Angiostrongylus vasorum is a metastrongyloid parasite of dogs and foxes which primarily parasitizes the pulmonary artery and its branches; it may also inhabit the right ventricle. Natural A. vasorum infection has been reported in a number of European countries including Ireland. Denmark, southwest France. Spain. Italy and pans of the UK. In the UK the majority of reported cases have been confined to southwest England and south Wales. The parasite was first encountered In the UK in racing greyhounds imported from Ireland.
Life cycle of Angistrongylus vasorum
Adult worms lay eggs in the terminal pulmonary arteries after a pre-patent period of 38-60 days. The first stage larvae hatch into the alveoli, pass up the bronchial tree and are swallowed and shed in the faeces. Several species of slugs and snails act as intermediate hosts. Transmission of the infective L3 larvae is thought to occur if the snails are ingested or if the dog’s food becomes contaminated with faeces or slime. When ingested the third stage larvae migrate via the mesenteric lymph nodes and portal system to the liver before passing to the right ventricle and pulmonary arteries.
Clinical signs include tachypnoea, coughing, haemoptysis, anaemia and haematoma formation. Some dogs with natural or experimental infections either have no clinical signs referable to the disease or show only mild exercise intolerance, others may develop signs of acute right-sided heart failure and die within a few days of the onset of clinical signs.
A diffuse interstitial / bronchial pattern with patchy coalescing alveolar densities especially in the caudal lung lobes may be visible. The latter are thought to represent focal areas of pneumonitis and haemorrhage. The right atrium and right ventricle may become enlarged and the pulmonary arteries may appear truncated. Occasionally there is evidence of pulmonary emphysema or pneumothorax.
Identification of A. vasorum larvae in faecal samples is the best method of diagnosing patent infections. Occult infections occur and the absence of faecal larvae does not preclude A. vasorum infection, since adult worms shed eggs intermittently. Haematological findings are inconsistent; an eosinophilia associated with larval migration may be present and some cases may be anaemic. Serum protein electrophoresis may reveal beta 2 and / or gamma globulin peaks.
The increased bleeding tendency has been attributed to a consumptive coagulopathy resembling chronic compensated disseminated intravascular coagulation which results in low platelet numbers and prolongation of the activated partial thromboplastin time. Decreased factor V concentrations have been reported in experimental infections.
A. vasorum infections have been treated effectively with fenbendazole (20 mg kg-1 once daily for 10 days), levamisole (10 mg kg-1 daily for three days) and ivermectin (200 μg kg1 given subcutaneously as a single dose). The ability of these drugs to completely eliminate adult worms and / or larvae has yet to be determined.