Lungworm in dogs is relatively uncommon overall, but infection rates vary dramatically by region. In some areas, particularly parts of southern England and western Europe, lungworm is an emerging and increasingly recognized threat. In other regions, like northern Scotland or much of the central United States, cases are rare or nearly nonexistent. The parasite’s uneven distribution means your dog’s risk depends heavily on where you live and how much contact your dog has with slugs, snails, and foxes.
Prevalence by Region
The most concerning species for dogs is Angiostrongylus vasorum, a parasite that lives in the blood vessels of the heart and lungs. Two decades ago, it was only sporadically reported in parts of the UK, Denmark, France, Italy, and Spain. Since then, it has spread into new geographic areas across nearly all European countries, turning it into what researchers now call an emerging disease.
In the UK, prevalence is heavily skewed toward the south. A national survey of foxes (the main wildlife reservoir for the parasite) found a 7.3% infection rate overall, but that figure jumped to 23% in the South East while dropping to 0% in northern England and Scotland. Since foxes shed larvae into the environment through their feces, areas with high fox infection rates create hotspots where dogs are far more likely to encounter the parasite. The pattern is similar across Europe: hyperendemic pockets sit right next to zones with very low prevalence.
In North America, a different lungworm species called Oslerus osleri (formerly Filaroides osleri) is the more commonly reported type in domestic dogs. Infections have been documented in nine US states and four Canadian provinces. Wild canids carry it more frequently: about 23% of coyotes and 4% of wolves across North America test positive. In Australia, a form of lungworm causing neurological disease is becoming increasingly common in eastern regions, particularly around Sydney.
Which Dogs Are Most at Risk
Young dogs face the highest risk. In one large Australian study of 92 cases, 81% of affected dogs were under six months old. Puppies are more vulnerable partly because of their habit of chewing and eating things indiscriminately, which makes them more likely to swallow an infected slug or snail. They also may not yet be on preventive parasite treatments that could kill larvae before they mature.
Beyond age, the key risk factors are seasonal and behavioral. Cases tend to cluster earlier in the calendar year, when slug and snail activity picks up. Dogs that spend time off-lead outdoors, interact with wildlife, or drink from puddles and outdoor water sources face greater exposure. Dogs with a recent history of appropriate preventive treatment show lower infection rates.
How Dogs Get Infected
Lungworm larvae need an intermediate host to develop, and that host is almost always a slug or snail. Dogs pick up the parasite by accidentally or deliberately eating these creatures, or by swallowing larvae left behind in slime trails on grass, outdoor water bowls, or toys left in the garden. Frogs can also carry larvae and pose a risk to dogs that catch or mouth them.
The specific slug and snail species involved vary by region, but many are common garden varieties. Research in Hawaii found that 78% of one semi-slug species collected from residential properties carried a related lungworm parasite, compared to 24% of another slug species from the same yards. These creatures thrive under plant pots, compost piles, fallen leaves, and anywhere damp organic matter collects. They climb walls, drain pipes, and water tanks, and are attracted to pet food left outdoors.
For Oslerus osleri, transmission works differently. Pups typically catch it directly from their mother’s saliva or feces, often during grooming or nursing. Adult worms form nodules in the trachea, and hatched larvae are coughed up and swallowed, then passed out in feces where they can infect other dogs.
Symptoms and How Quickly They Appear
After a dog swallows Angiostrongylus vasorum larvae, the parasites travel through the gut wall into the bloodstream and eventually reach the heart and lungs. Larvae mature about 40 days after infection. Early signs are often subtle and easy to dismiss: a persistent cough, mild exercise intolerance, or occasional breathlessness.
As the infection progresses, symptoms can become more serious and varied. Dogs may develop bleeding problems, because the parasite interferes with blood clotting. This can show up as unexplained bruising, prolonged bleeding from minor cuts, blood in urine or feces, or nosebleeds. Some dogs develop neurological signs if bleeding occurs in the brain or spinal cord. Others show mainly respiratory symptoms like coughing, wheezing, and nasal discharge. The combination of heart and lung involvement means symptoms can look like heart disease, pneumonia, or a clotting disorder, which makes lungworm easy to misdiagnose.
How Serious Is It Without Treatment
Lungworm can be fatal. Data from veterinary referral centers, which tend to see the more severe cases, show a mortality rate of roughly 24%. In one Irish study, 17% of diagnosed dogs died before or within 24 hours of starting treatment. Some were euthanized after failing to respond to therapy or because the diagnosis was missed entirely.
The danger increases significantly when the infection is advanced or when clotting problems develop before treatment begins. Dogs that are diagnosed early and treated promptly generally have a much better prognosis, which is why awareness of the parasite matters even in areas where prevalence is moderate.
Diagnosis Challenges
One reason lungworm may be underdiagnosed is that standard fecal tests used for common intestinal worms don’t reliably detect it. A specialized technique called the Baermann test, which extracts larvae from a fecal sample, is more effective but not routinely performed. Blood antigen tests are also available and can detect the parasite even when fecal samples come back negative. If your vet suspects lungworm based on symptoms and your dog’s exposure history, they will likely use one or both of these methods rather than relying on a routine fecal float.
Prevention and Treatment
Monthly preventive treatments are the most practical way to protect dogs in areas where lungworm is present. Products containing moxidectin (a spot-on treatment) or milbemycin oxime (an oral tablet) are the most commonly recommended options. Neither achieves greater than 90% efficacy in a single dose against Angiostrongylus vasorum, so vets often verify that the infection has cleared and retreat if necessary. For Crenosoma vulpis, another lungworm species, a single dose of milbemycin oxime at the standard heartworm prevention dose is typically effective.
European veterinary guidelines from ESCCAP recommend tailoring prevention to each dog’s age, lifestyle, health status, and geographic risk. Dogs that spend a lot of time outdoors, live in known hotspot areas, or have a habit of eating slugs and snails generally warrant monthly preventive dosing rather than less frequent treatment. Reducing environmental exposure also helps: bringing in water bowls overnight, picking up toys from the garden, and clearing away leaf litter and compost where slugs congregate can lower the chances of your dog encountering larvae.