CONDITION
Angiostrongylus (Lungworm)
Angiostrongylus vasorum is a parasitic worm that lives in the blood vessels of the heart and lungs. Dogs become infected mainly by swallowing slugs or snails that carry the larval stage, and these can be very small and eaten by accident. Frogs can also carry the larvae. Foxes are also infected and act as a wildlife reservoir. Once inside the body, the larvae migrate to the pulmonary arteries, where they mature. Adult females lay eggs that lodge in the small blood vessels of the lungs and hatch into larvae, which break into the airways and are coughed up, swallowed and passed in faeces. Many dogs show no outward signs for weeks or months. When signs do appear, they can vary widely: some dogs develop a persistent cough or become tired more easily, others may bleed unusually after minor injuries or show unexplained bruising, and a smaller number experience more severe breathing difficulty or neurological changes such as seizures or loss of balance. Bleeding or neurological signs can appear suddenly, sometimes in a dog with no cough at all, and the infection can be fatal. The variety of presentations can make the picture unclear at first. This page explores what lungworm infection can look like in practice, how the parasite affects different body systems, the tests used to detect it, and the treatment approaches available. It also considers prevention strategies and what tends to happen after treatment begins.
Why this matters now
Angiostrongylus vasorum, commonly called French heartworm or lungworm, has become increasingly prevalent across the UK over the past two decades. Dogs acquire infection by ingesting slugs or snails, including very small ones swallowed by accident whilst eating grass, drinking from puddles or outdoor water bowls, or playing with toys left outdoors. Whether contact with slime alone leads to infection in everyday settings has not been established. Younger dogs appear more commonly affected, possibly due to exploratory behaviour. The infection is seen particularly in southern England and Wales and in areas with established fox populations. Transmission can be higher in warm, wet conditions when slugs and snails are more active, but infection can occur year-round. Angiostrongylus vasorum is not known to infect people, and dogs and foxes are its usual hosts. Domestic cats are at low risk, with only isolated infections reported, and the lungworm that commonly affects cats is a different parasite, Aelurostrongylus abstrusus.
After ingestion, larvae migrate through the body, eventually reaching the heart and pulmonary arteries where adult worms develop. Eggs laid by adult females lodge in the small blood vessels of the lungs and hatch into larvae that break into the airways, causing inflammation. Adult worms typically take several weeks after infection to begin producing larvae, and some dogs remain free of obvious signs for months. Clinical signs can range from mild coughing to life-threatening bleeding disorders, depending on worm burden and individual response. The course can be gradual, or it can appear suddenly with marked breathing difficulty, a bleeding episode or neurological signs, sometimes in a dog that has shown no cough at all. Bleeding and neurological signs can worsen over hours, and a dog can have markedly abnormal clotting while still appearing bright. Angiostrongylosis can be fatal, through bleeding into the lungs or brain, severe breathing compromise, or pulmonary hypertension and right-sided heart failure, and sudden death has been reported.
Signals & patterns
Early signals
Coughing
A persistent cough develops as larvae migrate through lung tissue and cause inflammation.
Reduced exercise tolerance
Dogs tire more easily during walks or play.
Weight loss
Gradual weight loss may occur despite maintained appetite.
Breathing changes
Faster or more laboured breathing, particularly after exertion. Breathing that is fast or effortful at rest can reflect bleeding into the lungs or more extensive inflammation, and bleeding into the lungs can make breathing worse within hours.
Nosebleeds or unexplained bruising
Small amounts of blood from one or both nostrils, or bruising without an obvious knock, can occur when the parasite affects blood clotting, sometimes before any cough has been noticed. A nosebleed that stops by itself does not mean clotting has returned to normal. It can be the first visible sign of a clotting disorder that may go on to cause bleeding into the lungs, chest, brain or spinal cord, and the degree of clotting disruption cannot be judged from the outside.
Later signals
Unexplained bleeding
Bruising, prolonged bleeding from minor wounds, bleeding from the gums, or blood in the stool, vomit or urine without obvious cause. Bleeding into the chest, abdomen or lungs may not be visible from the outside, and can show instead as weakness, pale gums, breathing difficulty or collapse. Survival is lower in dogs that develop bleeding problems than in those that do not.
Pale gums
Anaemia from chronic infection or blood loss causes visible pallor.
Collapse or weakness
Collapse, fainting or profound weakness can reflect significant internal bleeding, or pulmonary hypertension straining the right side of the heart. Sudden death has been reported.
Neurological signs
Bleeding into the brain or spinal cord can cause seizures, blindness, incoordination, a head tilt, paralysis or a change in behaviour. These signs can appear suddenly, and bleeding within the brain can be fatal.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis involves specific tests to detect the parasite, along with assessment of any complications. Early detection before severe signs develop improves outcomes.
Faecal Baermann test
Antigen blood test
Clotting tests
Chest radiographs
Complete blood count
Options & trade-offs
Treatment involves antiparasitic medication to kill the worms. Supportive care addresses complications. Prevention through regular prophylaxis is highly effective.
Antiparasitic treatment
Specific products that kill Angiostrongylus worms. In the UK, licensed options include a moxidectin-containing spot-on applied as a single dose, with a check around 30 days later because some dogs need a second application, and milbemycin oxime tablets given once a week for four weeks. Fenbendazole given daily over a longer course is also used, although it is not licensed in the UK for Angiostrongylus and would be prescribed under the veterinary cascade.
Trade-offs: Several licensed products are effective. Worm death can temporarily worsen inflammation before improvement. Multiple treatments may be needed.
Treatment of coagulopathy
Fresh frozen plasma or blood transfusion for dogs with bleeding disorders, together with rest and restricted activity to reduce the risk of trauma. Vitamin K may be given while anticoagulant rodenticide poisoning has not been excluded, but the clotting disorder caused by lungworm is not due to vitamin K deficiency.
Trade-offs: May be lifesaving in severe cases. Addresses the dangerous complication whilst anthelmintics work. Transfusion carries risks, including reactions, and clotting often improves once the infection is treated, although the time course varies and some dogs need repeated clotting checks.
Anti-inflammatory medication
Corticosteroids to reduce lung inflammation caused by dying parasites.
Trade-offs: May be used alongside antiparasitic treatment in some cases. Reduces inflammatory response that contributes to signs.
Supportive care
Oxygen therapy, rest, and monitoring for dogs with respiratory compromise.
Trade-offs: Severity determines level of support required. Many dogs recover with treatment, but survival is lower in dogs with bleeding problems or severe breathing compromise, and deaths can occur despite treatment.
Monthly prevention
Regular prophylactic treatment to prevent establishment of infection.
Trade-offs: Highly effective at preventing disease. Various products are available. Considered particularly in areas where the parasite is common. Not all wormers are active against Angiostrongylus, and missed or delayed doses can allow infection to establish.
Common misconceptions
"My dog does not eat slugs, so cannot get lungworm"
Dogs often swallow slugs or snails accidentally, including very small ones, through grass eating, drinking from outdoor water bowls, or playing with garden toys. Infected molluscs also release some larvae into their slime, although how often this leads to infection in everyday settings has not been established. Intentional consumption is not required.
"Lungworm is rare in the UK"
Angiostrongylus is now endemic in many areas of the UK, mainly southern England and Wales, with cases also recorded elsewhere, including parts of Scotland. Risk varies by region.
"Standard wormers prevent lungworm"
Not all wormers are effective against Angiostrongylus. Specific products licensed for lungworm prevention are required. Regular broad-spectrum wormers may not provide protection.
"A negative test means my dog definitely does not have lungworm"
Larvae are shed intermittently, so a negative faecal test does not rule out infection, and antigen blood tests can be negative early in infection or when few worms are present. A dog can also have a serious clotting disorder while still appearing well. Results are interpreted alongside the dog's signs and other findings.
Related conditions
Heartworm Disease in Dogs
Heartworm (Dirofilaria immitis) also lives in the pulmonary arteries and heart and can cause cough and exercise intolerance, which is why Angiostrongylus vasorum is sometimes called French heartworm. Heartworm is spread by mosquitoes and is not endemic in Great Britain, so it is relevant mainly to dogs that have travelled from or been imported from endemic regions, whereas lungworm is acquired in the UK from slugs and snails. The two are separated with specific blood tests.
Pulmonary Thromboembolism
Angiostrongylus infection damages the walls of the pulmonary arteries, and worms, including dying worms, together with clots that form around them can block branches of these arteries, a form of pulmonary thromboembolism. The overlapping respiratory signs and the shared mechanism of vascular injury mean the two conditions can sometimes occur together or be considered during investigation of unexplained breathing difficulty.
Pulmonary Hypertension
Chronic inflammation and obstruction of the pulmonary arteries caused by adult lungworms can lead to sustained elevation of blood pressure in the lung circulation. This relationship means that dogs with longstanding or severe Angiostrongylus infection may develop pulmonary hypertension as a consequence.
Chronic Bronchitis in Dogs
Both conditions can present with persistent cough and exercise intolerance, and both involve inflammation of the airways or lung tissue. In practice, chronic bronchitis may be considered as part of the differential picture when a dog presents with a cough of unclear origin, particularly before parasitic testing has been completed.
Intestinal Parasites
Angiostrongylus is not an intestinal worm, but its larvae pass out in faeces, so faecal testing overlaps with screening for intestinal parasites. Routine faecal flotation used to find the eggs of intestinal worms does not reliably detect lungworm larvae, which need the Baermann method, and many wormers aimed at intestinal worms do not protect against Angiostrongylus.
Anticoagulant Rodenticide Toxicity
Lungworm can cause a bleeding disorder that closely resembles anticoagulant rat poisoning, with nosebleeds, bruising, blood in the stool or urine, breathing difficulty from bleeding into the chest or lungs, or collapse. The two are separated with history, clotting tests and parasite testing rather than by appearance, and both can progress to fatal bleeding.
Eosinophilic Bronchopneumopathy
Eosinophilic bronchopneumopathy can cause a persistent cough, raised eosinophils and patchy changes on chest radiographs, a picture that overlaps with angiostrongylosis. Parasite testing is typically part of investigating this pattern, because the two are managed very differently.
Several of the most serious features of lungworm infection cannot be assessed at home. A dog's ability to clot can be markedly reduced while the dog still seems well, and bleeding into the lungs, chest, abdomen, brain or spinal cord may show only as weakness, pale gums, breathing difficulty, collapse, or a sudden change in behaviour or balance. In a dog with a cough, or a dog with access to slugs and snails, nosebleeds, unexplained bruising, blood in the urine or stool, prolonged bleeding from a small wound, or any neurological change point towards a clotting disorder that can progress to fatal bleeding, sometimes within hours, and whose severity can only be measured with blood clotting tests. The same bleeding pattern can follow ingestion of anticoagulant rat poison, and the two are separated with history and blood tests rather than by appearance. For prevention, not all wormers are active against Angiostrongylus, and reducing access to slugs and snails, including bringing outdoor water bowls and toys inside, may lower exposure.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS