Organism The nematode Strongyloides ster-coralis, which is morphologically similar to the hookworms. Far less common is S. fiilleborni.

Clinical features There are several alternative cycles of development and it is the type of cycle which determines the nature and degree of pathological change and hence the clinical features.

An infective filariform larva develops in warm moist soil, penetrates the skin, and follows the same internal route as the hookworms to the final resting site in the small intestine. However, no eggs are passed to the outside, only rhabditiform larvae are found in the faeces. If environmental conditions are favourable, a free-living cycle takes place, with the rhabditiform larvae developing into adults in the soil. This cycle can be repeated and the number of potential parasites increases with each completed cycle. If conditions change, filariform larvae are produced or if unsuitable for the free-living cycle, then the rhabditiform larvae passed in the faeces change directly into filariform larvae. Direct autoinfection can also occur, with the rhabditiform larvae penetrating the intestinal mucosa to enter the blood stream without ever leaving the body. Swallowed larvae can as well complete their development by entering the body through the intestinal mucosa (Fig. 10.4). Achlorhydria, as occurring in malnutrition, makes infection easier by the oral route.

It is the abnormal cycle of autoinfection that can lead to wandering larvae producing linear urticaria (larva currens) or 'eosinophilic lung'. Larva currens can persist for periods in excess of 40 years. Immunocom-promised persons, such as those with HIV infection or malignant disease, can get widespread dissemination of worms with serious consequences.

Diagnosis is made by finding the rhabditi-form larvae in the faeces or in the aspirate of the duodenal string test. Serological tests can be of value, but where positive, repeat stool examinations should be made.

Transmission is from direct penetration of the skin by infective stage (filariform) larvae or being swallowed from contaminated food, water or fingers. Soil is contaminated by faeces deposited directly on the ground or inadequately buried.

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