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Epidemiology and Pathology

Although there are variations between the species of worms which infect man, there is a common pattern. The adults do not multiply in man, are long lived (3-30 years), and produce large numbers of eggs. Because of repeated reinfection, the worm burden tends to increase, and the immunity given by the initial exposure is not complete. Many of the eggs remain embedded in human tissues and organs and the ill health of infected persons represents the total combination of reactions to the parasites and the eggs (Fig. 2.2), varying with the anatomical site, the individual response of the host, and the intensity of the infection. The clinical disease in particular reflects the acuteness or chronicity of the infection and whether there are few or many worms.

Fig. 2.2 A,B. The life cycle of the schistosomes (bilharzia).

Schistosomiasis is primarily an infection of the vascular system, from where it spreads to damage multiple organs. The worms (blood flukes) live and copulate in the portal vein and its mesenteric tributaries or, for S. haematobium, in the vesicle plexus of veins. The adult worms are strictly intravenous parasites, attached to the wall of the vein by two suckers (Fig 2.3C). The female spends much of her life in the male gynecophoral cleft and they migrate together along the vessel wall, against the blood stream, with the aid of their suckers. Some worms, however, lose their hold and become emboli.

The male worm is shorter and wider than the female (Fig 2.3A, B). The species vary in size but are in the range of 16 mm x 0.75 mm for the male and 25 mm x 0.3 mm for the female. The female leaves the male after copulation and migrates further upstream (retrograde) to reach the smallest venule possible. There the eggs are deposited. The various species have different egg laying capacities which have been estimated to be as high as 3,500 eggs per day in the case of S. japonicum and a mere 300 eggs per day in the case of S. mansoni. Within the individual species, the substrains vary also. For example, the South African and Tanzanian strains of S. mansoni produce far fewer eggs than similar species of the same schistosome in Egypt and Brazil.

Fig. 2.3. A Adult Schistosoma mansoni flukes: the shorter, wider male is on the left with the longer, more slender female on the right. B Male and female S. japonicum worms copulating. The longer female lies in the gynecophoral canal of the male. C The sucker near the end of the male S. mansoni by which it attaches itself to the wall of the vein in the portal system. The dark testes can be seen above the beginning of the gynecophoral canal, on the right.

Each species has a preferential site for depositing its eggs, although unexpected distribution can occur. The eggs of S. haematobium are usually deposited in the wall of the urinary bladder or ureters. The eggs of S. mansoni are usually deposited in the rectum or large intestine. The eggs of S. japonicum are usually deposited in the small intestine, the proximal large bowel, or the rectum. However, it is not unusual for the eggs of S. haematobium to be in the walls of the bowel, and the eggs of S. mansoni or S. japonicum are occasionally found in the bladder.

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Copyright: Palmer and Reeder