SCHISTOSOMIASIS

Summary

First infection with schistosomiasis usually occurs during the early school years and is a frequent cause of absenteeism. It is not uncommon for 85 percent of a school's student population to be infected in some highly endemic areas in Africa.

In addition to its effect on children, schistosomiasis has a major impact on the agricultural workforce and on national economic productivity. In Egypt, where 20 percent of the people are infected, economic losses due to lost work are estimated to exceed $500 million a year.

Freshwater snails serve as intermediate hosts for the parasite. ne larvae of the parasite penetrate the unbroken skin of humans who enter the water in which infected snails live. Ile cycle is continued when people infected with schistosome worms deposit urine or fecally borne eggs into the water.

Development, both planned and unplanned, has resulted in a number of changes in the epidemiology of the disease that threaten to expand the infected population, reduce productivity and minimize development gains. Water development schemes, including dam building and irrigation systems, have created new breeding sites for snails. Intensive agriculture has encouraged people to migrate to urban and peri-urban areas that are ill-prepared to meet their needs for sanitation and water. In these areas, snail-infested streams and canals are often the most convenient water sources. New agricultural systems that emphasize irrigation, double cropping and other intensive cultivation practices have increased farmers' exposure to infection.

Praziquantel, a safe, single-dose, oral medication for all species of schistosome, was introduced in the early 1980s and is now the primary tool for controlling the disease. Recent reports about its reduced efficacy in localized situations have raised concerns about tolerance or perhaps resistance. Significant control of schistosomiasis also can be achieved through environmental improvement, increased sanitation, and provision of safe water.

Schistosomiasis, or Bilharziasis, is a disease that affects an estimated 200 to 300 million people in 75 countries of Africa, Latin America, Asia and the NEddle East. It is caused by flatworms, or flukes of the genus Schistosoma. The disease is transmitted to humans when they enter water contaminated with infected freshwater snails, which serve as intermediate hosts of the parasite.

The morbidity associated with schistosomiasis results from the mechanical and toxic irritation caused by eggs lodged in blood vessels and by granulomata, which are localized tissue reactions to eggs. As the immune system recognizes the egg as a foreign body and tries to destroy it, scar tissue develops around the egg, forming a granuloma.

There is a direct relationship between the number of eggs in tissues and the severity of symptoms. The numbers of eggs are, in turn, directly proportional to the total worm burden and the duration of the infection. For those who experience a daily infection over a period of several years, the total worm burden may be enormous. People who enter snailinfested waters infrequently may be lightly infected and completely asymptornatic. The type of pathology experienced by an infected individual is also a function of the location of the egg-producing worms, which varies with the species of schistosome.

Some of the more common pathological changes seen in chronic schistosomiasis infections include bleeding into the intestine or urinary system, liver enlargement, periportal (symmers) fibrosis and spleen enlargement. Other conditions associated with the disease are heart enlargement due to decreased blood flow through the lungs, and esophageal varicose veins that rupture and bleed. A connection between the chronic urinary form of schistosomiasis and bladder cancer is suspected.

There are five species of schistosomes known to infect humans with sufficient regularity to be considered human diseases: Schistosoma mansoni, Schistosoma japonicum, Schistosoma haematobium, Schistosoma mekongi and Schistosoma intercalatum.

Two forms of schistosomiasis infection are recognized in humans: urinary schistosomiasis caused by Schistosoma haematobium and intestinal schistosomiasis caused by S. mansoni, S. japonicum, S. mekongi and S. intercalatum.

With S. haematobium, the major pathologic lesions occur in the wall of the urinary bladder, often extending to the ureter, kidney, urethra and genitalia. The most conspicuous early sign of this disease is blood in the urine, which tends to subside or even disappear as the lesions become advanced. Frequency, incontinence and pain upon urination are constant and increasingly distressing factors. Lower abdominal pain, bladder colic, and weakness also are characteristic of this disease.

The major pathological lesions for infections with S. mansoni, S. japonicum, S. mekongi or S. intercalatum initially occur in the wall of the intestine, particularly the cecum, colon and rectum. Dysentery accompanied by abdominal pain is quite often a severe complication. Enlargement of liver and spleen, as well as arterial obliteration in the lungs, are late complications that may have secondary effects upon the heart. S. japonicum is generally considered the most serious of the species responsible for intestinal schistosomiasis, primarily because this species produces more eggs.

Schistosome worms enter the human body as forked-tailed larvae, or cercariae, which actively penetrate the unbroken skin of people who venture into water in which these larvae are suspended. The human host may experience itching and rash after the larvae has penetrated the skin. If the numbers of penetrating larvae are exceptionally large, the person may develop pulmonary symptoms as the larval worms migrate through the lungs. This stage is accompanied by shortness of breath and cough, and, on rare occasions, an intense, acute and sometimes fatal immunological reaction known as Katayama syndrome.

Adult schistosome worms are found in pairs within the deeper blood vessels of the human body. The females produce eggs, which are passively transported by the bloodstream to the tissues of the body. The majority of the eggs become lodged in the intestine, bladder, liver and spleen.

A small percentage of the eggs are extruded into the lumen of the urinary system or the intestine and are eliminated with the urine or feces. When those eggs are deposited into fresh water, they hatch, releasing thousands of highly motile, ciliated larvae called miracidia.

The life cycle within the snail is the same for all schistosome species. The miracidium actively searches out its specific snail host, penetrates snail tissue, changes into a mother sporocyst and gives rise to a daughter sporocyst. This sporocyst migrates to the digestive gland or reproductive organ, where further multiplication occurs and a new larva, the cercariae (the stage infective to man), is produced. It takes approximately four weeks at 26-28 OC for the miracidium to produce cercariae within the snail body. After maturing, the cercariae emerge from the snail and enter the surrounding water. One miracidium produces several thousand cercariae.

Freshwater snails are considered the intermediate hosts of schistosome infection, rather than vectors, because transferring the infection requires no physical contact between man and snail.

Many of the intermediate host species, whether aquatic or amphibious, are highly resistant to drying and may repopulate an environment in as little as 50 days after desiccation.

Snails of the genus Biomphalaria serve as the intermediate hosts of S. mansoni in Africa, southwest Asia, and the Americas. Snails of the genus Bulinus serve as the intermediate hosts for S. haematobium in Africa, Southwest Asia, and Europe, as well as for S. intercalatum in Africa. The genus Oncomelania serves as the intermediate host for S. japonicum in Asia, while Tricula aperta serves as the intermediate host for S. mekongi in Southeast Asia.

The world distribution of infection with the five species of schistosome includes 200-300 million residents of 79 countries. An estimated 600 million people are at risk of infection.

S. mansoni occurs in 53 countries of Africa, the Middle East, South America and some islands in the Caribbean. S. haematobium is endemic in 53 African and Eastern Mediterranean countries. S. mansoni and S. haematobium are co-endemic in 40 countries of Africa and the Middle East. S. japonicum occurs in the People's Republic of China, Indonesia and the Philippines, while S. mekongi is endemic in two Southeast Asian countries. Small foci of S. haematobium are reported in India and Portugal.

More recently, the parasite has been reported to occur in agricultural areas in Sao Tome and Principe, Jordan and Oman.

Various species of animals are found naturally infected with species of schistosomes that commonly infect humans. In the Orient, dogs, rats, cattle, pigs, sheep and goats are important in perpetuating human infections with S. japonicum. Primates are found naturally infected with S. haematobium, but their role in the epidemiology of human disease is not considered a strong one.