Historical Background

Strongyloides stercoralis might be described as 'the military worm'. It was first described by a military physician in soldiers returning from war and in recent times much work on diagnosis, immunology and epidemiology has been conducted on veterans of World War II and Vietnam. Louis Normand (1876) first described Strongyloides stercoralis in French soldiers returning from Cochin China (now Vietnam), who were suffering intractable diarrhoea. This was an important breakthrough, since at last a plausible causative agent of the disease known as 'diarrhoea of Cochin China' was found. Very soon after his discovery, Normand realised that the stages which were initially isolated could also be found in several organs throughout the body at autopsy. Bavay (1878), who was a professor of pharmacy in the Navy, named the parasite Anguillula stercoralis and provided detailed descriptions of the worm. It is interesting to note that in this short time Bavay was also able to describe free-living adult parasites, which he found would develop in vitro if the stool was kept at favourable conditions (Grove, 1989a).

Normand continued his quest to describe the possible causative agent of 'diarrhoea of Cochin China' and at an autopsy in 1876 he removed a worm from the intestine that was about 2 mm long and that appeared different from those seen in stool or previous autopsies. He sent this specimen to Bavay, who concluded that it was a separate species, which he named Anguillula intestinalis (Bavay, 1877a). In addition, he and Normand, for the first time, saw larval stages in faecal culture that were longer than those previously seen, had a notched tail and an oesophagus that extended to about half the length of the body (Bavay, 1877b). This description of the infective filariform larvae of S. stercoralis was interpreted as a larval stage of A. intestinalis (Grove, 1989a).

From these military beginnings, several workers took an interest in the parasite. Notable among these was Laveran (1877), who confirmed the findings of Bavay (1877a,b) and Normand (1876). He also concluded that there were two separate species, 'A. stercoralis', which produced larvae in the stool and adults in the external environment, and 'A. intestinalis', which lived in the gut and produced larvae with notched tails in vitro. However, these findings were overturned when Grassi and Parona (1879), working in Italy, found that the parasitic form (A. intestinalis) laid eggs in the intestinal lumen that hatched to release larvae identical to those of'A. stercoralis'. Furthermore, these larvae did not develop into free-living adults, as described by earlier workers, but into notched-tailed larvae. Grassi (1879) suggested a new genus, Strongylus, for the parasite and later Strongyloides intestinalis,

Principles and Practice of Clinical Parasitology

Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd which was readily accepted, since the genus Anguillula already existed for eels (Grove, 1989a).

A confusing observation was made by Perron-cito (1881), who cultured free-living adults from larvae that were identical to those described by Normand (1876). He observed that they laid eggs that hatched to release rhabditiform larvae, which moulted into notched-tailed larvae identical to those of 'A. intestinalis' (Grove, 1989b). Therefore, these notched-tailed larvae were now known to develop in two ways; first, directly from larvae collected from patients with 'A. intestinalis' and second, from faecal culture of free-living adults of 'A. stercoralis'. It was Leuckart (1883) who suggested that this was the same parasite with two separate modes of development and who suggested the name 'Rhabdonema strongyloides'.

One of Leuckart's students, Loos, observed that some of the newly passed rhabditiform larvae could develop into free-living adults, which in turn produced eggs and more rhabditi-form larvae that developed into filariform larvae, whilst others developed directly into filariform larvae in the same faecal cultures (Grove, 1989b). Golgi and Monti (1884) brought this to the attention of the academic community with arguments on whether or not environmental conditions could influence the mode of development.

Among the observations made by Bavay (1877b) was the absence of parasitic males in the intestines of humans. This raised the question of whether or not these disappeared soon after fertilisation or whether the females were hermaphroditic. Grassi (1882) suggested that the females were parthenogenetic. However, Kreis (1932) reported finding parasitic male worms in the faeces of dogs and humans. Faust (1933) also reported that male worms were found in the respiratory tree and that mating may have taken place there before the parasite become embedded in the gut mucosa.

After several name changes to the causative agent of 'diarrhoea of Cochin China', Stiles and Hassal (1902) suggested that the correct name for the organism should be Strongyloides stercoralis, to give precedent to the first species name. The International Commission on Zoological Nomenclature (1915) accepted this.

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