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Fig. 2.4 A-D. The three main species of schistosomes. A Multiple eggs of S. mansoni (press preparation from a colon biopsy). B Single eggs of S. mansoni from a stool specimen, showing the prominent lateral spine. (Stained with iodine; courtesy of Barbara Drake, Hackensack) C An S. haematobium egg with a short terminal spine, seen in a urine specimen. D An S. japonicum egg in stool, showing a tiny lateral spine. (Courtesy of John Hall).

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Fig. 2.5 A-D. Slowly moving fresh water in the tropics is the major source of schistosomiasis. A A West African river with villagers gathering sand, washing clothes and becoming infected. B The Egyptian Nile, source of life...and death. C The shells of Australorbis glabratus snails (Biomphalaria spp) from the island of Saint Lucia in the Caribbean. These snails are natural hosts for S. mansoni. D Oncomelenia snail shells. These mollusks are intermediate hosts for S. japonicum.

Eggs of all species are oval, and vary in size from 120 to 60 µm. All eggs have a spine by which they become fixed in the tissues (Fig. 2.4), although whether this is the function or purpose of the spine is unknown. S. haematobium has a narrow, finely tapered terminal spine, whereas S. mansoni has a thick lateral spine, about 20 µm long. The spine of S. japonicum is very small and may not be seen. The shells of S. haematobium eggs are not acid-fast, while those of S. mansoni are; this helps to distinguish the eggs in tissue sections. There are other schistosome eggs with terminal spines, e.g. S. mattheei, which are very common in cattle in the tropics (92% of cattle at slaughter in Zimbabwe). They may be recovered from both the stools and urine of man, and accurate identification is essential because they seldom cause clinical symptoms.

The shell of the schistosome egg consists of chitin and, when mature, each egg contains an embryo called a miracidium. Two possible fates await the eggs:

1. They may fail to attach to the wall of the capillaries and become emboli, traveling and lodging almost anywhere in the body. Many accumulate in the liver, but if there is already portal hypertension, with a collateral circulation, the eggs may be carried to the lungs, brain and other organs. They provoke the production of discrete granulomas in which the eggs are trapped and incarcerated: eventually they disintegrate and phagocytes remove their contents. The granulomas heal, forming a nodular scar within which are the calcified remnants of the eggs and shells. Granuloma formation is initiated by the alpha tumor necrotizing factor, and the higher the alpha factor , the more granulomas are produced. Unencapsulated eggs do not stimulate granuloma formation, but secrete a toxin which causes severe liver damage. The acute syndrome (Katayama disease) is the result of the circulating immune complexes, not a reaction to mature worms or eggs.

The extent of the tissue reaction depends on many factors, including host genetics, allergy, load of infestation, and repetitive infections; but, whatever the reaction, the result is fibrosis. When this occurs in a vessel, especially in the lungs, there is blockage with damage both to the intima and the wall. A dumbbell-shaped granuloma develops which eventually recanalizes to form small arteriovenous fistulas and sometimes aneurysms. These are the angiomatoids which are characteristic of pulmonary schistosomiasis (Figs. 2.59-2.61). Elsewhere the progressive fibrosis, combined with obliterative endarteritis, causes the urinary, bowel, or hepatic changes of chronic schistosomiasis.

Dead worms embolizing to the liver and elsewhere, especially during treatment (or for other reasons), may also initiate inflammatory and fibrotic reactions; thus granulomas can be caused by dead worms, as well as by eggs.

Solitary granulomas may be large enough to produce symptoms, which will depend on the anatomical location. The granulomas in the spinal cord may lead to paraplegia, or in the colon, resemble an ameboma or carcinoma. Schistosome papillomas within mucosa represent a similar hyperplastic granulomatous reaction and can be found in the colon, bladder, cervix or body of the uterus. No organ escapes, not even the heart.

Another type of reaction may form around a few eggs and is mainly allergic. There is infiltration by lymphocytes, plasma cells, and large numbers of eosinophils. The vessels become dilated and congested and the lymph nodes enlarged. The severity of such allergic reactions depends on the load of infection and on repeated infection: allergic reactions are responsible for the "acute phase" which is seen clinically.

The diffuse effects of schistosomiasis may also be caused in large part by allergy. Repeated infections also enhance the immune response, so that more eggs are trapped in the tissues and larger granulomas are formed around each egg. The diet of the infected person influences the parasites, by affecting the defense mechanisms: low levels of vitamins D and C make the individual less resistant to schistosomal infection, but also make the ova abnormal.

2. The alternative is that the enclosed miracidium secretes an irritating lytic substance which produces an acute necrotizing arteriolitis. (There is a hypersensitivity element in this reaction.) In conjunction with mechanical forces related to peristaltic activity, the eggs then leave the host through the wall of the intestine or urinary tract by passing through the wall of the blood vessel. They escape into the lumen of the gut or urinary tract: if the miracidia lie deeper in the tissues, they produce tiny abscesses (sometimes referred to as "egg abscesses") which may rupture in the same way.

When the eggs have been released from the host and reach fresh water, they hatch and release the miracidia, which require a specific snail as their host (Fig. 2.5). The snails are found in slow-moving streams, especially in water which is used for irrigation, such as in paddy fields or sugar plantations, and (where endemic) in almost any open water, natural or artificial (Fig 2.5). One of the difficulties in controlling schistosomiasis is that the snail eggs are transported in the feathers of water birds, and thus spread widely. The snails are sensitive to water temperature, altitude, and many other factors: for example, they are not infected by the miracidia if the water temperature is below 15°C. The miracidia are attracted to the snails chemically and swim towards them at a rate of 7 m per minute. (The snail's pace is not recorded.) Each species requires a particular genus of snail (mollusk) usually one of the many species in the family Planorbidae, most commonly the genus Biomphalaria for S. mansoni and the genus Bulinus for S. haematobium. There are two other genera, Planorbarius and Ferrissia, which are of limited importance. In Asia the snails are species of Oncomelania and Tricula, the latter particularly acting as host for S. mekongi. Other freshwater snails serve as hosts for nonhuman schistosomes, contributing to schistosomal dermatitis. Each species need particular snails, but species have adapted to the local environment so that the host for S. mansoni in Africa is not the same as the snail which it inhabits in South America.