Lungworms are parasitic nematodes that take up residence in the airways and lung tissue of cats, causing respiratory signs that range from a mild, lingering cough to life-threatening breathing failure. The most common culprit worldwide is a small worm called Aelurostrongylus abstrusus, though several other species are increasingly recognized as contributors to feline respiratory disease. Because the symptoms overlap heavily with asthma, bronchitis, and other common conditions, lungworm infections are underdiagnosed in many parts of the world, and the true number of affected cats is almost certainly higher than reported.
Which Worms Are Involved
Aelurostrongylus abstrusus has long been considered the primary lungworm of domestic cats. Adults are thin, roughly centimeter-long worms that embed in the small airways and lung tissue. But research over the past decade has drawn attention to other species that had been largely ignored. Troglostrongylus brevior and Troglostrongylus subcrenatus, Oslerus rostratus, and Capillaria aerophila (also called Eucoleus aerophilus) have all been found in domestic cats, and their actual role in causing disease is still being clarified.1Trends in Parasitology. Neglected feline lungworms: a new-old challenge in small animal practice Mixed infections involving more than one species can and do occur, which complicates both diagnosis and treatment.
Wild felids serve as a reservoir. A study of European wildcats in Germany found that cardiopulmonary nematodes were common in the wild population without appearing to seriously harm it overall. Domestic cats likely pick up infections through shared prey species, so areas where wild and domestic cat territories overlap carry higher risk.2PubMed Central. Cardio-pulmonary parasites of the European wildcat (Felis silvestris) in Germany
How Cats Get Infected
Cats do not typically pick up lungworms directly from another cat’s feces. The life cycle requires an intermediate host, usually a slug or snail. An infected cat coughs up larvae, swallows them, and passes them in feces. Slugs or snails ingest the larvae, and those larvae develop into an infective stage inside the mollusk. When a cat eats the slug or snail, the larvae migrate from the gut through the bloodstream to the lungs, where they mature into adults and start producing their own larvae. The whole cycle from ingestion to the appearance of larvae in feces takes roughly five to six weeks.
Many cats never touch a slug deliberately, but they do hunt. Birds, rodents, lizards, and frogs that have eaten infected slugs or snails can act as transport hosts, carrying the infective larvae in their tissues. An outdoor cat that catches and eats a mouse or a small bird may ingest lungworm larvae without ever encountering a mollusk. This is the most common real-world route of infection for domestic cats with access to the outdoors.
One species breaks the usual rules. Troglostrongylus brevior can apparently pass directly from a queen to her suckling kittens, a route that does not require any intermediate host at all. Researchers have documented observational evidence of this direct transmission from mother to nursing offspring, a discovery that changed the understanding of how quickly very young kittens can become infected.3PubMed. Evidence for direct transmission of the cat lungworm Troglostrongylus brevior (Strongylida: Crenosomatidae) Later investigation also pointed to transmammary transmission, where larvae pass through the queen’s milk, as a plausible route.4PubMed Central. Transmammary transmission of Troglostrongylus brevior feline lungworm: a lesson from our gardens
Recognizing the Symptoms
The signs of lungworm infection in cats can be maddeningly nonspecific. Many cats show a persistent cough, sometimes productive, that does not respond to antibiotics. Beyond coughing, respiratory signs include labored breathing, rapid breathing, wheezing, open-mouth breathing, and sneezing. Mucopurulent nasal discharge, meaning thick and sometimes yellowish, is common. General signs like lethargy, weight loss, and depression round out the picture.1Trends in Parasitology. Neglected feline lungworms: a new-old challenge in small animal practice
In a clinical study of 26 cats with confirmed lungworm infections, the most frequently recorded signs were coughing, increased lung sounds on auscultation, difficulty breathing (including orthopnea and open-mouth breathing), and rapid breathing. Two of those 26 cats had no symptoms at all and were only identified through routine testing.5PubMed Central. Single and mixed feline lungworm infections: clinical, radiographic and therapeutic features of 26 cases (2013-2015) That is worth emphasizing: a cat can carry lungworms and shed larvae in its feces without showing any outward sign of illness. Subclinical infections are a real phenomenon, and they help the parasite persist in an area because no one thinks to test for it.
The severity of symptoms depends heavily on the worm species involved and the burden of infection. A cat harboring a handful of A. abstrusus adults may cough occasionally and otherwise seem fine. A young kitten with a heavy T. brevior infection, on the other hand, can develop respiratory failure. In the most severe cases, lungworm infection can be fatal.6PubMed Central. Lungworm disease in cats: ABCD guidelines on prevention and management
Why Kittens Are at Higher Risk
Troglostrongylus brevior is a particular concern in kittens younger than six months of age. The disease it causes in these young cats tends to be significantly more severe than what is typically seen with A. abstrusus in adults.4PubMed Central. Transmammary transmission of Troglostrongylus brevior feline lungworm: a lesson from our gardens Part of the problem is the direct transmission route from queen to kittens: because the parasite can bypass the slug-and-snail step entirely, kittens can become infected in their first weeks of life, before their immune systems have any ability to mount a meaningful defense.
Feral and stray kittens are especially vulnerable. They often harbor heavy parasite burdens of multiple species simultaneously, and they tend not to receive veterinary care until respiratory signs are already advanced. A case report described a five-month-old feral kitten that developed worsening respiratory signs, including rapid breathing, coughing, and wheezing, even after standard deworming treatment with fenbendazole at a shelter.7PubMed Central. Respiratory distress associated with lungworm infection in a kitten Cases like this illustrate that treatment does not always produce an immediate turnaround, and that young cats with severe infections may need supportive care alongside antiparasitic drugs.
Getting a Diagnosis
The overlap between lungworm symptoms and feline asthma or chronic bronchitis makes clinical suspicion the first hurdle. If your cat has access to the outdoors, lives in an area where lungworms circulate, and has a chronic cough or breathing difficulty that does not improve with standard respiratory treatments, lungworm should be on the list of possibilities. Clinical guidelines recommend that lungworms be included in the differential diagnosis for any cat in an endemic area showing respiratory signs and abnormalities on chest X-rays, and that a fecal examination should be the first diagnostic step for cats at risk.5PubMed Central. Single and mixed feline lungworm infections: clinical, radiographic and therapeutic features of 26 cases (2013-2015)
The gold standard for finding lungworm larvae in feces is a technique called the Baermann method. A fresh fecal sample is placed on gauze over warm water, and the larvae migrate down through the gauze into the water over about 24 hours, where they can be collected and examined under a microscope. A comparative study found the Baermann technique to be the most sensitive fecal test for detecting A. abstrusus, outperforming standard fecal flotation methods.8PubMed. Comparison of bronchoalveolar lavage fluid examination and other diagnostic techniques with the Baermann technique for detection of naturally occurring Aelurostrongylus abstrusus infection in cats Standard flotation, the type of fecal test most veterinary clinics run as a matter of routine, picks up lungworm larvae less reliably. If your vet runs a routine fecal and finds nothing but you still suspect lungworm, it is worth asking specifically for a Baermann test.
One frustrating diagnostic wrinkle: larvae are shed intermittently, and a single negative fecal result does not rule out infection. Repeated testing can improve the odds of catching a positive sample. Larvae can also be found in tracheal swabs and bronchoalveolar lavage fluid, but these methods tend to be less sensitive than fecal testing.6PubMed Central. Lungworm disease in cats: ABCD guidelines on prevention and management
Molecular diagnostics are an emerging option. PCR-based tests show high specificity and sensitivity and have the advantage of being able to detect infection during the prepatent period, meaning the window between when a cat ingests larvae and when it starts shedding them in feces.6PubMed Central. Lungworm disease in cats: ABCD guidelines on prevention and management During this prepatent phase, which lasts roughly five to six weeks, fecal tests will come back negative even though the infection is actively developing. Molecular tests are not yet widely available in general veterinary practice, but they represent a genuine step forward for difficult cases.
Treatment Options
The two main drug classes used against feline lungworms are benzimidazoles and macrocyclic lactones. Fenbendazole, a benzimidazole, has historically been one of the most commonly used treatments, typically given orally over multiple days. Among the macrocyclic lactones, moxidectin (often combined with imidacloprid in a spot-on formulation) and milbemycin oxime are frequently used. Emodepside, available in combination with praziquantel as a spot-on, is another option. Specific treatment protocols and which drug a vet reaches for first will depend on the worm species involved, the severity of infection, and what formulations are available locally.
Most cats with mild to moderate infections respond well to antiparasitic treatment, with coughing and respiratory signs resolving over days to weeks as the worms die and the lung tissue heals. But treatment is not always straightforward. When large numbers of worms die simultaneously in the lungs, the inflammatory response to the dying parasites can temporarily worsen respiratory signs before things improve. This is especially relevant in cats with heavy burdens. Supportive care, such as oxygen supplementation, bronchodilators, or anti-inflammatory drugs, may be needed alongside the antiparasitic itself. The role of the host’s own immune response in clearing the infection and in determining which cats actually benefit most from prophylactic versus therapeutic treatment remains an area of active investigation.9BioMed Central / Parasites & Vectors. Updates on feline aelurostrongylosis and research priorities for the next decade
The kitten case mentioned earlier, where respiratory signs worsened after fenbendazole treatment, is a useful cautionary example.7PubMed Central. Respiratory distress associated with lungworm infection in a kitten A worsening clinical picture after starting treatment does not necessarily mean the drug failed. It can reflect a robust inflammatory reaction as dead worm material is cleared from the lungs. Veterinarians familiar with lungworm will be watching for this possibility and prepared to manage it.
Preventing Infection
The most effective prevention strategy combines limiting exposure to intermediate and transport hosts with regular antiparasitic treatment. Keeping cats indoors eliminates the most common route of infection, since indoor cats do not hunt rodents, birds, or lizards, and are unlikely to encounter slugs. For cats that go outdoors, the risk is real, and routine prevention with an appropriate antiparasitic is the most practical tool available.
A controlled study demonstrated that monthly application of a spot-on formulation containing imidacloprid and moxidectin reliably killed early larval stages in the lungs, preventing lung damage and the establishment of a patent (egg-or-larva-producing) infection with A. abstrusus.10PubMed Central. Efficacy of imidacloprid 10%/moxidectin 1% spot-on formulation (Advocate®) in the prevention and treatment of feline aelurostrongylosis This is a significant finding because it means the product was not just treating established infections but was intercepting the parasite before it could damage the lungs at all. Monthly dosing was key to sustained protection.
Not every broad-spectrum dewormer covers lungworms effectively. Many routine deworming products sold over the counter target intestinal worms like roundworms and hookworms but have limited or no activity against the nematodes that live in the respiratory tract. If your cat is at risk, talk to your vet specifically about lungworm coverage rather than assuming your regular dewormer handles it. The distinction matters, and it is one of the more common blind spots in standard preventive care for outdoor cats.
Why Lungworm Gets Missed
Underdiagnosis is the central practical problem with feline lungworm. Several factors contribute. First, the symptoms are generic. A coughing cat is far more likely to be worked up for asthma or bronchitis than for a parasitic infection, especially in regions where veterinarians do not routinely think about lungworm. Second, the standard fecal flotation test used in most clinics is less sensitive for lungworm larvae than the Baermann technique, so infections can be missed on routine screening.8PubMed. Comparison of bronchoalveolar lavage fluid examination and other diagnostic techniques with the Baermann technique for detection of naturally occurring Aelurostrongylus abstrusus infection in cats Third, intermittent larval shedding means even the Baermann test can return a false negative on any given day. And fourth, many infected cats are subclinical, silently shedding larvae and maintaining the parasite in the local environment without anyone suspecting they carry worms.
Geography adds another layer. Lungworm has historically been considered a problem mainly in parts of southern and central Europe, but reports continue to expand the known range. Changing climate, movement of cats across borders, and increased testing in regions that previously did not look for lungworm all contribute to a shifting picture. If you have recently moved or adopted a cat from a different region, its lungworm risk profile may be different from what your local vet typically encounters.
Mixed Infections and Diagnostic Complexity
Cats can be infected with more than one species of lungworm simultaneously, and mixed infections complicate both the clinical picture and the choice of treatment. The 26-cat case series noted that mixed infections were observed alongside single-species infections, with radiographic and clinical features that did not always clearly point to one parasite over another.5PubMed Central. Single and mixed feline lungworm infections: clinical, radiographic and therapeutic features of 26 cases (2013-2015) From the owner’s perspective, this matters because a cat that responds partially to treatment might still harbor a second species that requires a different drug or a longer course.
Species identification under the microscope requires some expertise. The first-stage larvae of A. abstrusus and T. brevior look similar but have morphological differences at the tail end that an experienced parasitologist can distinguish. Misidentification is a genuine concern in clinics without specialized parasitology support, and it can lead to assumptions about prognosis and treatment that do not match the actual parasite involved. Molecular testing sidesteps this problem entirely by identifying the species from its DNA, but access to these tests is still limited in general practice.
Slugs, Snails, and Indoor Risks
Strictly indoor cats are at much lower risk, but “lower” does not mean zero in every scenario. Slugs and snails can enter ground-floor homes, particularly in damp climates, and a playful cat may well bat one around. Cats that live in enclosed outdoor spaces like catios or screened porches can encounter mollusks that climb in. These are edge cases, not common transmission routes, but they are worth mentioning for owners who assume indoor living completely eliminates the possibility.
A more realistic concern for indoor cats is the “occasional escapee” scenario. A cat that gets out for even a few hours and catches a mouse has had a potential lungworm exposure. The prepatent period of five to six weeks means symptoms, if they develop, will not appear for over a month after the escape. By then, the connection between the brief outdoor adventure and the new cough may not be obvious.
For multi-cat households where some cats go outdoors and others stay in, the risk to indoor cats remains low as long as they are not eating intermediate or transport hosts. Feline lungworms are not spread through casual contact, shared litter boxes, or grooming. The larvae passed in feces need to be ingested by a slug or snail before they become infective to another cat (with the exception of T. brevior‘s queen-to-kitten route). So an indoor cat living with an outdoor cat is not at meaningful risk from household contact alone.
Chest X-Rays and What They Show
Radiographs are often part of the diagnostic workup for a cat with chronic respiratory signs, and lungworm infection produces some characteristic patterns. The most typical finding is a diffuse bronchial and interstitial pattern, sometimes described as looking like a mix of small nodules and thickened airways scattered throughout the lungs. In more severe cases, alveolar patterns indicating fluid or consolidation may be visible. These findings are suggestive but not unique to lungworm. Asthma, other infections, and even some cancers can produce overlapping radiographic pictures. The X-ray raises suspicion; the fecal test confirms it.
Radiographic abnormalities can persist for weeks after successful treatment because the inflammation and tissue changes take time to resolve even once the worms are gone. A follow-up X-ray taken too soon may show lingering changes that do not necessarily indicate treatment failure. Veterinarians familiar with the condition will typically recheck fecal samples rather than relying solely on imaging to judge whether the infection has cleared.