The spread of Angiostrongylus vasorum across the UK over the past two decades alongside media campaigns and research into its epidemiology, diagnosis and prevention has led to raised awareness of this parasite. In comparison, the impact of feline lungworms on the health of UK cats has been relatively overlooked with knowledge gaps persisting in their UK distribution and pathogenicity. While cats are not infected with A. vasorum, they can be affected by two lungworms of veterinary significance that have been demonstrated to be endemic in the British Isles. These are Aelurostrongylus abstrusus and Eucoleus aerophilus.
Aelurostrongylus abstrusus has a worldwide distribution and is thought to be endemic in most European countries including the UK and Ireland. It has an indirect life cycle with first-stage larvae (L1) passing out in the faeces of cats and molluscs acting as intermediate hosts. A number of reptiles, amphibians and birds can act as paratenic hosts, making hunting cats at greater risk of infection (Traversa and Di Cesare, 2013). Adult worms live in the lung parenchyma and small bronchioles with small foci in the lung tissue, although larger foci of up to 1cm with wider areas of consolidation can occur. Muscular hypertrophy and hyperplasia of the bronchioles, alveolar ducts and pulmonary arteries are typical pathological changes.
Although A. abstrusus infections are commonly of low pathogenicity and reports of fatal cases are rare, infected cats commonly present with respiratory signs. The most common clinical presentation is a mild to moderate chronic cough, but many other signs may be present including sneezing, wheezing, mucopurulent nasal discharge and dyspnoea, with or without tachypnoea. These in turn may lead to lethargy, anorexia and weight loss. Fatal cases are rare but can occur, especially in kittens and the immune suppressed. A fatal case of aelurostrongylosis was recently reported in a UK kitten with an exceptionally high worm burden (Dobromylskyj et al., 2019), demonstrating the need for A. abstrusus to be considered as a differential for severe as well as mild respiratory signs in both adult cats and kittens.
Diagnosis relies upon Baermann faecal analysis (Figure 1) for the detection of L1 larvae. In experienced hands this can be highly specific but is relatively insensitive as larvae are only shed intermittently and it requires examination of fresh faecal samples over three consecutive days to improve sensitivity. L1 larvae of A. abstrusus are typically 360 to 400μm in length with a kinked “s”-shaped tail, dorsal and ventral incisures and a knob-like projection (Figure 2). In cats that have travelled to continental Europe, Troglostrongylus brevior should be considered as a differential.
T. brevior L1 larvae are morphologically very similar to A. abstrusus and distinguishing between them is a specialised task (the “s”-shaped tail is less kinked in T. brevior). Indeed, the wide variation in lengths reported in the length of T. brevior L1 larvae (300 to 500μm) may be due to A. abstrusus L1 larvae being misdiagnosed as T. brevior.
Treatment and prevention
Fenbendazole is licensed for the treatment of A. abstrusus infection and is effective at 50mg/kg daily for at least three days, although longer periods of treatment (up to seven days) may be required. In recent years, spot-on solutions containing eprinomectin (Broadline) and emodepside (Profender) have also gained treatment licences for A. abstrusus, offering treatment flexibility to improve compliance. Other spot-on preparations have been demonstrated to have 100 percent efficacy against the parasite, including moxidectin (Advocate) and selamectin (Stronghold) but their use is off licence (Traversa et al., 2009).
This apparent susceptibility to a wide range of anthelmintics in routine prophylactic use against a range of other parasites may explain why prevalence in domestic populations of cats remains low when compared to higher prevalence in feral cat populations. Lifestyle, however, is also likely to play a role in this variation with indoor cats having been demonstrated to have a lower prevalence of infection (Elsheikha et al., 2019). Response to treatment is often excellent with reversal of pathological changes (with the exception of the muscular hypertrophy) and resolution of clinical signs.
With the potential for fatal cases and chronic respiratory changes, preventative treatment is warranted in cats at high risk of infection. The parasite has been demonstrated to be present in the English and Irish cat population at low prevalence and therefore is likely to be present at least focally across the whole of the UK. A recent study found prevalence to be significantly higher in cats living in the south east of England, but it is more likely that lifestyle is a greater risk factor for infection in cats than geographical location (Elsheikha et al., 2019).
Prevention should be considered in cats with a history of hunting and those with outdoor access. Emodepside is licensed for use in cats at risk of infection and it is likely that routine use of emodepside or a macrocyclic lactone will have a protective effect against clinical aelurostrongylosis. Limiting access to paratenic and mollusc hosts by keeping cats indoors is also likely to be an effective preventative measure but not always practical.
Canine and feline respiratory infection caused by E. aerophilus is sporadic across Europe and most cases are subclinical. Clinical cases in cats, however, have been reported (Barrs et al., 2000; Foster et al., 2004). In Europe, the nematode is commonly found in wildlife, with foxes being a reservoir in the UK and Ireland (Morgan et al., 2008; Wolfe et al., 2001) but recently it has been identified in companion animals including cats. Knowledge of epidemiological data (eg range of hosts and geographic distribution) of E. aerophilus in Europe is fragmentary but it would appear that cats are occasionally infected due to reservoirs of infection maintained in wildlife hosts such as foxes where prevalence can be high.
Typical clinical signs associated with infection include a cough (productive or unproductive), sneezing and dyspnoea, with or without tachypnoea. E. aerophilus infection should be considered as a differential in cats presenting with these signs. Diagnosis is achieved by identification of the lemon-shaped, slightly asymmetrical bipolar plugged eggs by faecal flotation (Figure 3). Shedding of these eggs may be intermittent and faeces should be tested over three consecutive days.
Treatment and prevention
There is no product licensed for the treatment or prevention of E. aerophilus infection and data on efficacy of anthelmintics against the parasite are lacking. One recent study, however, demonstrated a spot-on preparation of moxidectin (Advocate) to have close to 100 percent efficacy against infection (Traversa et al., 2012). Prevention of exposure to environmental infection is difficult without keeping cats indoors, but this should be considered in repeatedly infected cats or the use of a monthly moxidectin/imidacloprid preparation off licence where this is not practical.
Lungworm infections can cause significant lung disease and are often overlooked in cats presenting with acute and chronic respiratory signs. They should be considered as a differential in cats with relevant clinical presentations, especially those that hunt or have regular outdoor access. Recommendation of monthly emodepside or macrocyclic lactone use is warranted as a preventative measure in cats at higher risk of infection or switching to an indoor lifestyle where this is practical. Pet travel and climate change will only make feline lungworm infections more clinically relevant in future years, with the possibility of exotic feline lungworms such as T. brevior establishing. Veterinarians must therefore be prepared to encounter feline lungworms in practice and give accurate preventative advice to clients to help keep UK cats free from parasitic respiratory disease.