Soil-mediated Helminthiases

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Soil-transmitted helminthic infections are of two types; the hookworms which undergo a cycle of development in the soil, the larvae being infective, and a second group of nematodes which merely survive in the soil as eggs that have to be ingested in order for the cycle to continue. The geographical distribution of the hookworms is limited by the requirements of the developing larvae for warmth and humidity. Generally speaking, the second type can occur not only in the tropics and subtropics, but also in temperate regions. All these helminthiases provide an index of the level of hygiene and sanitation in a community since they depend for their dispersal on the indiscriminate deposition of faecal material on the ground, the use of untreated night soil as an agricultural fertiliser and similar unsophisticated human habits. In temperate as in other areas, those infections that are spread directly, ie through the ingestion of eggs, are common in microenvironments which favour such spread, notably homes for mentally subnormal people, refugee camps, orphanages, etc. The provision of adequate sewage disposal facilities virtually excludes these diseases.

The hookworm infections are transmitted through soil-dwelling infec­tive larvae that penetrate the skin. Faecal contaminated soil in the neighbourhood of human habitations or on farmland is the source of infection for the barefooted inhabitants. Conversely, the use of footwear greatly reduces the prevalence of hookworm infection. Barefooted rubber tappers in Malaya commonly acquire infection from contaminated soil. Larvae of a number of animal hookworm species do not mature in man but the invasive larvae produce a transitory skin eruption as they migrate (cutaneous larva migrans). Visceral larva migrans may be due to infections with eggs of the dog or cat roundworm {Toxocara canis etc). Here too the larvae do not mature in man but may set up inflammatory reactions in the viscera, especially the liver, or in the eye.

Generally speaking, the degree of harm done to the host is related to the worm burden in these infections. Hookworm disease results when large numbers of adult worms are present and the loss of blood due to the worms cannot be balanced because the host's diet is deficient in iron, etc. Moreover multiple intestinal helminthic infection is the rule in many areas. Heavy infections with Ascaris may result in intestinal obstruction. Trichuris infection of the large bowel can lead to rectal prolapse in infants.

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