Strongyloidosis

Strongyloidosis is an intestinal parasitism caused by the threadworm, Strongyloides stercoralis. The parasite, occurring in dogs, primates and man, is found throughout the moist tropics, as well as in temperate areas where poor sanitation or other factors facilitate the occurrence of faecally transmitted organisms. In some parts of the world, notably Africa and New Guinea, human infections caused by S. fülleborni have been reported. In Africa, the latter is primarily a parasite of primates, but in New Guinea, no animal host is known. S. stercoralis is unique among zoonotic nematodes, in that larvae passing in the faeces can give rise to a free-living generation of worms which, in turn, give rise to infective larvae. This life history alternative (i.e. heterogonic development) acts as an amplification mechanism, increasing the population of infective larvae in the external environment. The infective larvae are active skin penetrators; infection per os , while possible, is probably of limited importance. Because the parasitic female’s eggs hatch internally, a potential for autoinfection exists when precociously developing larvae attain infectivity while still in the host. This is another virtually unique feature of S. stercoralis infections in both its human and animal hosts. Autoinfection can occasionally escape control by the host, with massive re-penetration and larval migration. This can cause pulmonary or cerebro-spinal strongyloidosis as well as fulminant intestinal parasitism. Control of canine strongyloidosis has been achieved in kennels by strategic use of anthelmintics. Given the lack of epidemiological information community-based programs to control human strongyloidosis have not been attempted. The growing importance of human strongyloidosis depends upon the unique ability of S. stercoralis to replicate within its host and to behave as a potentially fatal opportunistic pathogen in immunocompromised hosts, particularly in those receiving corticosteroids.