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Aelurostrongylosis of the cat

Last updated October 30, 2012

Aelurostrongylosis (feline lungworm) is a parasitic disease caused by Aelurostrongylus abstrusus, a metastrongylid nematode that, in its adult stage, infests the bronchioles, alveolar ducts and alveoli of the cat, its definitive host. It is often considered a self-limiting and/or asymptomatic disease, despite the fact that it can be associated with relevant clinical symptoms and can sometimes be fatal, particularly when the infected animal is very young or frail for other reasons.  Over the last decade there has been an increase in reports of this infestation both in epidemiological studies and in clinical cases in Italy, as in other European countries and extra-European ones. These reports bear witness to the important role that A. abstrusus plays in feline clinical practice and to the possible emergence of the parasite in various different geographical regions.

 

THE PARASITE 


The adult stages of A. abstrusus are very small, having a diameter of about 50-80 µm and length of 4-8 mm (males) or 9-10 mm (females). They live mainly in the nodules that form in the bronchial tree, the alveolar ducts and the alveoli of the definitive host, the domestic cat. Some other wild felines and, occasionally, canine species can become the definitive host for A. abstrusus. After mating the female adult lays eggs that mature and hatch in the pulmonary parenchyma releasing first stage larvae (L1). The L1 (Fig. 1) actively migrate up through the respiratory tree in about 15 days, reaching the trachea, where, helped by the coughing of the infested animal, are pushed into the larynx and pharynx and, subsequently, swallowed. The L1 pass unharmed through the gastrointestinal tract of the cat and, in the faeces, reach the exterior where they continue their development in intermediate hosts, which are various species of terrestrial gastropod molluscs (e.g., Agriolimax spp., Helminthoglypta spp., Helix spp., Helicella spp., Levantina  spp., Limax spp., Cernuella spp.). The larvae moult twice within the intermediate host, in about 40-60 days depending on the species of mollusc and environmental temperature, reaching the third larval stage (L3), which is the parasite of the definitive host. Other animals besides the cat, such as birds, reptiles and small rodents, can act as paratenic hosts, in which the parasite remains alive and infestant, but does not continue its development.

These animals can ingest the mollusc and, with it, the larvae of A. abstrusus, which localise to various districts where they survive for long periods waiting for their biological cycle to be re-activated. The parasite’s life cycle continues when a receptive cat eats the infested slug or snail or prey acting as a paratenic host of the nematode. Following ingestion, the L3 penetrate the gastrointestinal mucosa of the animal and, through the lymphohaematic circulation, reach the pulmonary parenchyma, where they moult twice more in about 2 weeks and, subsequently, reach sexual maturity and continue their biological cycle. Following mating, the female nematode starts to lay eggs about 1 month post-infection (p.i.). The L1 begin to be present in the faeces from the 5th to 6thweek p.i. and elimination lasts from months up to 1-2 years.

The factors influencing the presence and distribution of feline aelurostrongylosis are mainly the climate and the ubiquitous spread of the intermediate and paratenic hosts, besides the receptivity of the feline population, given the temperament and habits of this species of animal. The availability of paratenic hosts (e.g., small birds, mice, lizards) affects the epidemiology of the parasitosis to a greater extent than that of the intermediate hosts because the cat is more easily infected by eating vertebrate prey rather than by eating the molluscs themselves, which are less palatable. Furthermore, the paratenic hosts offer the parasite the possibility of being present in the territory throughout the whole year and, unlike the molluscs, their availability is not subject to the problems of survival related to seasonal changes in the climate. Factors intrinsic to the definitive host which  favour infection are the age of the cat and its life style, as well as access to prey or, more generally, the external environment where it can hunt molluscs and/or paratenic hosts or play with them, ingesting infected tissues. Cats are particularly receptive to infection by A. abstrusus when they have never previously been in contact with the parasite and are, therefore, sensitive from an immunological point of view. The rate of infection is higher in young cats which live in rural or peri-urban areas, with access to non-domestic environments, and with the possibility of coming into contact and ingesting intermediate and/or paratenic hosts.

A abstrusus is present worldwide and in Europe is endemic particularly in Mediterranean regions, although it has also been reported in central and northern areas of the continent (e.g., the Netherlands, Denmark, United Kingdom, Norway, Germany) and eastern parts (e.g., ex-Yugoslavia, ex-Czech Republic, Poland, Hungary). In Italy, the nematode is present throughout the country, with the rate of infection reaching 20% in some regions (e.g., Lazio, Abruzzo and Puglia).

 

PATHOGENIC ROLE AND SIGNS


The signs of aelurostrongylosis of the cat vary greatly and, in most cases, are non-specific and the same as those occurring in other feline respiratory disorders, such as viral and bacterial infections, and parasitic diseases caused by fungi (e.g., blastomycosis, cryptococcosis, histoplasmosis), protozoa (e.g. toxoplasmosis) or helminths (e.g. paragonimiasis, pulmonary capillariasis), lymphoplasmacytic or allergic rhinitis, presence of foreign bodies, nasopharyngeal polyps, asthma, eosinophilic bronchopneumonia, and malignancies. Above all, if the animal is adult and in perfect health and/or the parasite load is low, the infection can be asymptomatic or only mildly symptomatic, or be self-limiting.

When present, the signs are caused by the host’s immune response to the cuticular antigens released during the moults from the L3 larval stage to the adult parasite, from the deposition of eggs, capable of causing an inflammatory cell infiltration and formation of nodules, abscesses and granulomata, and by the mechanical and traumatic effect of the L1 as they migrate actively through the respiratory tree.

The clinical picture is usually chronic, with signs occurring from 1 month up to 7-24 months p.i. even though, obviously, the timing of the appearance and duration of the signs cannot be confined into a precise time schedule. The course of aelurostrongylosis is more severe in immunocompromised and immunodeficient animals, in young cats and in those with a high parasite load. Repeated infections, even when caused by a low number of parasites, can lead to a severe clinical picture and to the death of the host. However, when repeated infections occur with a lower parasite load, the host can counteract the pathogenic evolution of the parasitosis and the (re)-appearance of signs.

The most common sign is a mild to moderate chronic, persistent cough that is resistant to antibiotic therapy, often not productive and which worsens after physical exercise or clinical manipulation. The cough is often accompanied by a nasal discharge and sneezing, nasal and/or oculo-conjunctival mucopurulent discharge and, sometimes, fatigue, dyspnoea/tachypnoea, weight loss, loss of appetite, vomiting and diarrhoea. The symptoms may worsen, particularly between 6 and 13 weeks p.i., the period in which egg laying reaches its maximum. In the particularly severe cases, caused by a high number of adults and the simultaneous deposition of hundreds of eggs, cardiac complications, pneumothorax, paroxysmal cough, asthmatic crises and respiratory failure can occur, especially in the period immediately preceding death.

 

DIAGNOSIS


Given the non-specific signs, it is extremely difficult, and perhaps impossible, to make a clinical diagnosis of aelurostrongylosis. Nevertheless, the diagnosis can be suspected on the basis of the data reported and the clinical history, when compatible with the typical risk factors for A. abstrusus infection. Imaging techniques can provide valid support for a suspected diagnosis, but cannot make a definitive diagnosis, given that the lung alterations differ depending on the period of the infection and the degree to which the parenchyma is affected and, above all, because the aetiological agent cannot be identified. In any case, between 3 and 5 weeks p.i., radiological studies show slight bronchial opacity and focal alveolar lesions, while from 5 to 15 weeks p.i., a generalised alveolar pattern appears, particularly in young cats and those with severe infections. Between 11 and 40 weeks p.i. the more serious changes become evident, such as increased interstitial opacity, thickening of the bronchial walls, increase in the vascular pattern, particularly in the right caudal lobe, pulmonary congestion, enlargement of the lobar pulmonary arteries and pleural effusions.

The only way of making a definite diagnosis is to demonstrate the presence of A. abstrusus in the affected cat. However, independently of the method used, false negative results must always be considered, given that the larvae are never eliminated constantly by the host, particularly in the more advanced stages of the disease. During and after the first month from the start of the period of patency, the larvae are eliminated almost daily, while in the second month the number of eggs laid by the females can decrease and the larvae are present only intermittently. The L1 of A. abstrusus can be detected in bronchoalveolar lavage fluid, although almost always in low numbers and this diagnostic approach is, in any case, complicated in the cat. For this reason, copromicroscopic examination of the faeces, carried out on samples collected for 3 consecutive days or on alternate days during the period of 1 week, is the method of choice for diagnosing aelurostrongylosis.

The flotation method, using saturated solutions of sucrose, sodium chloride or zinc sulphate, with a specific gravity of 1200-1350, can, sometimes, enable the detection of larvae. Nevertheless, in most cases they cannot be identified because the solution, if left too long in contact with the sample, alters the normal morphology of the parasites and, therefore, makes them difficult to recognize (Fig. 2), or causes them to precipitate so that they do not float and cannot be found.

The gold standard for the diagnosis of feline aelurostrongylosis is the Baermann technique, which allows to isolate and identify larvae in their active and viable stage. This method exploits the marked hydrophilia of the L1 of A. abstrusus (Fig. 1) which, once isolated, must be identified on the basis of their morphometric characteristics (length of 300-390 mm) and tail morphology, with an S-shaped caudal extremity that has a short dorsal spine (Fig. 3).

Although the Baermann technique is the most accurate diagnostic method, its sensitivity is less than 100%, it is relatively time-consuming (it takes about 24-36 hours) and requires specific skills to identify the parasite. Furthermore, it can produce false negative results in the presence of an infection, both because elimination of the larvae is intermittent and because some cats can have symptoms even before the A. abstrusus reaches sexual maturity.

Molecular biology methods have recently been developed which are able to detect parasite DNA both in the faeces and in the mucus of infected cats, even when the samples are negative according to the classical diagnostic techniques. However, these approaches are not currently used in routine clinical practice.

 

TREATMENT


When the parasite load is low and symptoms absent or very mild, the infection may be self-limiting and, therefore, symptomatic and supportive therapy, aimed at keeping the inflammatory state and any bacterial inflammation under control, is sufficient. In contrast, in cases with clear or severe symptoms, or recurrent cases, antihelminthic treatment is necessary.  Over the years numerous compounds have been evaluated as treatments for aelurostrongylosis, with the benzimidazoles and macrocyclic lactones (macrolides) being the mostly widely used.

Fenbendazole at a dose of 50 mg/kg per os for 3-15 consecutive days is effective at eliminating the parasite in most cases although there are reports of the larvae reappearing in the faeces 2-3 weeks after the treatment. The macrocyclic lactones include ivermectin, which parenterally administered ‘off label’ is variably effective, depending on the dose, although cases of overdose and even death in kittens have been reported following the use of this compound. Selamectin, also administered ‘off label’ as a spot-on treatment, has poor efficacy unless followed by repeated administration of fenbendazole.

The efficacy and safety of three formulations containing different antihelminthic compounds were recently compared in the treatment of aelurostrongylosis: an oral meal containing fenbendazole 18.75%, administered at a dose of 50 mg/kg for 3 consecutive days, and two spot-on products containing imidacloprid 10%/moxidectin 1% and emodepside 2.1%/praziquantel 8.6%, applied as a single dose.

The three formulations were found to be safe and effective. The spot-on single dose emodepside was equally effective as repeated oral doses of fenbendazole (~99%) while the spot-on product containing moxidectin had an efficacy of 100%. Repeated administration of oral products can be difficult, particularly in unruly, wild or feral animals; while the spot-on preparations have the advantages of being easy to apply to the skin and requiring only a single dose.

 

Suggested readings


  1. Traversa D., Guglielmini C. 2008. Feline aelurostrongylosis and canine angiostrongylosis: a challenging diagnosis for two emerging verminous pneumonia infections.Veterinary Parasitology 157, 163-174.
  2. Traversa D., LiaR.P., BoariA., Di Cesare A., CapelliG., MililloP., Otranto D.2009. Indagine epidemiologica sull’aelurostrongilosi felina in centro e sud Italia. Veterinaria 1, 41-45.
  3. Traversa D., Di Cesare A., Milillo P., Lohr B., Iorio R., Pampurini F., Schaper R., Paoletti B., Heine J. 2009. Efficacy and safety of imidacloprid 10%/moxidectin 1% spot-on formulation in the treatment of feline aelurostrongylosis. Parasitology Research Suppl. 1, S55-62.
  4. Traversa D., Milillo P., Di Cesare A., Lohr B., Iorio R., Pampurini F., Schaper R., Bartolini R., Heine J. 2009. Efficacy and safety of emodepside 2.1%/praziquantel 8.6% spot-on formulation in the treatment of feline aelurostrongylosis. Parasitology Research Suppl. 1, S83-89.
  5. Traversa D., Di Cesare A., Conboy G., 2010.Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated. Parasites & Vectors2010, 3:62.

Vetpedia is translated by a team of expert scientific translators coordinated by Alberto Scalcerle (InterMed - Italian Association Medical Interpreters - coordinator) and Rachel Stenner (MA (Cantab) MB BS (Lon) - lead translator). 
For further information please contact: alberto.scalcerle@alice.it  www.scalcerle.net

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