Toxocariasis aetiology and management

Toxocariasis aetiology and management

Toxocariasis aetiology and management

Aetiology Toxocariasis is caused by infection with larval forms of Toxocaracanis or T. cati, which are primarily parasites of dogsand cats. The adult worms are found in the gut lumen andeggs are shed in the faeces. Humans are infected by ingestingeggs, and the larvae released burrow into the gut wallto enter vessels and disseminate. Larvae mature outsidethe host over 14 days and then are infective to the definitivehost or to humans.

Distribution, incidence and transmission The distribution is worldwide in dogs and cats, althoughthere is considerable variation in the frequency of animalinfection and contamination of the environment. Studiesin Britain showed that 17% of soil samples were contaminatedwith toxocara eggs. Moist soil conditions are moresuitable for transmission than hot, dry conditions.Children are most often infected. Infection is likely tooccur when fingers are contaminated with soil or sandcontaining eggs during play and then put in the mouth.Dog faeces deposited in public parks, especially aroundplaygrounds, represent a potential source of infection.Dog breeders and people who work in kennels are atrisk of infection. Toxocariasis due to T. cati is much lesscommon.

Pathology and pathogenesis Disease in humans relates to the number of infectinglarvae and the host response. Clinical manifestations aredue to dying and dead larvae, which evoke granuloma formationwith eosinophils, macrophages and lymphocytes.The eye, brain, liver, spleen and lungs may be involved intoxocariasis, but granuloma formation may occur in anyorgan of the body.

Clinical features Ocular toxocariasis and visceral larva migrans (VLM) aretwo clinical presentations of this disease, which is often asubclinical infection. Unilateral visual impairment is theusual symptom in ocular toxocariasis. Lesions directly onthe visual axis will cause severe impairment. A child maydevelop a squint. The granuloma can form in relation tothe lens and ciliary body, or on the retina itself. A cataractmay develop secondary to a granuloma affecting theciliary body. Visceral larva migrans is due to a heavy infectionwith larvae. Fever, anorexia, chills, night sweatsand weight loss are usual features. Examination showshepatosplenomegaly as the main physical sign. Pneumonitismay also be present.There is usually a marked eosinophilia in the peripheralblood in VLM, but eosinophilia is less common with oculardisease. Serological testing is valuable, the ELISA techniqueusing a toxocara secretory antigen being a sensitiveand specific test.

Differential diagnosis Toxoplasmosis usually causes bilateral choroidoretinitiswith destruction of the retina. Lymphoma, tuberculosisand sarcoidosis are usually considered in the differential diagnosis of VLM, although the gross eosinophilia isagainst the former conditions and supports a helminthic infection. The toxocara antibody test is strongly positive.

Management DEC is the usual treatment, giving initial doses of 50 mgand doubling the dose on alternate days till the maximum(lOmg/kg/day in three doses for 21 days) is reached.Ocular disease is not specifically affected by DEC becausethe worm is dead. There may be some spontaneous improvement as inflammation and granuloma size reduce.DEC is given in these cases to kill any worms that are stillmigrating.

Prevention and control Regular deworming of dogs, particularly pregnant bitchesand puppies, reduces the numbers of eggs contaminating the environment. Dog owners should try to ensure thattheir dogs defaecate somewhere where the faeces will not contaminate open spaces, playgrounds and parks.

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