Schistosomiasis

SCHISTOSOMIASIS

Schistosomiasis, also known as bliharzia or bilharziasis, is a parasitic infection caused by trematodes, also known as flatworms or flukes, of the genus Schistosoma. There are many species of animal schistosomes worldwide, with five responsible for the majority of human infections: S. haematobium, S. mansoni, S. japonicum, S. intercalatum, and S. mekongi. Schistosomiasis is the second most common human parasitic disease, malaria being the most common.

The World Health Organization estimates that 600 million people worldwide are at risk of infection, with 200 million already infected. Of these, over 120 million have a symptomatic infection. The disease is endemic in over seventy-five countries.

Schistosomes are blood flukes that have two distinct life-cycle stages: a sexual stage in mammals and an asexual stage in freshwater snails. Humans acquire infection when they come into contact with freshwater lakes and rivers containing infective schistosome larvae, called "cercariae." Cercariae penetrate the skin, migrate through the bloodstream and, in the case of S. mansoni, S. japonicum, S. intercalatum, and S. mekongi, come to rest in the mesenteric venous plexus. S. haematobium cercariae end up in venous plexus surrounding the urinary bladder. The male and female worms mature into adults and form a permanent mating pair that lives up to five years. Approximately six weeks after the initial infection, the females begin to produce between 200 and 3,000 eggs a day, depending on the species. Approximately half of the eggs are excreted in the urine or stool. If the excreted eggs reach freshwater, they hatch into free swimming miracidiae that will infect the appropriate snail species. Further development occurs within the snail and, after three to five weeks, a new generation of cercariae emerge from the snail ready to infect other mammalian hosts.

Most infections are asymptomatic. In a minority of cases, a transient illness may occur several weeks after the initial infection, known as Katayama fever, characterized by fever, cough, abdominal pain, and diarrhea.

Many eggs are not excreted and end up trapped in tissues. The host's granulomatous inflammatory response to these eggs is responsible for most of the damage associated with chronic schistosomiasis. S. haematobium eggs, which are mainly found around the bladder, can result in hematuria, ureteric obstruction, and bladder cancer. Eggs of the other species usually lodge in mesenteric vessels draining to the liver and cause periportal fibrosis, with the subsequent development of portal hypertension, splenomegaly, esophageal varices, and progressive liver dysfunction. Eggs in the bowel mucosa cause ulcerations and polyp formation leading to diarrhea and abdominal pain. When portal hypertension occurs, eggs are shunted to the lungs, where pulmonary hypertension may occur.

Diagnosis is made by finding the characteristic eggs in stool or urine. Because eggs may be excreted intermittently, several specimens should be examined. Occasionally, a rectal or bladder biopsy may be necessary. Serology is the most sensitive diagnostic tool and is particularly useful for detecting light infections. However, the antibody test does not distinguish between past and current infection, so it is not clinically useful in areas of high prevalence where individuals may have been successfully treated and then reinfected.

The drug of choice for treatment is praziquantel. Other options include oxamniquine for treatment of S. mansoni and metrifonate for S. haematobium. Treatment may reverse some of the long-term sequelae of infection, including fibrosis, especially in children.

Infection control is based on two strategies: reduction of transmission and reduction of morbidity. Reduction of transmission is accomplished by providing safe water supplies and proper sanitation facilities. Snail eradication is not an effective long-term strategy. Much of the focus of current schistosomiasis control strategies is to minimize the morbidity caused by the infection through mass treatment of at risk populations with praziquantel. This approach also leads to the reduction of egg output and transmission.

MARTHA FULFORD

JAY KEYSTONE

(SEE ALSO: Communicable Disease Control; Tropical Infectious Diseases)

BIBLIOGRAPHY

Ali El-Garem, A. (1998). "Schistosomiasis." Digestion 59:589–605.

Bica, I.; Hamer, D. H.; and Stadecker, M. J. (2000). "Hepatic Schistosomiasis." Infectious Disease Clinics of North America 14(3):583–604.

Dunne, D. W.; Hagan, P.; and Abath, F. G. C. (1995). "Prospects for Immunological Control of Schistosomiasis." Lancet 345:1488–1492.

Elliot, D. E. (1996). "Schistosomiasis, Pathophysiology, Diagnosis and Treatment." Gastroenterology Clinics of North America 25(3):599–625.

Lucey, D. R., and Maguire, J. H. (1993). "Schistosomiasis." Infectious Disease Clinics of North America 7(3):635–653.

Mostafa, M. H.; Sheweita, S. A.; and O'Connor, P. J. (1999). "Relationship between Shistosomiasis and Bladder Cancer." Clinical Microbiology Reviews 12(1): 97–111.

World Health Organization (1993). "The Control of Schistosomiasis: Second Report of the WHO Expert Committee." WHO Technical Report Series 803:1–86.

—— (1996). "Schistosomiasis." (Fact Sheet No. 115). Geneva: Author.