Veterinary Epidemiology and Zoonosis

VISCERAL LARVAL MIGRANS

Synonyms

• Toxocariosis

Type of zoonosis

• Saprozoonosis

Etiology

• Visceral larval migrans (VLM) is due to Toxocara canis, an ascarid of dogs and other canids, but in a few cases Toxocara cati in domestic cats has been implicated.

Reservoir and incidence

• Human infections are sporadic and occur worldwide.
• The reservoir mechanism for T.canis is latent infections in female dogs, which are reactivated during pregnancy.
• Transmission from mother to puppies is via the placenta and milk.
• The life cycle of T.cati is similar to T.canis, but transplacental transmission does not occur.

Transmission and life cycle

• Infection is generally in dirt-eating young children who ingest T.canis or T.cati eggs from soil or sand contaminated with animal faeces, most often from puppies.
• Direct contact with infected animals does not produce infection, as the eggs require a 3 to 4 week extrinsic incubation period to become infective; thereafter, eggs in soil remain infective for months to years.
• In humans, hatched larvae are unable to mature and continue to migrate through the tissues for up to 6 months. Eventually they lodge in various organs, particularly the lungs and liver and less often the brain, eyes and other tissues, where they produce eosinophilic granulomas up to 1 cm in diameter.

Disease in animals

• The first indication of infection in young animals is lack of growth and loss of condition.
• Infected animals have a dull coat and often are "pot-bellied".
• Worms may be vomited and are often voided in the faeces.
• In the early stages, pulmonary damage due to migrating larvae may occur; this may be complicated by bacterial pneumonitis, so that respiratory distress of variable severity may supervene.
• Diarrhoea with mucus may be evident.
• In severe infections of puppies, verminous pneumonia, ascites, fatty degeneration of the liver and mucoid enteritis are common.
• Cortical kidney granulomas containing larvae are frequent in young dogs.

Disease in man

• Migrating larvae induce fever, cough, wheezing, hepatomegaly and sometimes splenomegaly and lymphadenopathy are present.
• The acute phase may last 2 to 3 weeks.
• Leukocytosis is marked due to eosinophils.
• Hyperglobulinemia occurs when the liver is extensively invaded.
• Ocular toxocariasis results in eosinophilic granuloma of the retina that may be mistaken for retinoblastoma.

Diagnosis

• ELISA
• No parasitic forms can be found by faecal exam.

Treatment

• Thiabendazole, mebendazole or ivermectin can be given.
• Symptomatic treatment with corticosteroids, antibiotics, antihistamines and analgesics can be given.
• Treatment for ocular toxocariasis includes symptomatic treatment plus vitrectomy and laser photocoagulation.

Prevention and control

• Disease in humans is best prevented by periodic treatment of puppies, kittens and nursing dogs and cats.
• Children should be supervised to prevent pica, their hands should be washed after playing in soil and sand, and play areas should be protected from animal faeces.