Aelurostrongylus abstrusus — the feline lungworm — is a metastrongyloid nematode (roundworm) that parasitises the lower respiratory tract of domestic and wild felids worldwide. While historically considered an uncommon clinical problem, advances in diagnostic techniques have revealed a significantly higher prevalence than previously recognised in many regions, and the condition has attracted increasing veterinary attention as a cause of chronic or recurrent respiratory disease in cats.

This article covers the complete clinical picture of feline lungworm infection: the parasite’s complex life cycle, which cats are at risk, how to recognise and diagnose the infection, and the antiparasitic treatment options that are now available — including recently developed therapies that have substantially improved treatment outcomes.

The Parasite: Biology and Life Cycle

Understanding the life cycle of Aelurostrongylus abstrusus is important for understanding both why certain cats are at risk and how prevention and treatment work.

Adult Parasites and Egg-Laying

Adult Aelurostrongylus abstrusus worms inhabit the terminal bronchioles and alveolar ducts of the feline lung. Adult females are approximately 10 mm in length; males are shorter at around 7 mm. After mating, females deposit embryonated eggs into the respiratory tissue, where they hatch into first-stage larvae (L1). The larvae travel up the mucociliary escalator, are swallowed, and pass out in the cat’s faeces.

Intermediate and Paratenic Hosts

Aelurostrongylus abstrusus cannot complete its development directly from cat to cat. It requires intermediate and paratenic hosts to reach the infective stage:

• Intermediate host (obligate): Terrestrial snails and slugs (molluscs of various species) ingest the L1 larvae from contaminated soil and faeces. Inside the mollusc, larvae develop through L2 to the infective third-stage larvae (L3) over 2 to 3 weeks, depending on environmental temperature.
• Paratenic (transport) hosts: Small vertebrates — particularly birds, mice, voles, frogs, and lizards — ingest infected molluscs. L3 larvae encyst in the muscles and organs of these paratenic hosts without developing further. The paratenic host serves as a mobile reservoir that concentrates the infection.
• Definitive host (cat): Cats become infected by consuming infected molluscs directly (less commonly) or, more importantly, by hunting and eating paratenic hosts containing encysted L3 larvae. After ingestion, L3 larvae migrate from the gastrointestinal tract through the lymphatics and bloodstream to the lungs, where they undergo L4 to adult development over approximately 4 to 6 weeks.

Why Outdoor and Hunting Cats Are Most at Risk The life cycle requires ingestion of infected prey. Cats that hunt birds, mice, frogs, or lizards — all common paratenic hosts — are at substantially higher risk than indoor cats with no hunting access. Cats in rural areas, on farms, or with regular outdoor access should be considered at ongoing risk throughout the hunting season.

Prevalence

Prevalence varies considerably by geographic region and cat population studied. European studies using Baermann larval migration technique or PCR have reported infection rates of 2 to 17% in owned outdoor cats in endemic areas, with higher rates in feral and shelter populations. Infection is most common in regions with mild, moist climates that support mollusc populations. In the UK, Ireland, Italy, Spain, and parts of Central Europe, the infection is considered endemic. Prevalence in North America is less well characterised but likely underdiagnosed.

Clinical Signs

The severity of clinical signs in Aelurostrongylus abstrusus infection correlates with the larval burden — the number of infective larvae ingested — and the individual cat’s immune response. The spectrum ranges from subclinical infection detectable only on faecal examination to severe, life-threatening respiratory disease.

Mild to Moderate Infection

• Chronic intermittent coughing — often the most prominent presenting sign
• Sneezing
• Increased respiratory rate at rest
• Mild exercise intolerance
• Occasional mucopurulent nasal discharge

These signs may be episodic and are frequently misattributed to feline herpesvirus, calicivirus, or allergic bronchitis, leading to delayed or missed diagnosis.

Moderate to Severe Infection

• Persistent, productive cough
• Dyspnoea (laboured breathing)
• Tachypnoea (increased respiratory rate) at rest
• Open-mouth breathing in severe cases
• Weight loss and reduced appetite
• Lethargy

In the most severe cases — typically associated with heavy larval burdens or concurrent immunosuppression (FIV, FeLV, high-dose corticosteroid use) — cats can develop severe verminous pneumonia with diffuse pulmonary consolidation, respiratory failure, and death.

Diagnosis

Baermann Larval Migration Technique (Faecal Examination)

The Baermann technique is the standard diagnostic method for lungworm infection. Fresh faeces (ideally collected over 3 consecutive days and pooled, as larval shedding is intermittent) are suspended in warm water over a funnel, and larvae migrate through the sample under gravity and warmth. The sediment is then examined microscopically for L1 larvae.

L1 larvae of Aelurostrongylus abstrusus are distinctive: approximately 360 to 400 micrometres in length with a characteristic S-shaped tail with a small dorsal spine at the tip. This morphology distinguishes them from L1 larvae of Troglostrongylus brevior, another feline lungworm species that has been increasingly recognised.

The key limitation of the Baermann technique is its imperfect sensitivity — false negative results occur when larval shedding is at a low level or during the prepatent period (before adult worms begin laying eggs). Repeat testing and pooled faecal samples substantially improve sensitivity.

PCR Testing

Real-time PCR on faecal samples offers higher sensitivity than the Baermann technique and can detect infection during the prepatent period. It also allows species-level differentiation between Aelurostrongylus abstrusus and Troglostrongylus brevior. PCR is increasingly available through commercial veterinary diagnostic laboratories and is the preferred diagnostic method when available.

Thoracic Imaging

Chest radiographs in cats with moderate to severe lungworm infection typically show a diffuse mixed bronchial-alveolar-interstitial pattern, most prominent in the caudal lung lobes. Nodular opacities representing larval granulomas may also be visible. Computed tomography (CT) provides more detailed assessment of pulmonary lesion distribution and severity. Radiographic findings are not specific to lungworm and must be interpreted alongside parasitological results.

Bronchoalveolar Lavage (BAL)

Bronchoscopy with BAL can demonstrate larvae in airway fluid and is particularly useful when faecal tests are negative but clinical suspicion remains high — for example, in cats with chronic cough and a suggestive radiographic pattern. BAL also allows assessment of the inflammatory cell population, which typically shows eosinophilia in lungworm infection.

Treatment

Until relatively recently, treatment options for Aelurostrongylus abstrusus were limited in efficacy and licensing. The introduction of macrocyclic lactone-based products specifically licensed for feline use has substantially improved the treatment landscape.

Drug	Formulation	Dose / Regimen	Efficacy	Licensed for Cats
Emodepside + praziquantel (Profender)	Spot-on	Single topical application; may repeat after 4 weeks in heavy infections	High (>95% efficacy vs. adults)	Yes (EU and others)
Imidacloprid + moxidectin (Advocate/Advantage Multi)	Spot-on	Single application; repeat monthly for prevention	High; also effective against L3/L4 stages	Yes — lungworm indication
Selamectin (Revolution/Stronghold)	Spot-on	Monthly application	Moderate; good for prevention and mild infection	Yes — some countries
Fenbendazole	Oral	50 mg/kg q24h for 3 days, repeated every 2 weeks x3	Moderate; good for adjunct use	Off-label in most countries
Ivermectin	Oral/SC	0.2–0.4 mg/kg; not routinely recommended	Variable; narrow safety margin in cats	Not routinely recommended

Recommended First-Line Treatment

Emodepside/praziquantel spot-on (Profender) or imidacloprid/moxidectin spot-on (Advocate/Advantage Multi) are the preferred first-line treatments in most markets where these products are available with lungworm licensure for cats. Both are easy to administer topically, well tolerated, and have demonstrated high efficacy against adult Aelurostrongylus abstrusus in clinical and experimental studies.

In severe infections, a course of oral fenbendazole may be prescribed concurrently or as adjunctive therapy to address larval stages. Respiratory support (bronchodilators, oxygen supplementation) may be required in cats with severe verminous pneumonia until antiparasitic treatment takes effect.

Monitoring Response to Treatment

Clinical signs typically improve within 2 to 4 weeks of treatment as adult worm burden decreases and pulmonary inflammation resolves. Faecal Baermann examination or PCR should be repeated 4 to 6 weeks after treatment to confirm larval clearance. Thoracic radiographs may show persistent abnormalities for several months after parasitological cure due to ongoing resolution of granulomatous inflammation — this does not indicate treatment failure if the cat is clinically improving.

Prevention

For cats with outdoor access in endemic areas, monthly application of an antiparasitic product with demonstrated activity against Aelurostrongylus abstrusus (imidacloprid/moxidectin or selamectin) is the most effective preventive strategy. Restricting hunting behaviour — difficult in practice for outdoor cats but achievable through supervised outdoor time, leash walking, or enclosed outdoor runs — reduces exposure risk. Thorough cooking of any prey-derived food eliminates infective larvae.

Key Takeaways

• Aelurostrongylus abstrusus is an underdiagnosed cause of chronic cough and respiratory disease in outdoor and hunting cats
• Infection requires ingestion of infected molluscs or, more commonly, paratenic hosts (birds, mice, frogs, lizards)
• Pooled faecal Baermann technique or PCR are the diagnostic methods of choice; repeat testing improves sensitivity
• Emodepside/praziquantel (Profender) and imidacloprid/moxidectin (Advocate) are first-line licensed treatments with high efficacy
• Monthly antiparasitic spot-on application is recommended for ongoing prevention in outdoor cats in endemic regions
• Severe infections require more intensive therapy and respiratory support; always consider lungworm in cats with chronic cough and outdoor access

References

1. Traversa D et al. (2010). Feline lungworms unleashed: the past, the present and the future. Trends Parasitol 26(4):209–18.

2. Brianti E et al. (2012). Troglostrongylus brevior and Troglostrongylus subcrenatus in domestic cats. Parasitol Vectors 5:104.

3. Pennisi MG et al. (2015). Lungworm disease in cats: ABCD guidelines on prevention and management. J Feline Med Surg 17(7):626–36.

4. Conboy G (2009). Helminth parasites of the canine and feline respiratory tract. Vet Clin North Am Small Anim Pract 39(6):1109–26.

5. Traversa D, Di Cesare A (2013). Feline lungworms: what a dilemma. Trends Parasitol 29(9):423–30.

6. Crisi PE et al. (2017). Efficacy of emodepside/praziquantel spot-on solution against Aelurostrongylus abstrusus in cats. Parasitol Res 116(3):891–9.