Morphological description of Strongyloides stercoralis is based on the work of Little (1966) and Speare (1986). These works have been thoroughly reviewed by Speare (1989), Schad (1989) and Grove (1996).
Clinical parasitologists may never encounter a parasitic female (Figure 18c.l) of S. stercoralis throughout their careers, since this stage is very seldom seen in the stool and was to be removed from the intestine using special techniques. They may, however, be found in patients with very severe infections (Scowden et al., 1978). Parasitic females are 2.0-2.8 mm long and have an average diameter of 37 They are very slender and threadlike, giving rise to the common name, 'threadworm'. Their almost transparent nature makes them difficult to visualise with the naked eye, even when they are separated from faecal material. Anteriorly the worm is tapered and this portion contains the pharynx or 'oesophagus', which extends for about one-quarter of the body length. Food is drawn into the pharynx through the mouth (which is hexagonal in shape and contains six papillae), by the creation of negative pressure caused by its expansion. Sequential contractions and expansions pass the food backwards and force it into the intestine. The intestine is a single cell layer thick and passes almost to the posterior tip of the body, where it ends in a cuticular rectum, which opens to the outside through the anus on the mid-ventral line close to the tail.
A nerve ring surrounds the oesophagus and roughly divides it into a muscular anterior 25% and a posterior, mainly glandular 75%, with a small constriction in between. The pharynx is divided into two subventral pharyngeal glands which empty their secretions into the pharyngeal lumen and a dorsal gland which opens near the mouth (Little, 1966). Based on the site at which they empty their contents, the glands are thought to differ in function (Schad, 1989).
The excretory system consists of two longitudinal canals running the length of the worm, and these are joined just behind the nerve cord
by a transverse duct to give an H-shaped reproductive system. The duct is connected to a single excretory cell and together their contents are emptied to the exterior via a short excretory canal and excretory pore on the mid-ventral line.
The female reproductive system is quite prominent in S. stercoralis. The vulva is located in the posterior third of the body in the mid-ventral line. From a short vagina, uteri in which eggs can be readily seen extend anteriorly and posteriorly. The oviducts also extend anteriorly and posteriorly; the former branches up to the pharynx and then folds back on itself, whilst the latter extends almost to the tip of the tail before folding. Eggs in the uteri are aligned in single file and occupy most of the body of the worm where they occur.
Free-living adult females are easily seen in culture using a dissecting microscope. The most striking feature is the number of eggs they contain. With some patience, egg laying can be observed using the microscope. They are 1.0-1.5 mm longxca. 85 ^ in diameter and taper at both ends. The oesophagus (pharynx) is typical of free-living rhabditiform worms. It is attached to the terminal mouth by a small vestibule and is demarcated into three distinct regions; the procorpus, isthmus and bulb. The procorpus is the most anterior and is cylindrical in shape; behind this lies the narrow isthmus and the bulb, which leads into the intestine. The bulb of the pharynx is the so-called 'rhabditiform oesophagus' and is typical of microbiverous nematodes.
The reproductive system of the free-living female is very similar to that of parasitic females, except that numerous eggs are present in the uteri. The point where each uterus enters its oviduct is sperm-filled and acts as a seminal receptacle (Schad, 1989).
The free-living males of S. stercoralis are < 1.01.2 mm long x 55 ^ in diameter. This stage is rhabditiform in nature and has a similar mouth, oesophagus and intestine to the free-living female. Male worms are typically J-shaped, which is caused by the bending of the conical tail anteriorly during fixing. The reproductive system consists of a blindly ending testis at the anterior end and this is attached to a poorly demarcated vas deferens and seminal vesicle. Within the reproductive system, spermatogonia, spermatocytes and spermatozoa may be seen (Grove, 1996). Copulatory spicules, which are inserted into the female during mating, surround the cloaca. In addition to the spicules there are six pairs of caudal papillae and a single mid-ventral precloacal papilla (Grove, 1996).
Eggs of free-living females and parasitic are morphologically similar. They are thin-shelled, ellipsoidal and measure about 40 x 70 ^ (Little, 1966). The eggs may be fully embryonated when laid or may have undergone several cell divisions. Eggs of the parasitic female hatch in the crypts of Lieberkuhn and are rarely seen in stool (Grove, 1996).
First-stage larvae voided in faeces or recovered from intestinal fluids and those that develop from the heterogonic cycle are morphologically similar (Little, 1966). They measure ca. 250 ^ longx 17 ^ in diameter when passed in stool, but are 180-240^ longx 14-16^ wide when newly hatched. The pharynx of first stage larvae is similar morphologically to that described for
free-living adults. It extends throughout the anterior third of the body and is 89-94 ^ long (Little, 1966). Of importance is the short buccal cavity, which is central in the differentiation of S. stercoralis from hookworms. Whilst the buccal cavity of the S. stercoralis is shorter than the body is wide at the base of the buccal cavity (48 that of hookworms is about as long as the body in this region (6-19 (Speare, 1989). Also, the genital primordium is larger (about 4 in S. stercoralis than in hookworms and is refractile instead of cellular (Speare, 1989) (Figures 18c.2, 18c.3 and 18c.4).
Second-stage Rhabditiform Larvae
This stage is larger than the first stage and the major change is reorganisation of the head, with
Fig. 18c.4 Stool smear showing first-stage larva of hookworm. The genital primordium is inconspicuous and the buccal cavity is longer than in S. stercoralis. Photograph courtesy of Dr Ralph Robinson, University of the West Indies, Jamaica the shifting of the cephalic lobe from a dorso-ventral to a lateral position (Grove, 1989b). Larvae that are destined to become infective and filariform will undergo major changes, including lengthening of the oesophagus, with the end posteriad to the nerve ring becoming more glandular and less muscular. The nuclei of the three oesophageal glands become more apparent. Little (1966) reported that these cephalic changes also occur in rhabditiform larvae.
Third-stage Rhabditiform Larvae
Third-stage rhabditiform larvae are merely larger versions of second stage larvae.
Fig. 18c.5 Filariform (infective larva of S. stercoralis). Note the filariform oesophagus, which extends almost half the length of the worm and the characteristic notched tail. Photograph courtesy of Dr Ralph Robinson, University of the West Indies, Jamaica
Fig. 18c.6 Third-stage larva of Strongyloides stercoralis
This stage (Figures 18c.5, 18c.6) is long and slender and, unlike other stages, seems to be suited for rapid directional movement and percutaneous penetration. The worm measures 400-700 ^ long x 12-20 ^ in diameter. The cuticle is striated and is divided into two adjacent flanges called alae. The alae extend along the entire length of the worm and are thought to be important in stability during movement. They extend slightly beyond the tip of the larva and give the appearance of a notch under light microscopy (Little, 1966). Diagnostically, this feature is important in distinguishing Strongyloides from hookworm species, where the notch is absent. Examination at high magnification shows that the alae are paired and that the tail is in fact tetrafurcate (Little, 1966). Scanning electron microscopy of the tail of S. ratti has shown that there is a circular aperture surrounded by eight apertures (Zaman et a/., 1980). However, it is not known whether this is the case with S. stercora/is.
The mouth of the filariform larva is a small shallow pore, thought to be closed to particulate materials and liquids. There is no marked buccal cavity and the pharynx (oesophagus) begins almost immediately behind the mouth and extends for about 40% of the body length. The oesophagus is a thin cylindrical tube, without the bulb seen in rhabditiform stages (Schad, 1989). A nerve ring encompasses the oesophagus at about 25% of its length from the anterior end. Shortly behind this, the excretory pore opens to the exterior.
The sensory system consists of two pairs of minute sensilla that lie between the lateral alae and another pair at the level of the excretory pore. The latter pair are putative mechano-receptors, which may signal the worm when it enters a space too small to permit it to pass. A pair of putative chemoreceptors lies between the alae at a point half-way between the anus and the tail; these lateral sensilla are the phasmids in Strongy/oides (Schad, 1989).
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