Feline lungworm and heartworm

Introduction

There is a fairly common belief that while lungworm and heartworm are a threat to dogs, the risk to cats is marginal, and possible cases of infection are not clinically relevant. Veterinarians are well aware of the risk of angiostrongylosis and dirofilariosis, life-threatening cardiopulmonary diseases in dogs which are endemic in many parts of the world, but cats do not become infected with Angiostrongylus vasorum, while Dirofilaria immitis infection is much less clinically significant. However, the cat lungworm Aelurostrongylus abstrusus, previously considered rare and of low pathogenicity, is now regarded as the most prevalent respiratory nematode in domestic cats worldwide. In addition, Troglostrongylus brevior, recorded in southern Europe and until recently associated with wild felids, has now been recognized as a significant cause of severe bronchopneumonia in kittens and young cats, whilst Capillaria aerophila (syn. Eucoleus aerophilus), which is found in dogs and wild carnivores, is now increasingly being detected in cats. A brief overview of the clinical signs caused by feline parasitic cardiopulmonary infections is presented in Table 1.

Table 1. Clinical symptoms of lungworm and heartworm infections in cats.

 

Aelurostrongylus abstrusus

Extensive research into the epidemiology and diagnosis of A. abstrusus in pet cats over the past decade has widened awareness of the threat posed by this parasite, resulting in an increasing number of diagnosed cases of aelurostrongylosis, although it cannot be unequivocally stated that there has been a real increase in the prevalence and geographical expansion of the parasite. However, A. abstrusus remains the most prevalent lungworm worldwide in domestic cats, often simply referred to as the “cat lungworm” because it is considered to be species-specific 1,2,3 Aelurostrongylosis is one of the most clinically important feline parasitic diseases, as the nematode is endemic in many countries, with a reported prevalence of up to 35% (Table 2). A European survey has shown that aelurostrongylosis is a threat to all domestic cats that have regular outdoor access, irrespective of their age, gender or geographical location 4. A. abstrusus may also infect wild felids that prey on rodents and birds and live in sympathy with domestic cats 3; for example, the lungworm has been identified in European wildcats (Felis silvestris silvestris) in Italy (Figure 1), with a prevalence of 62.5% and the cause of severe lung damage 5.

Table 2. A summary of reports on the prevalence of feline cardiopulmonary parasites.

Life cycle

The adult nematode is small in length (males 5-10 mm, females up to 10 mm) and very thin (less than 100 µm or slightly thicker than a hair). It inhabits the respiratory bronchioles, alveolar ducts and alveoli, but also occurs in subpleural nodules in the lung parenchyma 1,3,6. The life cycle is indirect, with the intermediate hosts being terrestrial mollusks, but probably also aquatic snails and slugs. Amphibians, reptiles, birds and rodents play an important epidemiological role, as they become paratenic (reservoir) hosts when they eat infected mollusks. Cats become infected after ingestion of larvae (L3) in paratenic hosts, or (less frequently) by direct ingestion of infected mollusks (Figure 2). The prepatent period is typically 35-48 days but can be up to 63 days 7. The female worms lay eggs that hatch within the alveolar ducts; L1 larvae migrate to the pharynx and are then coughed up, swallowed and passed in feces. Once in the environment they penetrate snails or slugs, where they develop into infective L3 larvae. Excretion of larvae in the cat’s feces is most intense at 10-14 weeks after infection, but can last from several months to more than a year in individual animals 1. Shedding of larvae may be intermittent, which makes them difficult to detect, with infection in the host cat lasting for up to two years 8.

Epidemiology

The spread of A. abstrusus is favored by a humid environment and proximity to water 9, as L1 larvae can survive in moist habitats for up to 5 months 8. L3 larvae may overwinter in hibernated terrestrial snails (Helix aspersa) for at least 120 days, which significantly increases the likelihood of infection of paratenic hosts 6, and can also survive in dead snails for around 3 weeks 8. L3 larvae have been found in mucus released by infected snails and in water containing dead infected mollusks 10. In paratenic hosts, L3 larvae are viable and capable of infecting a cat for at least 12 weeks 8.

A cat’s lifestyle significantly influences the risk of infection, as animals with outdoor access are more prone to ingest A. abstrusus; the prevalence in free-roaming cats may reach 50% 1,6. It is not yet clear whether age is a risk factor for aelurostrongylosis, but large-scale studies conducted in 12 European countries showed that infections were most frequently diagnosed in animals between 6 months to 2 years of age 4. Extensive research in the United States has also suggested age to be a significant risk factor; one study showed 5.15% of cats aged 1-12 months and 0.65% of cats older than 12 months were positive for A. abstrusus 9. However, other surveys report lungworm to be more common in adult cats that have a greater hunting ability and hence an increased chance of ingesting larvae within the tissues of paratenic hosts 1.

Clinical signs

A. abstrusus generally causes mild clinical signs, but an infected cat can be asymptomatic or, if infection is severe, may develop signs of bronchitis and pneumonia 1,6,8, with adult parasites and hatching L1 larvae causing damage to the lung parenchyma. The most frequent signs are mild to intense coughing, sneezing, wheezing, dyspnea, tachypnea, open-mouthed abdominal breathing and mucopurulent nasal discharge. Apathy, lethargy and weight loss has also been described 1,8,11. Fatalities are rare but can occur in severe cases, especially in young, weakened or immunosuppressed individuals 1.

Diagnosis

Diagnosis of aelurostrongylosis relies upon the detection of L1 larvae in the feces (Figure 3), with the Baermann method regarded as the gold standard, as it has much higher sensitivity than the classical flotation technique 2,6,9,11 and also provides quantitative information on the level of larval burden, which correlates with the severity of the disease. However, it is not possible to diagnose infection in the pre-patent stage, and because larval shedding is intermittent, the test should be performed three times to exclude false negative results. A major limitation of the Baermann test is that it takes 12-24 hours to perform, and the fecal sample must be fresh (or refrigerated for no more than a few days) to obtain motile larvae 2. The flotation method is less sensitive 9, and larvae may also be damaged in the saturated solutions 2,6. An alternative option is the recently developed FLOTAC method, which can identify larvae in preserved or frozen fecal samples 1,6. The larvae can be also identified from tracheal washes, but bronchoscopy and bronchoalveolar lavage are invasive techniques, especially in cats with respiratory signs, and not reliable for lungworm detection 2. Identification of A. abstrusus larvae requires experience; the length of the larva is not a sufficient feature for their identification and differentiation from T. brevior, so a complete morphological and morphometric characterization is necessary, alongside (if possible) molecular confirmation 2,3. Although the reported length of L1 is 300-400 μm, a broader range is 210-495 μm, which coincides with the values for T. brevior (203-521 μm) 2,3,4. A characteristic morphological feature of A. abstrusus L1 is the caudal section, with its kinked “S”-shaped tail with dorsal and ventral incisures and a terminal knob-like projection 2,6,12.

An alternative diagnostic method is genetic testing, for example using nested PCR 13. Serological tests have been developed to detect antibodies to A. abstrusus, but commercial in-clinic rapid tests are not currently available 2,9.

Imaging (thoracic radiography, computed tomography) should be performed as a preliminary investigation in cases of suspected pulmonary inflammation, which may show disseminated interstitial and peribronchial changes. However, confirmation of a parasitic etiology is still required, and differentiation from other feline lung nematodes is advisable 2,6.

Treatment

Possible treatment options to control a diagnosed infection of A. abstrusus are summarized in Table 3. Formulations containing moxidectin, eprinomectin or emodepside are licensed in many countries, with a dosing regimen that varies with the product. At least one spot-on product containing imidacloprid 10%/moxidectin 1% is licensed in Europe for adulticidal therapy of A. abstrusus in cats 2. A recent study has shown that a single dose of a topical formulation of 2 mg moxidectin + 40 mg fluralaner/kg is fully effective in preventing aelurostrongylosis for at least 12 weeks in 5-7-month-old cats 14. Another report confirmed that a spot-on moxidectin 1% (1 mg/kg) + imidacloprid 10% (10 mg/kg) product administered monthly was efficacious in preventing lung damage and patent A. abstrusus infection, while for 100% effective treatment of an existing infection three applications at monthly intervals were required 15.

Table 3. Possible options for treatment and prevention of feline lungworm and heartworm.

^P - only to prevent infection (larval treatment); +? – efficacy based on single study/no data on efficacy
Note: not all products will be licensed for use in all countries.

 

There are only preliminary data indicating efficacy of milbemycin oxime and selamectin against A. abstrusus, but both molecules in products currently marketed for cats were found effective in stopping larvae shedding and allowing clinical recovery after a single administration 2. Oral fenbendazole, approved in some countries for treating aelurostrongylosis, is effective at a dose of 50 mg/kg daily for at least three days 2,12, or 20-50 mg/kg for 5-15 days 8.

Troglostrongylus brevior

The recent large survey performed in Europe confirmed T. brevior to be the second most frequently detected feline lungworm species, reporting that out of 210 affected cats, 78.1% were infected with A. abstrusus and 19.5% infected with T. brevior 4. T. brevior has a limited geographic distribution and is uncommon in cats, but it has been increasingly diagnosed and is more widespread than previously thought (Table 2) 4. The risk of feline troglostrongylosis has been recognized in the last decade, with case reports in domestic cats in southern European regions where the parasite is endemic in wildcats, which are considered to be the natural hosts of the parasite 3. T. brevior has been found in cats in the Mediterranean region (Italy, Greece, Albania, Bulgaria, and the major Mediterranean islands) 3,4 but there are also reports of it in a Polish cat and in wildcats from Germany and Romania 16. Cases of troglostrongylosis are mainly in kittens and young adults, where – in contrast to aelurostrongylosis – the disease is often severe and sometimes fatal 2,3. An Italian study of 575 domestic cats reported T. brevior infection in 18.2% of kittens under 6 months of age and in 3.2% of cats aged 6-24 months, while no infection was found in cats over two years 17.

Life cycle and clinical signs

The life cycle of T. brevior is very similar to that of A. abstrusus, involving terrestrial mollusks as intermediate hosts and lizards, birds and small mammals as paratenic hosts. A recent study in Italy noted T. brevior larvae in the land snail Cornu aspersum, and in rats and lizards 18. The nematode inhabits the cat’s upper respiratory tract (i.e., the trachea and bronchi) 2,13,17,19, but a feature of this parasite’s infection is the possibility of vertical transmission from queen to offspring; mature T. brevior have been found in two 18-day-old kittens with severe respiratory clinical signs, indicating that larvae could have passed to them via the queen’s colostrum or milk 18. Such early patent infection in kittens suggests that transplacental transmission cannot be excluded; a pre-patent period of 24 days has been recently identified after experimental infection of 5-6-month-old kittens with L3 larvae from paratenic hosts 16.

As noted above, animals most susceptible to T. brevior infection are cats under a year of age with outdoor access 13,17. Clinical presentations include bronchitis with significant catarrhal exudate from the airways, sneezing, coughing, dyspnea, tachypnea and ocular discharge. Other non-specific signs, such as anorexia, hyperthermia or hypothermia, dehydration, poor body conditions, apathy and lethargy, are also reported 2,13,19. In adult cats, the infection may be subclinical or cause only mild symptoms 2.

Diagnosis and treatment

In cats with respiratory signs and outdoor access, a first step in the diagnostic work-up should be fecal examination by Baermann test for L1 larvae 13, as discussed above for A. abstrusus. Adult T. brevior specimens are approximately 14 mm long and 0.4-0.5 mm wide, much wider than A. abstrusus. In endemic southern Europe areas mixed infections with both parasites should be considered, which often results in a more severe clinical presentation and more substantial radiographic alterations than in single species infections 2.

At present most countries only have a licensed topical product containing eprinomectin for treatment and prevention of feline troglostrongylosis with monthly administrations 2,4,13,20 (Table 3). However, high efficacy of a single administration of a combination containing fipronil, (S)-methoprene, eprinomectin and praziquantel is reported in domestic cats 4. A recent controlled study in 5-6-month-old kittens showed eprinomectin in the same multi-component product to be 100% efficacious against developing larval and adult T. brevior, and prevented the occurrence of clinical signs when administered during the prepatent period 16. In pilot trials under natural conditions, moxidectin and emodepside also appear to be successful. Moxidectin 1% spot-on proved to be highly effective at stopping shedding of T. brevior larvae after a single administration in naturally infected cats, and a topical product containing emodepside was 100% efficacious with two applications two weeks apart in reducing fecal larvae and achieving a clinical recovery 2. If the parasite causes severe dyspnea in kittens and young cats, and because sudden death is a risk, appropriate supportive care measures are essential in these cases whatever anthelmintic is chosen 13.

Capillaria aerophila

C. aerophila, along with A. abstrusus, is seen as the most important cause of parasite-induced respiratory infection in felids 4. C. aerophila has low host specificity and will infect both canids and felids, with the red fox considered its main reservoir. The worms are located in the submucosa of the trachea, bronchi and bronchioles, with the female nematode laying eggs which are the coughed up, swallowed and passed in the feces, reaching the infective stage after 30-45 days. The life cycle is direct, so cats usually become infected by ingesting larvated eggs, but may also become infected by eating earthworms, which act as paratenic hosts 6. After hatching in the intestine, the larvae reach the lungs via the circulatory system, with a prepatent period of around 40 days. C. aerophila is considered to have low prevalence in cats, with a range in domestic European cats with regular outdoor access reported to be between 0.6-10.8% (Table 2), and infection being most frequently found in animals aged from 6 months to 2 years 4.

In most infected cats, C. aerophila induces chronic bronchitis, with clinical signs including coughing, sneezing, dyspnea and tachypnea. Diagnosis relies on standard fecal flotation to identify the characteristic lemon-shaped, slightly asymmetrical eggs with bipolar plugs (Figure 4), but intermittent excretion of eggs can make diagnosis difficult, so fecal samples should be examined for three consecutive days. Apart from anything else, it is important to diagnose and control infections in cats and dogs due to the zoonotic potential of C. aerophila 6.

At present, only a topical product containing moxidectin 1% is licensed in some countries for treating feline capillariosis 2,21. However, eprinomectin in combination with fipronil, (S)-methoprene and praziquantel has proved to be 100% effective in treating naturally infected cats with a single administration 4, and emodepside appears to control the parasite in field trials 2.

Prevention of lungworm

Feral and free-roaming cats and those with outdoor access are at a higher risk of infection with A. abstrusus, T. brevior and C. aerophila. Keeping cats indoors to limit access to paratenic and mollusk hosts is likely to be effective at preventing infection, but is not always practical 6,12 and does not comply with good animal welfare. The appropriate prevention of feline lungworm is via the regular use of anthelmintics, as recommended for dogs and cats in areas endemic for heartworm. Monthly applications with emodepside or macrocyclic lactone is justified in cats with an identified risk of aeluro- and/or troglostrongylosis, and to prevent the development of chronic pulmonary changes as a result of undiagnosed disease 6.

Heartworm

Unlike dogs, cats are less susceptible to infection with D. immitis, with only a few parasites reaching the pulmonary artery and developing into the adult stage; the life cycle is shown in Figure 5. The pre-patent period lasts longer in cats (7-9 months) than in dogs (6-7 months), with microfilariae developing in only 20-50% of affected cats and lasting for 1-3 months. This makes detection of larvae in blood difficult. Infections correspond to areas endemic for heartworm in dogs (i.e., the tropics and subtropical zones, and in adjacent temperate zones), and in Europe D. immitis infections in dogs are usually found in the Mediterranean countries, but also in areas further north (Germany, Austria, Hungary, Czech Republic, Slovakia and Poland) 8,22. In the United States, heartworm is considered at least regionally endemic in each of the contiguous 48 states, and overseas territories and protectorates (Hawaii, Puerto Rico, US Virgin Islands and Guam) 23.

Clinical signs

In most cats, heartworm infection is asymptomatic for a long time, and frequently goes undiagnosed; spontaneous self-cure is not uncommon. Serious signs can appear when juvenile heartworms establish in the pulmonary artery (3-4 months post infection) or when natural death of the adult parasites occurs. Cats may develop an acute illness with respiratory signs (coughing, dyspnea, hemoptysis) and vomiting, but sudden death can occur in asymptomatic cats. Feline heartworm disease is now recognized as a significant pulmonary syndrome, the so-called “Heartworm Associated Respiratory Disease” (HARD), with clinical signs that can include anorexia, lethargy, coughing, tachycardia, vomiting, diarrhea, blindness, convulsions, collapse and sudden death 22. Ectopic localization from aberrant migration of larval stages into the central nervous system, body cavities or femoral artery are rare (but more common than in dogs), and may cause neurological manifestations (e.g., blindness, ataxia, paresis) and bleeding.

Diagnosis

Detection of microfilariae in the blood – a basic diagnostic test for canine heartworm infection – is very difficult and unlikely to be successful in cats, due to the short-term, low-level microfilaremia. The most reliable option is to use a combination of various diagnostic methods, including imaging 8. Serological testing to detect antigens from female D. immitis is highly specific, but negative results do not exclude existing infection; false negatives may be obtained when only male worms or immature stages are present. Anti-D. immitis antibodies can be found 2-3 months after initial infection, but long-term persistence of antibodies in self-cured cats may lead to a high false-positive rate.

Thoracic radiography is indicated for both diagnostic and prognostic reasons, and cardiac ultrasound should be performed for all suspected feline heartworm infections, as its sensitivity and specificity are very high, with scanning allowing direct visualization of the parasites within the right ventricle and pulmonary artery 22.

Prevention

The elimination of adult D. immitis is not recommended in cats for several reasons; firstly, there is no drug approved for the removal of adult D. immitis in cats, and melarsomine (the arsenical compound used to treat canine heartworm) is unsafe for cats. Secondly there is a high risk of pulmonary thromboembolism and anaphylactic reactions following the death of heartworms in treated cats, and thirdly most cats do not show clinical signs of heartworm infection and will spontaneously self-cure. However, supportive therapy (e.g., prednisolone) is recommended to reduce the consequences of respiratory distress in animals that show more severe clinical signs. Infected cats should be monitored (via thoracic radiography and echocardiography) and tested for antigens (and antibodies) in order to decide on further treatment and to determine if self-curing.

In areas endemic for D. immitis, because of the risk of potentially fatal HARD and the lack of licensed adulticidal drugs, prophylactic larval treatment is recommended (Table 3). This involves monthly dosing throughout the mosquito season with either spot-on (selamectin, moxidectin, eprinomectin) or oral (milbemycin oxime) preparations 8. Cats moving from non-endemic to endemic regions should be treated within 30 days of arrival in the risk area 22.

Conclusion

The geographical distribution of feline lungworms is regional, with endemic spots often associated with the occurrence of wild felids. In feral and stray cat populations the prevalence may be high, with the animals acting as a parasite reservoir of lungworms in some environments. Lung parasites should be suspected in cats with respiratory disease that show no clinical improvement with antibiotic therapy, but all pet cats with a history of hunting and regular access to outdoors that present with either acute or chronic respiratory signs should be tested for lungworm via fecal sampling. Prognosis should be based primarily on physical examination and the severity of the clinical signs. Lastly, it is prudent to remember that detection of cats with C. aerophila is important, as it is a zoonotic parasite which may cause severe pulmonary disease in humans.