory conditions closely.

• Coughing: sometimes mistaken for hairball retching, which looks similar.
• Wheezing: an audible sound on breathing out.
• Increased respiratory rate: faster breathing at rest.
• Increased effort: visible abdominal movement with each breath.
• Sneezing or nasal discharge: in some cats.
• Reduced activity: less play, less jumping, more sleeping.
• Weight loss: or failure to thrive in younger cats.
• Open-mouth breathing: an emergency sign in any cat.

The overlap with feline asthma is substantial, and the distinction matters because the treatments are entirely different. A cat with suspected asthma should reasonably be tested for parasites as part of the workup where lungworm occurs locally — most obviously one who hunts or has outdoor access, but an indoor-only cat should not be ruled out on lifestyle alone.

Owners frequently misinterpret feline coughing. A cat who crouches, extends the neck, and makes a repetitive retching movement without producing a hairball is usually coughing rather than vomiting, and a video of the episode is one of the most useful things you can bring to an appointment.

How Veterinarians Diagnose It

Diagnosis can be genuinely challenging, because larvae are shed intermittently and a single negative test does not exclude infection.

Faecal testing using a specialised technique designed to recover larvae is a mainstay. Because shedding is inconsistent, samples are usually collected over three consecutive days and submitted together, which substantially improves the chance of detection. Standard faecal flotation used for intestinal worms is not reliable for lungworm.

Blood tests that detect specific markers of infection are available for the canine parasite in some regions and can give a rapid answer, sometimes in the consulting room. Their availability varies, and your veterinarian will advise on what applies locally.

Chest radiographs show changes in lung pattern that support the diagnosis, though they are not specific to lungworm alone. Where airway disease is prominent, sampling fluid from the airways under anaesthesia can recover larvae directly and simultaneously assess the type of inflammation present — this is particularly valuable in cats where asthma is the alternative diagnosis.

In dogs with bleeding signs, clotting tests are important both to confirm the abnormality and to guide safe management, since a dog with impaired clotting needs careful handling during any procedure.

Treatment and Recovery

Treatment is prescribed by your veterinarian and involves specific antiparasitic medication effective against lungworm. Many dogs and cats respond well, and the prognosis for uncomplicated cases treated promptly is generally good.

Supportive care may be needed alongside, particularly for animals with significant respiratory compromise or, in dogs, with bleeding problems. Severely affected patients may require hospitalisation, oxygen, and careful monitoring.

One point deserves emphasis: killing large numbers of parasites at once can itself provoke a strong inflammatory reaction in the lungs. This is why treatment should always be veterinary-directed and monitored rather than attempted with an over-the-counter product, and why your veterinarian may recommend a particular protocol or a period of observation.

Follow-up testing after treatment confirms that the infection has cleared. Rest during recovery is usually advised, and exercise is reintroduced gradually according to your veterinarian's guidance. In dogs where bleeding has occurred, clotting is rechecked before any elective procedure.

Reinfection is entirely possible after successful treatment, because immunity does not develop reliably. Prevention therefore matters just as much after an episode as before one.

Prevention That Actually Works

This is the section that changes outcomes, and it rests on one question most owners have never asked.

Ask your veterinarian directly whether your pet's current parasite prevention covers lungworm. Many popular worming products do not, because they are formulated for intestinal parasites. An owner can worm scrupulously every three months and have zero lungworm protection, and this is one of the most common gaps in otherwise excellent preventive care.

Where lungworm occurs in your region, prevention typically means a prescribed product with the relevant coverage, given at the interval your veterinarian specifies — often monthly, because the treatment interval needs to match the parasite's development timeline rather than a general worming schedule.

1. Ask specifically whether your product covers lungworm, and if not, discuss one that does.
2. Give preventive treatment at the exact interval prescribed; gaps matter more here than with intestinal worms.
3. Bring toys, chews, and water bowls indoors overnight rather than leaving them on wet grass.
4. Pick up faeces promptly from your garden and from public areas to interrupt the life cycle.
5. Discourage eating grass, and supervise puppies who investigate everything with their mouths.
6. Provide fresh indoor water rather than letting your dog drink from puddles or outdoor containers.
7. Consider keeping cats in overnight, which reduces hunting and several other risks at once.
8. Be more vigilant after wet weather, when slug and snail activity increases.

It is worth being clear about what does not prevent lungworm. No food, supplement, wellness drop, or grooming product prevents or treats parasitic infection. General health supplements may have a place in a normal routine, but they play no part in parasite prevention, and relying on them instead of a prescribed preventive leaves a pet unprotected.

Myths and Facts

• Myth: routine worming covers everything. Many common wormers treat intestinal worms only and provide no lungworm cover.
• Myth: your dog has to eat a whole slug. Accidental ingestion of tiny slugs or contact with slime is enough.
• Myth: one negative faecal test rules it out. Larvae are shed intermittently; repeated sampling is often needed.
• Myth: it spreads from dog to dog. Dogs become infected by eating a slug or snail, not from contact with another dog; in cats there is also a prey route and a described queen-to-kitten route.
• Myth: an indoor cat cannot possibly have it. Risk is far lower without outdoor hunting, but it is not zero — an indoor cat may catch a slug, snail, or mouse that gets inside, and kittens can be infected by their mother.
• Fact: bleeding can be the first sign in dogs. Unexplained bruising or prolonged bleeding warrants investigation.
• Fact: prevention is straightforward once the right product is used. The main obstacle is not knowing that coverage differs.

When to Contact Your Veterinarian

Respiratory and bleeding signs both belong in the category where waiting rarely helps.

• Book an appointment for: a persistent cough, reduced exercise tolerance, wheezing, weight loss, or a cat with repeated retching episodes.
• Call the same day for: unexplained bruising, prolonged bleeding from a small wound, a nosebleed, blood in urine or faeces, or pale gums.
• Seek emergency care for: laboured breathing, open-mouth breathing in a cat, collapse, seizures, or severe uncontrolled bleeding.

It is also worth a non-urgent call simply to ask whether your pet's current parasite prevention covers lungworm. It is a two-minute conversation that closes one of the more common gaps in preventive care.

For any medical emergency, contact your veterinarian or your nearest emergency veterinary hospital immediately.

The Practical Takeaway

Lungworm is the parasite that slips through the gaps in otherwise careful preventive care, and it does so for a completely avoidable reason: owners assume that worming means worming, and products differ. The single most valuable action available here is asking one question at your next appointment.

Beyond that, the practical measures are small and cheap. Toys and bowls indoors overnight. Faeces picked up promptly. Fresh water rather than puddles. Puppies supervised in the garden. Cats in overnight where that is workable. None of it is demanding, and together it meaningfully reduces exposure.

And know the signs that do not look like a lung problem. A dog who bruises easily or bleeds too long from a trivial cut, in a region where this parasite occurs, deserves to have lungworm considered — because that is the presentation most likely to be attributed to something else until it becomes serious.