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Angiostrongylosis: an emerging disease

by
08 November 2017, at 11:06am

LIBBY SHERIDAN hears about a nematode which is getting a foothold in the UK

ANGIOSTRONGYLUS vasorum is a nematode with an indirect life-cycle which infects dogs and foxes. 

It was first identified as long ago as 1853 in France, but now is prevalent across most northern European countries with predictions of wider spread across North America and Canada, East Africa and northern Japan and with recent identification of the parasite as far away as Australia.

Life-cycle 

The complete life-cycle of this parasite is not yet known. The adult worm lives in the right side of the heart and pulmonary vasculature and hence is commonly known as “heartworm” or “lungworm”. 

Dr Sue Shaw from the University of Bristol urged vets during her lecture at the recent London Vet Show to refer to the parasite as “angio” or “heartworm” to avoid confusion with other lungworms. 

Angiostrongylosis has the potential to be fatal and can present through a number of clinical signs other than coughing. 

Eggs produced by adult females are washed into the pulmonary arterial circulation where they develop to L1 larvae. The larvae migrate through the vascular endothelium into the bronchial tree, from where they are coughed up, swallowed and passed in the faeces. 

L1s are then ingested by an intermediate host (a slug or a snail) or by a paratenic host such as a frog. Moulting takes place within the intermediate host to become infective L3 larvae. Ingestion of the L3 by the dog or fox can take place either by eating the slug or snail (the most common scenario) or more rarely, by oral contact with the slime excreta of the mollusc. 

From the gut the L3 migrates and matures to L5, finally residing in the right side of the heart, the exact route being unknown. The pre-patent period can be as low as 28 to 40 days in optimal conditions. 

Risk factors

Slugs represent the highest likelihood for ingestion. The burden of larvae within them can vary and there can be risk to a dog from ingesting even a small number. These molluscs are strongly coprophagic, and many are minute and easily missed. They enjoy climbing to the top of grass and hence Dr Shaw was keen to advise vets to ask pet owners, “Does your pet eat grass?” rather than “Does your dog eat slugs or snails?” 

Age is a strong risk factor with young dogs less than two years being most at risk. Cases have been identified in puppies as young as 14 weeks. 

Clinical presentation 

It is thought that some dogs tolerate low burdens. Others show clinical signs, the most common of which are coughing, dyspnoea, exercise intolerance and coagulopathy. 

Rarer clinical presentations include neurological manifestations resulting from haemorrhage or aberrant migration of larvae, with acute CNS involvement, acute cervical or lumbar pain and hind limb paresis identified amongst other signs. 

Non-specific signs such as anorexia, weight loss and vomiting and diarrhoea can also be seen. Sudden death as a result of arterial embolism can occur. 

Cardio-respiratory signs

The adult worm provokes a strong immunological and inflammatory response. Pulmonary oedema, haemorrhage, emphysema and interstitial pneumonia can be seen and fibrosis can result from granulomatous inflammation. 

Coagulopathies 

Some dogs can present with coagulation defects in the absence of other clinical signs. In endemic areas such as Wales, it is common for dogs to be tested or treated for infection prior to neutering or surgery. Spontaneous bleeding or prolonged bleeding post trauma can be seen and should raise suspicion. 

Patients present with a combination of coagulopathies and it is unknown whether these are host or parasite induced. None of them is pathognomic for the disease and investigation is difficult. 

Signs of bleeding can include episcleral haemorrhage, internal haemorrhage (such as haemothorax), haemoptysis, epistaxis and mucosal and subcutaneous haemorrhages. 

Diagnosis 

The presentation of a young dog with clinical signs and no worming history allows a tentative diagnosis to be made. Older dogs, however, can be represented and watched for. 

Again, there are no pathognomic changes seen on haematology or biochemistry profiling. Globulin concentrations are often elevated and eosinophilia and neutrophilia is commonly seen. Thrombocytopaenia may be seen with prolonged one stage prothrombin time (PT) and activated partial thromboplastin time (APTT). Hypercalcaemia is also reported due to granuloma formation. 

Confirmation is by identification of the L1 larvae in the dog’s faeces by the Bayermann technique. Routine flotation technique in practice will underestimate the disease and Dr Shaw urged practices to use an external lab for identification, as the test lacks sensitivity due to intermittent shedding of larvae in faeces and is highly operator-dependent. 

Three consecutive samples should be examined. The larvae have a distinctive kink in their tails but can be confused with other L1 lungworm larvae. Bronchoalveolar lavage can also yield larvae for detection. Unfortunately, in some cases, despite strong clinical suspicion, the disease may not be confirmed and larvae not demonstrated. Thoracic imaging, while not diagnostic, can help with providing information on the severity of the disease. 

Because of the difficulty in identification and diagnosis, much effort is being directed into finding a serological or molecular test. Work ongoing in both Switzerland and at the University of Bristol looks promising and may provide a solution within the next one to two years. 

Treatment 

Supportive treatment will depend on the body system involved. Blood component therapy may be used in some animals. Treatment of the parasite, however, seems to be the most important factor in quickly managing the haematological abnormalities.

Several treatment options are available. Strong efficacy is reported from the use of moxidectin in combination with imidocloprid (Advocate, Bayer), 0.1ml/kg of 2.5% spot-on (2.5mg/kg), as a single treatment followed by a further vet examination and repeat treatment 30 days later for a proportion of cases. 

The slower systemic uptake of a spot-on product may have advantages in helping to decrease the risk of rapid killing of a large worm burden and subsequent overloading of the pulmonary circulation with proteinaceous material. 

Milbemycin 0.5mg/kg is available in the UK in combination with praziquantal as Milbemax (Novartis). Sole treatment with milbemycin once weekly for four weeks is recommended but this drug is not available in the UK as a sole preparation. Work done on the combined preparation suggests efficacy when used once weekly for four weeks but the drug is unlicensed at this frequency. 

Fenbendazole is unlicensed for use against angiostrongylus but efficacy has been shown. Dose rates of 20-50mg/kg per os once daily have been suggested for a duration of between five and 21 days. 

Prevention 

At this time, there are no products licensed in the UK for preventive treatment. The demonstration and evaluation of larval kill by a parasiticide will be required and is difficult at this time. Research is active, however, and may lead to the development of a preventive treatment within the next 12 months.