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Epidemiology and Pathology

Man is the major host for Strongyloides, but dogs and chimpanzees can also be infected. The adult female worm is 2 to 2.5 mm long; the adult male is 0.7 mm long (some parasitologists believe the adult male parasite does not exist; in any event, it is not a tissue parasite in man). The adult female worms are usually lodged in the submucosa and lamina propria of the duodenum and upper jejunum. The parthenogenetic female can reproduce even without the male and may deposit 30-50 eggs a day for several months. The eggs (50 x 30 Ám) closely resemble those of the hookworm, but the ova of S. stercoralis are seldom present in stools. The eggs hatch into rhabditiform larvae which enter the lumen of the small bowel and are discharged in feces.

In the soil, particularly where moist and heavily contaminated with fecal material, the first-stage rhabditiform larvae molt four times to become free-living adult worms which oviposit and perpetuate the life cycle (called a heterogonic or indirect cycle). These first-stage, nonmigratory larvae are 350 Ám long and have a rhabditiform esophagus which terminates in a distinct bulb shape near the anterior end of the worm, where it joins the intestine. These larvae feed on soil, bacteria and debris and require 4 molts to reach the adult stage. The fertilized female lays partially embryonated ova which hatch, and the process is repeated. Under tropical conditions, multiplication may continue and the ground may be heavily contaminated. When unfavorable conditions develop, such as drying out of the soil, reduction of food supply, or temperature change, the rhabditiform larvae, instead of maturing into adult worms, develop into filariform larvae (called the homogenic or direct cycle) which are 650Ám long and have a longer straight esophagus. This form is quite mobile and infective to man; the larvae enter the skin by penetrating it on contact. If these filariform larvae do not come in contact with a host within a few days, they will die.

The life cycle of S. stercoralis in man (Fig. 13.1) begins when the infective filariform larvae come in contact with skin, usually the sole of' the foot, and penetrate it, causing mild local erythema and inflammatory reaction. The larvae then enter the cutaneous blood vessels and lymphatics and are carried through the bloodstream or lymph into the right side of the heart and then to the lungs. They pierce the walls of the pulmonary capillaries and enter the alveolar sacs, where they molt again. (This is the standard migratory route, though Schad et al have shown the pulmonary route is just one of several possibilities.) During their transit from the vascular bed to the respiratory tree, the larvae produce some bleeding, inflammation, and eosinophilia in the lung. These changes may be minimal or severe. Occasionally some female worms may invade the respiratory mucosa instead of traveling towards the glottis and may cause a pneumonitis. The vast majority of worms migrate up the bronchial tree to the pharynx aided by coughing, spill into the esophagus, and are swallowed.

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Fig. 13.1 (A and B) Life cycle of Strongyloides stercoralis.

On reaching the duodenum and jejunum they burrow into the mucosa and settle to feed, live and oviposit. Occasionally the mucosa of other parts of the gastrointestinal tract from the stomach to the anus may be invaded. The females mature in the crypts of the small intestine and invade the superficial layers of the mucosa, but do not penetrate beyond the muscularis mucosa. The eggs are laid, probably by some process of parthenogenesis (unisexual reproduction) and hatch, allowing the larvae to escape into the lumen of the gut (Fig. 13.2).

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Fig. 13.2 (A) Strongyloides stercoralis eggs in various stages of development in crypts of human jejunum. (B) Strongyloides larva (center), hookworm egg (left) and Trichostrongylus egg (right) in a fresh stool specimen from a single patient. (A,B courtesy of Dr. Herman Zaiman) (C) Rhabditiform larva of S. stercoralis in a fresh stool smear. (D) Infective filariform larva in the muscular wall of the colon.

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Copyright: Palmer and Reeder