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Two hypotheses were tested to identify the mechanism(s) by which chronic Strongyloides stercoralis infections are maintained in experimental dogs as a model to explain delayed onset recrudescence in humans. Investigations tested the hypotheses that chronic infections result from 1) periodic reactivation of third-stage larvae from a reservoir of dormant parasites outside the gastrointestinal tract or 2) the periodic rejuvenation of postreproductive female worms remaining from a previous infection, lodged in the mucosal crypts. Populations of parenteral larvae survived in mature experimentally infected female dogs for 66 days; individual worms survived for 88 days, but there was no evidence that these larvae re-established patent, adult worm infections. Late in these infections, female worms were present in greater than predicted numbers with no evidence that autoinfection had occurred, suggesting that some postreproductive worms were long-lived. In separate trials, long-lived spent females were once again capable of producing viable larvae when the host was treated with corticosteroids.