CONDITION
Angiostrongylosis (Lungworm)
Angiostrongylosis is an infection caused by a parasitic roundworm, Angiostrongylus vasorum, which lives in the pulmonary arteries and right side of the heart in dogs. The parasite is spread when a dog swallows infected slugs or snails, or comes into contact with their slime trails. Once established, the worms can affect the heart, lungs, and blood clotting system in a range of ways. Owners often arrive on this page because their dog has developed a persistent cough, breathing difficulty, or unusual tiredness on exercise. Some dogs show bleeding problems—nosebleeds, bruising, blood in the stool—or, less commonly, neurological signs such as weakness or seizures. The pattern can be subtle at first, or it can be sudden and marked. Angiostrongylosis is now seen regularly across many parts of the UK, particularly in areas where foxes (another host) are present. This page explores what owners may observe at home, what is happening in the body, how the infection is investigated, and what approaches exist to treat it. The goal is to help you understand the shape this condition can take, without advising on what to do in your specific situation.
Why this matters now
Angiostrongylosis can affect dogs at any age, though younger dogs and those with outdoor access to areas frequented by slugs and snails tend to be at higher risk. The infection has become more widely distributed across the UK over the past two decades, particularly in southern England, Wales, and parts of Scotland, often in areas where fox populations are established. Seasonality plays some role—transmission can be higher in warm, wet conditions when molluscs are more active—but infection can occur year-round. Dogs that eat slugs or snails, drink from water bowls left outside, or mouth grass and toys contaminated with slime trails may be exposed.
The time between exposure and the appearance of signs is typically several weeks, as the larvae migrate through the body and mature into adult worms in the pulmonary arteries. Some dogs remain free of obvious signs for months, while others develop respiratory or bleeding problems relatively quickly. The course can be gradual—a mild cough that slowly worsens, subtle exercise intolerance—or it can appear more suddenly, with marked breathing difficulty or a bleeding episode. The pattern in an individual dog often depends on the worm burden, the strength of the immune response, and which systems are most affected.
Signals & patterns
Early signals
Persistent cough
A cough that does not settle after a few days, often dry or slightly productive, may be one of the first signs an owner notices. The cough can be worse after exercise or excitement, and tends to persist even when the dog otherwise seems well.
Reduced exercise tolerance
The dog may tire more easily on walks, lag behind, or choose to stop and rest when they would previously have kept going. This can be subtle at first and is sometimes attributed to age or weight, particularly in dogs where the change is gradual.
Increased breathing effort
Breathing may become faster or more laboured, particularly during or after activity. Some dogs breathe more obviously at rest, with visible chest movement or an impression that each breath requires more work.
Occasional nosebleeds
Small amounts of blood from one or both nostrils, sometimes noticed on the floor or the dog's bedding, can occur when the parasite affects blood clotting. The bleeding may stop on its own and recur intermittently.
Later signals
Marked respiratory distress
Breathing difficulty can become more pronounced, with the dog struggling to catch their breath, breathing rapidly even at rest, or showing reluctance to lie down. This pattern suggests more extensive lung involvement or complications such as fluid accumulation or clot formation.
Widespread bleeding
Bleeding may appear in several forms—blood in the stool or vomit, extensive bruising under the skin, blood in the urine, or prolonged bleeding from minor wounds. This reflects more severe disruption to the clotting system.
Neurological changes
Some dogs develop weakness, incoordination, seizures, or changes in behaviour if larvae migrate to the central nervous system or if small clots affect blood flow to the brain or spinal cord. These signs are less common but can appear suddenly.
Collapse or weakness
Episodes of collapse, profound lethargy, or an inability to stand can occur in more advanced cases, particularly if the heart or lungs are severely compromised or if there is significant internal bleeding.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a careful history—when the signs started, whether the dog has access to areas where slugs or snails are present, and what has been observed at home. Physical examination may reveal clues such as abnormal lung sounds, pale gums, or signs of bleeding. From there, testing is guided by the pattern of signs and the findings on examination, with the aim of confirming the presence of the parasite and understanding which systems have been affected.
Faecal analysis
Complete blood count
Coagulation profile
Radiography
Angiostrongylus antigen or antibody test
Options & trade-offs
Management of angiostrongylosis typically involves treating the parasitic infection itself, supporting the body systems that have been affected, and addressing any complications such as bleeding or breathing difficulty. The combination of approaches used in an individual dog depends on the severity of signs, which systems are involved, and how the dog responds as treatment progresses. What is workable for one household or one animal may differ from what suits another.
Anthelmintic treatment
Medications that kill the Angiostrongylus worms are the foundation of treatment. Several anthelmintic drugs are used, often given as a course over multiple weeks. The goal is to eliminate adult worms and larvae, stopping further damage and allowing the body to begin repair. In some cases, treatment may be repeated or extended depending on the response and whether larvae continue to be detected.
Trade-offs: Killing the worms can sometimes provoke an inflammatory reaction as the parasites die, which may temporarily worsen respiratory signs in some dogs. Close observation during the early phase of treatment can help identify any such response. Not all dogs clear the infection after a single course, and repeat testing may be needed to confirm resolution.
Anti-inflammatory medication
Corticosteroids or other anti-inflammatory drugs may be used to reduce lung inflammation and suppress the immune response that contributes to respiratory signs. These medications can ease coughing and breathing difficulty, and may also help reduce the inflammatory reaction that can occur as worms are killed during anthelmintic treatment.
Trade-offs: Anti-inflammatory drugs can dampen the immune system, which raises questions about timing—some clinicians delay their use until anthelmintic treatment is underway, while others use them earlier if respiratory distress is marked. The approach taken depends on the individual presentation and the balance of risks in that particular dog.
Supportive care for bleeding
Dogs with coagulopathy may need measures to manage or prevent bleeding. This can include rest and reduced activity to minimise the risk of trauma, medications that support clotting, or, in severe cases, transfusion of blood products such as plasma or whole blood to replace lost red cells and clotting factors.
Trade-offs: Transfusion carries risks, including immune reactions and the possibility of transmitting infections, and is typically reserved for dogs with life-threatening bleeding or profound anaemia. Clotting function often improves once the parasitic infection is treated, but the time course can vary, and some dogs require ongoing monitoring until coagulation normalises.
Oxygen therapy and respiratory support
Dogs with marked breathing difficulty or low blood oxygen levels may benefit from supplemental oxygen, either through a mask, nasal prongs, or time in an oxygen-enriched enclosure. This support allows the lungs to work more efficiently while anthelmintic and anti-inflammatory treatments begin to take effect.
Trade-offs: Oxygen therapy requires the dog to remain relatively still and calm, which can be stressful for some animals. It is a temporary measure rather than a solution to the underlying infection, and is most useful during the acute phase while other treatments are starting to work.
Preventive medication
After treatment, ongoing use of preventive anthelmintic medication can reduce the risk of reinfection. Several monthly preventive products licensed in the UK are effective against Angiostrongylus, and can be considered for dogs in areas where the parasite is present or for those with high exposure risk.
Trade-offs: Prevention is not the same as cure, and existing infections must be treated before preventive protocols are started. Compliance can be a challenge for some owners, and no preventive is entirely without the possibility of occasional breakthrough infection, particularly if doses are missed or delayed.
Common misconceptions
"If my dog has had angiostrongylosis once and been treated, they cannot get it again."
Infection with Angiostrongylus does not confer lasting immunity. A dog that has been successfully treated can be reinfected if they are exposed again to larvae in slugs, snails, or contaminated environments. This is one reason ongoing preventive medication may be considered for dogs in areas where the parasite is common.
"A negative faecal test means my dog definitely does not have lungworm."
Larvae are shed intermittently and not always in detectable numbers, so a single negative Baermann test does not rule out infection. Some dogs with angiostrongylosis never shed larvae in quantities that can be reliably found in stool samples. Blood tests for antigen or antibody, along with clinical signs and other findings, are often used alongside faecal analysis to build a fuller picture.
"Only dogs that deliberately eat slugs or snails are at risk."
Dogs can be exposed through contact with slime trails on grass, toys, water bowls, or vegetables left outside. Molluscs are often small and easily overlooked, and a dog does not need to consume a whole slug or snail to ingest infective larvae. Incidental contact with contaminated surfaces can be enough for transmission to occur.
Related conditions
Chronic Bronchitis in Dogs
Chronic bronchitis can present with a persistent cough similar to that seen in angiostrongylosis, and distinguishing between the two often requires specific testing for the parasite. In some cases, lungworm infection may contribute to or coexist with chronic airway inflammation.
Aspiration Pneumonia
Both angiostrongylosis and aspiration pneumonia can cause coughing, difficulty breathing, and changes visible on chest imaging. The two conditions may occasionally occur together if a dog with lungworm develops weakness or neurological signs that increase the risk of inhaling food or saliva.
Anticoagulant Rodenticide Toxicity
Angiostrongylosis can interfere with normal blood clotting in a way that may mimic rodenticide poisoning, with dogs showing nosebleeds, bruising, or blood in the stool. Distinguishing between the two typically requires a combination of history, clotting tests, and parasite screening.
Babesiosis
Babesiosis, like angiostrongylosis, can cause bleeding problems and anaemia due to destruction of red blood cells or disruption of clotting mechanisms. Both are parasitic infections that may present with lethargy, pale gums, and systemic signs that require laboratory testing to differentiate.
Atrial Fibrillation
Adult Angiostrongylus worms live in the pulmonary arteries and right side of the heart, and chronic infection can sometimes place strain on the cardiovascular system. In rare cases, this may contribute to abnormal heart rhythms or signs of heart strain, particularly if the parasite burden is high or infection is prolonged.
Angiostrongylosis sits within a broader picture of respiratory and clotting health, and understanding how these systems can be affected by infection may be useful context. The Immune & Inflammatory Health pillar explores how the body responds to parasites and other challenges, and how those responses shape the signs an owner observes. If your dog has been diagnosed or is being investigated for angiostrongylosis, the conversations that unfold over the coming weeks—about prevention, monitoring, and the balance of risks in your particular environment—can help shape a sustainable approach.
Last reviewed: 12 September 2026 · Dr Alastair Greenway MRCVS