of the adult worm by Bilharz, the life cycle of snail fever was known. The discharged eggs, on reaching freshwater, release a swimming larva, the miracidum. Miracidia are short-lived, but if they encounter a suitable snail, they penetrate the soft tissues (usually the foot), migrate to the liver, and change in form (sporocyst); and for 6 to 7 weeks, by asexual reproduction, the numbers of parasites increase. During this time the snail sheds thousands of fork-tailed cercaria, which can swim and directly penetrate human skin, and in 5 to 8 weeks they develop into adult worms.
Snail fever, the disease
Schistosomes differ from other flukes (trematodes) in that the sexes are separate and they inhabit the blood vessels. The adult worms are ~10 mm in length, and the stouter males have a groove running lengthwise, called the gynecophoric canal, where the female normally resides (Fig. 3.2E). It is this groove in the male that is the basis for the worm’s generic name Schistosoma, meaning “split body.” Both males and females have two suckers at the head end of the worm, and the more anterior one surrounds the mouth. (Bilharz mistakenly took the two suckers for two mouths, and thus he called the worm Distomum, “two mouths.”) The schistosome adults, in sexual union, live in blood vessels (veins) close to the bladder and small intestine. Mating occurs in the gynecophoric canal, and then the paired worms move “upstream” into smaller veins, where the female worm deposits the fertilized eggs. The pathology of schistosomiasis is due not to the adult worms themselves but to the eggs. Each day hundreds of embryo-containing eggs move across the walls of the veins into the bladder or intestine, aided by the host’s inflammatory response, and in the process eggs become enclosed in a small tumor called a granuloma. It is the passage of eggs through the bladder wall that results in bleeding and gives the telltale sign of hematuria. Once in the bladder or intestine, the egg becomes freed of the granuloma and is eliminated from the body either with the urine or in the feces.
More than two-thirds of the eggs, however, fail to work their way out of the body and are washed back in the veins, and by means of the bloodstream they scatter throughout the body, where they accumulate in various organs. Accumulation is greatest in the liver and spleen. The piling-up of eggs blocks the normal blood flow, and this leads to tissue death. The egg also acts as an irritating foreign substance that the body attempts to wall off by surrounding it with a fibrous capsule. The egg-laden liver eventually becomes filled with scar tissue. In the bladder blood fluke, S. haematobium, the scarred areas block the migration of eggs through the lower bowel tissues, and more eggs are swept back into other sites. The earliest signs of infection, fever, chills, sweating, headache, and cough, occur within 1 or 2 months. Six months to a year later the accumulation of eggs produces organ enlargement, especially the liver and spleen; the enlarged and cirrhotic liver causes the abdomen to become bloated, appetite diminishes, blood loss leads to anemia, and there is dysentery (Fig. 3.3).
Figure 3.3 Two young boys infected with blood flukes
Schistosomiasis is an arithmetic disease: the severity of its symptoms and cumulative damage are directly related to the number of worms present, and the latter depends on the degree of exposure. In heavy cases there may be hundreds of worms, and the adults may live for 20 or 30 years. Clearly, with time and increased invasion by cercaria, a person becomes more and more debilitated. Yet over the centuries the adult inhabitants of areas where the disease is endemic, such as Africa, developed some measure of immunity largely as a result of continuous exposure; Europeans and Americans with no such immunity suffer more-severe symptoms as a result of higher burdens of worms.
Where snail fever is found
Wars and human migrations carried the blood flukes of the East African lakes to the Nile River, and from there it was distributed along the trade routes to most of the continents of the world. Although in 1902 Manson believed schistosomiasis to be a disease unique to Africa, he had to revise his thinking when he discovered an Englishman, who had resided in the West Indies but had never been to Africa, passing eggs with a lateral spine. In 1908, Piraja da Silva, living in Bahia, Brazil, wrote that the schistosome common in the Americas was probably introduced from Africa by West African slaves beginning as early as 1550. Indeed, Bahia was one of the ports of entry for the African slaves, and more recently it has been suggested that, under the Dutch (1630-1654), Recife may also have been an important slave entry point. Although snails native to the Caribbean and South America have been found to be effective vectors (and different from the snail species in Africa), snails introduced from Africa have also been important in transmission.
Schistosomiasis has not been eliminated. It is estimated that at present there are 240 million people infected with schistosomes and 700 million people at risk. Ninety percent of the cases are found in sub-Saharan Africa, resulting in >200,000 deaths annually. S. japonicum is found in Southeast Asia and the western Pacific, as well as China, the Philippines, and Indonesia. S. haematobium and S. mansoni are both found in 43 countries in Africa, but the latter species is also found in the Americas (Brazil, Suriname, Venezuela, and the Caribbean).
People who come to the freshwater pools to work, bathe, drink, wash clothes, and swim may also use the water for elimination of their body wastes. Individuals may be infected and reinfected almost daily as they paddle through the cercaria-infested waters that they have come to use as their outdoor toilets. Schistosomiasis remains one of Africa’s greatest tragedies. The highest incidence occurs in African children. In some instances, technology has expanded the numbers of cases. Indeed, every new irrigation scheme and each new dam may pose a new threat.
The Aswan High Dam of Egypt, begun in 1960 with Soviet financing and engineering and requiring 30,000 Egyptians toiling around the clock, was completed in 1971. The High Dam, by controlling the level of water in Lake Nasser, has brought electricity to many parts of Egypt as well as making four crops per year possible through year-round irrigation, but it has also created conditions favorable for the schistosome-carrying snails. Before High Dam construction there was already perennial irrigation in the Nile Delta and the prevalence of schistosomiasis was 60%, whereas in the 500 miles of river between Cairo and Aswan when there was annual flooding the prevalence was 5%. Some 4 years after the dam was completed, the average prevalence between Cairo and Aswan increased 7-fold (35%; range, 19 to 75%).
Schistosomiasis is generally a disease associated with agriculture, but it has also been a military problem ever since the days of Napoleon. During World War II, when U.S. troops stormed ashore on the Pacific island of Leyte in October 1944, they were unaware that in addition to being attacked by Japanese bullets they were also being invaded by the cercaria of S. japonicum. By January 1945 the first cases were diagnosed, and in the end 1,700 men were put out of action at a cost of 300,000 fighting man-days and $3 million. Five years later, 50,000 Chinese communist soldiers prepared for an invasion of Taiwan, but schistosomiasis became so widespread among the troops that the campaign was abandoned and the island was retained by Chiang Kai-Shek’s forces.
Where did schistosomiasis originate? It probably first occurred in animals living in the rain forests and lakes of East Africa and then spread together with its vector snails along the Nile and out into the Middle East and Asia via the trade routes. (Blood flukes occur in birds and mammals other than humans. Indeed, “swimmer’s itch,” or cercarial dermatitis, is found in lakes and along the seashore in Michigan, Minnesota, Wisconsin, New Jersey, and New England, as well as in other parts of the world, and is caused by cercaria [Fig. 3.2G] that normally infect aquatic birds and mammals. The skin rash and pustules are the result of their failure to continue their migration past human skin.)
