Fig. 274.—Strongyloides stercoralis: mature filariform larva showing long transparent œsophagus, slender granular intestine and characteristic tip to the tail ending in two small points. × 620. (After Looss).
This cycle of development holds good for Strongyloides stercoralis of tropical origin (Bavay, Leuckart, Leichtenstern, Zinn). In the European Strongyloides the free-living generation, as a rule, is absent (Grassi, Sonsino, Leichtenstern, Braun); the rhabditis-like larvæ evacuated with the fæces are transformed into the strongyloid or filariform type of larva (in cultures which are easily made) which will only become adult if introduced into man.
So that we have these two cycles: (A) (1) ♀ parasitic, (2) eggs, the rhabditiform larvæ in fæces, (3) free-living strongyloid or filariform larva, (4) ♀ parasitic. (B) (1) (2) (3) as before, then (4) adult ♀ and ♂, free living, (5) eggs, (6) rhabditiform larva, (7) strongyloid or filariform larva, (8) ♀ parasitic.
Infection of man results not only from direct entry into the stomach but also, according to van Durme and Looss, through the skin.
Occurrence in Man.—As already mentioned, Strongyloides stercoralis was first observed in persons suffering from so-called Cochin China diarrhœa. From the enormous numbers of parasites evacuated with the fæces, the cause of the disease was apparently evident. It appeared, however, that only some of the soldiers returning from Cochin China and Martinique, and suffering from diarrhœa, harboured Strongyloides (Chauvin). Breton made the same observations in Cochin China and found that only 10·4 per cent. of cases of chronic dysentery, and 8·8 per cent. of chronic diarrhœa, show Strongyloides. Normand, moreover, found that only a few of the Europeans residing in Cochin China are exempt from S. intestinalis, yet the people exhibit no intestinal symptoms; if, however, from any cause a catarrhal condition of the intestine supervenes the condition is changed, the parasites appear in larger numbers, and the disorder is considerably intensified.
S. intestinalis, besides being present in the Indo-China region, also occurs in the Antilles, in Brazil, Africa, and Europe; in 1878 it was discovered in Italy by Grassi and C. and E. Parona; in 1880 it was also found in the labourers working at the St. Gothard tunnel. It was imported into Germany, Belgium, and the Netherlands by Italian labourers. One sporadic case has been observed in East Prussia, and the worm has also been reported from Siberia.
In mammals the following species are found: Probstmayria (Strongyloides) vivipara, Ransom, 1907, in Equus caballus; Strongyloides fülleborni, v. Linst., in Anthropopithecus troglodytes and Cynocephalus babuin.
Their development is, so far as is known, the same as that of Strongyloides stercoralis (v. Linstow, Centralbl. f. Bakt., Path. u. Infektionsk., 1905, Orig. xxxviii, p. 532).