Lungworm is a parasitic infection of the airways and lung tissue in cats, caused most commonly by a tiny nematode called Aelurostrongylus abstrusus. Cats pick it up by eating infected snails, slugs, or the small animals that have themselves eaten those mollusks, and the worms then take up residence deep in the respiratory tract. The infection ranges from completely silent to life-threatening, depending on the worm burden, the cat’s age, and whether other health problems are in the mix. Because the symptoms overlap heavily with asthma and other respiratory diseases, lungworm is underdiagnosed and often not on an owner’s radar until a cat is seriously struggling to breathe.
Which Parasites Are Involved
Aelurostrongylus abstrusus is by far the most common lungworm in domestic cats worldwide.1PubMed Central. Lungworm disease in cats: ABCD guidelines on prevention and management But it is not the only one. Two other species now get serious attention from veterinary parasitologists. Troglostrongylus brevior was once thought to mainly infect wildcats, but it has increasingly turned up in domestic cats, especially kittens and young strays.2PubMed Central. Diagnosis and management of lungworm infections in cats: Cornerstones, dilemmas and new avenues Capillaria aerophila rounds out the trio and carries the added concern of being zoonotic, meaning it can occasionally infect humans.3PubMed Central. Clinical-parasitological Screening for Respiratory Capillariosis in Cats in Urban Environments A cat can also harbor more than one species at the same time, which complicates both diagnosis and treatment.
How Cats Get Infected
The life cycle of A. abstrusus requires an intermediate host, and that role falls to snails and slugs. Adult worms living in the cat’s lungs produce larvae that travel up the airways, get swallowed, and pass out in feces. A snail or slug ingests those larvae, and the parasites develop inside the mollusk. In theory a cat could get infected by eating a slug directly, but in practice the more likely route involves a paratenic (transport) host: a mouse, bird, frog, or lizard that has eaten an infected snail or slug and now carries the larvae in its tissues.4PubMed. Intermediate and paratenic hosts in the life cycle of Aelurostrongylus abstrusus in natural environment When a cat catches and eats one of those animals, the larvae are released in the gut, penetrate the intestinal wall, and migrate through the bloodstream to the lungs. There, they mature into adults, mate, and begin producing first-stage larvae of their own. The whole cycle from ingestion to larvae appearing in the cat’s feces takes roughly five to six weeks.
This explains why outdoor cats and avid hunters are at highest risk. A strictly indoor cat that never encounters a rodent or slug has almost no chance of exposure. But even a cat that occasionally slips outside, or one that shares a yard with slugs that wander onto patios, has some risk.
What Lungworm Looks Like in a Cat
Many cats with light infections show no symptoms at all, and the parasite is only discovered incidentally. When the worm burden is heavier, though, the signs are respiratory: a chronic cough that does not respond to antibiotics, wheezing, labored breathing, nasal discharge, and sometimes sneezing. In severe cases you may notice your cat breathing with an open mouth or see the abdomen visibly working with each breath. These signs look a lot like feline asthma, bronchitis, or even pneumonia, which is why lungworm frequently gets missed on the first veterinary visit.
Kittens tend to have it much worse. A case report described a five-month-old feral kitten that developed rapid breathing, coughing, and wheezing severe enough to constitute respiratory distress; bronchoalveolar lavage confirmed heavy inflammation with parasitic larvae present in the airway mucus.5PubMed Central. Respiratory distress associated with lungworm infection in a kitten Because kittens have smaller airways and less robust immune systems, even a modest number of worms can cause serious trouble.
Why Kittens Face Greater Danger
The vulnerability of young cats goes beyond simply having smaller lungs. Research on Troglostrongylus brevior found that in a large sample of young cats, nearly 90 percent of the lungworm-positive kittens were infected with this species rather than the more common A. abstrusus. Infection with T. brevior was strongly associated with cats six months old or younger, and the prevalence dropped steeply in cats between six and twenty-four months, becoming essentially undetectable after age two.6PubMed. Troglostrongylus brevior: a feline lungworm of paediatric concern This suggests that immune maturation plays a role in clearing or preventing T. brevior infection, while very young cats remain highly susceptible.
At necropsy, kittens infected with T. brevior showed worms physically present in the trachea and bronchi, surrounded by a dense inflammatory response.7PubMed. Pathological and histological findings associated with the feline lungworm Troglostrongylus brevior That kind of airway obstruction in a small kitten can become fatal quickly, which is why veterinary parasitologists increasingly flag T. brevior as a pediatric concern. If you are fostering or adopting feral kittens, lungworm should be part of any respiratory workup, not an afterthought.
Diagnosing Lungworm
The classic diagnostic method is the Baermann technique, a fecal flotation procedure that concentrates first-stage larvae from a stool sample. It works well when larvae are being shed, but the shedding can be intermittent, so a single negative result does not rule out infection. Repeated sampling over several days improves accuracy.
Chest X-rays help assess how much lung damage is present and can sometimes point toward lungworm specifically. In a study of 26 cats with confirmed lungworm, the most common radiographic patterns were interstitial and bronchial changes, with alveolar patterns appearing in more advanced cases.8PubMed Central. Single and mixed feline lungworm infections: clinical, radiographic and therapeutic features of 26 cases (2013-2015) The trouble is that these patterns are not unique to lungworm. Asthma, bacterial pneumonia, and even heart disease can produce similar-looking X-rays. CT scans offer more detail: in one study of 14 naturally infected cats, CT revealed ground-glass opacities, multiple pulmonary nodules, enlarged lymph nodes, and subpleural thickening, findings that were sometimes invisible on standard radiographs.9PubMed Central. Computed tomography, radiology and echocardiography in cats naturally infected with Aelurostrongylus abstrusus CT is not routine in most veterinary clinics, but when a cat’s respiratory disease is not responding to treatment, it can provide the clue that tips the diagnosis.
Molecular tools such as PCR can confirm the species involved and detect mixed infections that fecal examination alone might miss. A survey of cats in Greece used both fecal microscopy and serology (ELISA), and found that serological testing flagged infections in cats whose stool samples came back negative, and vice versa, highlighting that no single test catches every case.10Parasitology Research. Feline lungworms in Greece: copromicroscopic, molecular and serological study In areas where lungworm is common, combining methods gives the most reliable picture.
Treatment Options
Several antiparasitic drugs effectively kill feline lungworms, but the details matter. A spot-on formulation of imidacloprid and moxidectin (sold as Advocate) has been extensively studied. A single application reduced worm burden but did not reliably clear the infection; three monthly applications, however, proved highly effective and safe.11PubMed Central. Efficacy of imidacloprid 10%/moxidectin 1% spot-on formulation (Advocate®) in the prevention and treatment of feline aelurostrongylosis This underscores a point your vet will likely emphasize: a single dose is usually not enough. You need to complete the full course.
Another spot-on combination, emodepside with praziquantel (Profender), has also shown strong results. In a controlled field study, treated cats went from active larval shedding at baseline to a complete elimination of larvae within about four weeks, and remained negative for the entire monitoring period afterward.12PubMed Central. Controlled field study evaluating the clinical efficacy of a topical formulation containing emodepside and praziquantel in the treatment of natural cat aelurostrongylosis An earlier trial comparing the same product to oral fenbendazole (Panacur) found both achieved roughly 99 percent efficacy.13PubMed. Efficacy and safety of emodepside 2.1%/praziquantel 8.6% spot-on formulation in the treatment of feline aelurostrongylosis Fenbendazole has the advantage of being inexpensive and widely available, but it requires daily oral dosing over several days, which any cat owner knows can be a battle.
The choice between these options often comes down to practicality. Spot-on products avoid the daily pill fight. Fenbendazole is the go-to in shelter settings where cost is a factor. Your vet will pick based on the severity of infection, the cat’s temperament, and what is available locally.
When Treatment Itself Causes Problems
One counterintuitive aspect of lungworm treatment is that the cat can temporarily get worse after the drug starts working. When large numbers of worms die in the airways simultaneously, the immune system mounts an intense inflammatory response to all that dead parasite material. The kitten described earlier actually developed worsening respiratory distress after beginning standard fenbendazole treatment at a shelter.14PubMed Central. Respiratory distress associated with lungworm infection in a kitten In that case, the airway lavage revealed heavy neutrophilic and eosinophilic inflammation, indicating an aggressive reaction to dying larvae.
This post-treatment flare is more of a risk in heavily infected cats and in kittens. Vets sometimes prescribe a short course of corticosteroids alongside the antiparasitic drug to dampen the inflammatory surge. If your cat is being treated for lungworm and seems to breathe worse for a few days after starting medication, contact your vet promptly. It does not necessarily mean the drug is failing; it may mean the drug is working but the body’s cleanup process is overwhelming the airways.
Prevention Strategies
Because the infection route runs through prey animals and mollusks, the most straightforward prevention is keeping your cat indoors. That is not realistic for every cat, though. For cats that go outside and hunt, monthly preventive treatments with moxidectin-containing products have shown excellent results in experimental settings. In studies using a fluralaner-moxidectin spot-on (Bravecto Plus), monthly application before experimental infection reduced larval shedding by more than 99.9 percent compared to untreated controls, and almost completely prevented lung damage.15PubMed Central. Efficacy of Bravecto® Plus spot-on solution for cats (280 mg/ml fluralaner and 14 mg/ml moxidectin) for the prevention of aelurostrongylosis in experimentally infected cats A follow-up study confirmed these findings using a multi-diagnostic approach, showing that treated cats did not seroconvert (meaning their immune systems never even mounted a response to the parasite), while all untreated controls developed severe pulmonary changes.16PubMed Central. Efficacy of Bravecto® Plus spot-on solution for cats (280 mg/ml fluralaner and 14 mg/ml moxidectin) in the prevention of feline Aelurostrongylus abstrusus infection evaluated in a multi-diagnostic approach
If your cat is an active hunter, talk to your vet about adding a lungworm-effective product to the regular parasite-prevention rotation. Many standard flea-and-worm preventives do not cover lungworm, so you cannot assume your cat is protected just because it is on a monthly treatment. Check the label or ask specifically.
Stray and Colony Cats as Reservoirs
Unsupervised outdoor cats maintain the parasite in the environment. A study comparing stray colony cats with shelter cats found that roughly a third of colony cats were shedding A. abstrusus larvae, compared to about 3 percent of shelter cats, a statistically significant difference.17PubMed Central. Stray Cat Colonies Lacking Health Surveillance and Management Pose Infection Pressure for Aelurostrongylus abstrusus on Sympatric Domestic and Wild Felids Those colony cats are continuously passing larvae in their feces, which are then picked up by snails and slugs, keeping the local cycle going. The implication is that even if your own cat is indoor-only, a nearby feral colony can raise the baseline risk in your area by boosting the number of infected intermediate hosts in the environment.
A molecular survey of cats in Romania illustrates how common the parasite can be in populations that lack regular veterinary care: about 30 percent tested positive for A. abstrusus by PCR, and roughly 7 percent also carried T. brevior.18PubMed Central. Molecular Survey of Metastrongyloid Lungworms in Domestic Cats (Felis silvestris catus) from Romania: A Retrospective Study (2008–2011) These are not edge-case numbers. In parts of southern and eastern Europe especially, lungworm is a mainstream feline health issue, not a curiosity.
Co-infections and Immunocompromised Cats
Lungworm on its own can range from mild to severe, but the picture gets harder when another disease is running in parallel. Cats co-infected with feline immunodeficiency virus (FIV) and A. abstrusus tend to present with more severe symptoms: lethargy, loss of appetite, pale gums, labored breathing, coughing, and nasal discharge. In a report from a Bulgarian veterinary clinic, three FIV-positive cats with concurrent lungworm infections required combined antiparasitic, antibiotic, and fluid therapy before their condition improved.19Tradition and Modernity in Veterinary Medicine. CLINICAL CASES OF AELUROSTRONGYLUS ABSTRUSUS AND FELINE IMMUNODEFICIENCY VIRUS CO-INFECTION IN CATS FIV suppresses the immune system, so the cat is less able to keep the worm burden in check, and the inflammation caused by the worms may be harder to resolve. Any cat known to be FIV-positive that develops respiratory symptoms deserves a thorough lungworm workup, even if the initial suspicion is an upper respiratory infection.
Is Lungworm Spreading?
There is growing concern that feline lungworm is expanding its geographic range. Researchers have flagged several possible drivers, including climate change (warmer, wetter conditions favor snails and slugs), shifting wildlife populations, and the movement of cats across borders through rescue and adoption networks.20PubMed Central. Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated In parts of Europe where lungworm was historically rare, case reports and surveys have been turning up positive cats with increasing regularity. T. brevior, once considered a parasite of wildcats in Mediterranean regions, has been documented in domestic kittens far from its original range, which suggests it is either spreading or was always more widespread than previously recognized.
Whether similar expansion is happening in other parts of the world is harder to say. In regions where slug and snail populations are abundant year-round, the infrastructure for transmission has always been there, and underdiagnosis may account for what looks like a recent increase. Either way, the practical message is the same: if your cat goes outside and hunts, lungworm belongs on the list of things to think about, wherever you live.
The Zoonotic Angle
A. abstrusus and T. brevior are not known to infect humans, so direct transmission from your cat to you is not a concern with those species. The exception is Capillaria aerophila, which can, in rare cases, infect people. This parasite is more commonly found in wild carnivores like foxes but has been documented in urban pet cats, and close contact between infected cats and humans creates at least a theoretical pathway for transmission.21PubMed Central. Clinical-parasitological Screening for Respiratory Capillariosis in Cats in Urban Environments Human cases of respiratory capillariosis are exceedingly rare, but they have been reported, and they present with cough and airway inflammation that can mimic other conditions. The risk is highest for immunocompromised people living in close quarters with outdoor cats in areas where C. aerophila circulates.
For the vast majority of cat owners, zoonotic transmission from feline lungworm is not something to lose sleep over. Standard hygiene practices, regular deworming of your cat, and prompt veterinary attention for respiratory symptoms in your pet are more than sufficient to keep the risk negligible. The more pressing concern remains the health of the cat itself, especially if it is young, immunocompromised, or living in an environment with high snail and rodent exposure.