Disease History, Characteristics, and Transmission
Bilharzia (bill-HAR-zi-a), or schistosomiasis (SHIS-toe-SO-my-uh-sis), is an infection that usually results in organ damage and is caused by parasitic worms of the genus Schistosoma. This disease is mostly restricted to developing countries in which the parasites are endemic. However, infections have been recorded in developed countries, usually due to travel, immigration, or the entrance of refugees. Schistosomiasis can be acute, in which a common symptom is a fever appearing six to eight weeks following infection and disappearing within a few months; or chronic, in which organ damage occurs as a result of the immune system attacking parasite eggs retained in the body's organs. Chronic schistosomiasis is more common and usually does not appear until months or years after infection.
Treatment of schistosomiasis is effective and safe, involving a course of oral medications. Infection can be prevented by avoiding infected water bodies and by treating water before bathing or drinking. Attempts to treat infected populations and to control infection have had positive results within the past decade.
WORDS TO KNOW
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
SCHISTOSOMES: Blood flukes that infect an estimated 200 million people.
Disease History, Characteristics, and Transmission
Humans have suffered from schistosomiasis for thousands of years, with cases recorded in the period of the Egyptian pharaohs. However, the parasite causing this disease was not recognized until the nineteenth century. In 1851, Theodor Bilharz (1825–1862) first discovered a schistosome (a parasitic trematode worm) in infected people. Since then, a number of species of this parasite have been found to cause schistosomiasis, and their mode of infection and life cycle has been determined.
In humans, schistosomiasis is primarily caused by one of three types of Schistosoma parasites: S. mansoni, S. haematobium, and S. japonicum. There are also other, more localized species, such as S. mekongi, and S. intercalatum, which also cause human infections. While infection by these parasites usually results in some form of schistosomiasis, some species cause severe dermatitis, notably cercarial dermatitis.
There are both acute and chronic forms of this infection. Acute symptoms usually appear six to eight weeks after exposure to the parasite. The most common acute syndrome is called Katayama fever with symptoms including fever, loss of appetite, weight loss, abdominal pain, blood in the urine, weakness, headaches, joint and muscle pain, diarrhea, nausea, and cough. An initial symptom, usually occurring within days of exposure, is itchy skin. Acute symptoms usually disappear after a few weeks, although, some cases can be fatal. Chronic symptoms are more common than acute, and appear months to years after exposure. Chronic symptoms arise as a result of the body's immune system responding to the parasite's eggs. These eggs become lodged in various areas of the body depending on their species. Organ damage usually occurs as a result of the immune system responding to egg retention. The most commonly infected areas of the body are the urinary and intestinal systems, and damage to the bladder, intestines, spleen, and liver can occur. In rare cases, eggs may lodge in the spinal cord or brain, which can lead to seizures and paralysis.
Fresh water becomes contaminated with the eggs of Schistosoma parasite when a human who has the disease urinates or defecates in the water. The parasites hatch and are then ingested by freshwater snails, which are intermediate hosts during the parasite's life cycle. Following excretion from the snail, parasites can live in freshwater for 48 hours. During this time, they may come into contact with another human host and they can penetrate human skin within seconds. Once inside a host, the parasite develops into male and female worms that breed and lay eggs within blood vessels. While half of these eggs are excreted in urine or feces, the other half remain in the body and cause schistosomiasis symptoms. Excreted eggs hatch as soon as they enter fresh water, resulting in contamination of the water body. The cycle begins again if snails are present in the contaminated water.
Scope and Distribution
Schistosoma parasites are not found in the United States, but they are endemic to 74 developing countries. They are found in: Africa, the Caribbean, the Middle East, southern China, and Southeast Asia. Schistosomiasis is a major health risk, particularly within rural areas of Central China and Egypt. About 200 million people are estimated to be infected with Schistosoma parasites worldwide. While the majority of those suffering from this disease are found in countries where the parasite is endemic, some cases are found in other countries such as the United States and Great Britain as a result of travel, immigration, and entry of refugees into uninfected countries.
The majority of infected people tend to be rural agricultural workers who come into frequent contact with contaminated fresh water. In addition, a large number of children are infected. Across 54 countries, an estimated 66 million children are infected with the parasites. In one region alone, Lake Volta in Ghana, 90% of children in some villages are infected.
Treatment and Prevention
Treatment for schistosomiasis is effective and safe, usually involving a one to two day course of oral medications. Depending on the type of infection, one of three drugs is usually used. Praziquantal can be used for all forms of infection; oxamiquine is exclusively used for intestinal infections in Africa and South America; and metrifonate is used to treat urinary infections. Re-infection is possible after treatment, although the risk of serious organ damage is reduced as a result of treatment.
Because schistosomiasis is caused by a freshwaterborne parasite, the most effective prevention methods involve avoiding or treating contaminated water. Since the parasite penetrates the skin within seconds, avoiding contact with any potentially contaminated water bodies, such as lakes, rivers, and dams, will prevent infection. This includes avoiding swimming, bathing, and working in these water bodies. Fresh water that has been filtered, or heated to at least 150°F (65.5°C), is suitable for bathing. Water held in storage for 48 hours is also suitable for bathing as the parasite only lives without a host for this length of time. To ensure drinking water is free of parasites, filtering or boiling for at least one minute removes or kills the parasites.
Vigorous towel drying may also prevent parasite penetration, if the body has only been briefly submerged in contaminated water. However, this method is not recommended as a reliable means of prevention.
Long-term prevention of parasite infection involves controlling the occurrence of infection. Methods of control include educating people on parasite transmission; supplying clean water to regions where the parasite is endemic; diagnosing and treating infected people; controlling freshwater snails, the parasite's intermediate host; and increasing sanitation in infected regions.
IN CONTEXT: SCHISTOSOME DISTRIBUTION
“Human contact with water is thus necessary for infection by schistosomes. Various animals, such as dogs, cats, rodents, pigs, horse and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi.”
“ Schistosoma mansoni is found in parts of South America and the Caribbean, Africa, and the Middle East; S. haematobium in Africa and the Middle East; and S. japonicum in the Far East. Schistosoma mekongi and S. intercalatum are found focally in Southeast Asia and central West Africa, respectively.”
SOURCE: The Centers for Disease Control & Prevention; National Center for Infectious Diseases. Division of Parasitic Diseases.
Impacts and Issues
Schistosomiasis infection primarily occurs in developing countries. This is due to the fact that the parasites that cause schistosomiasis are endemic to developing countries, but the conditions of life in these regions also play an important role in the incidence and spread of the disease. Poverty; lack of awareness (both in terms of mode of infection and treatment methods); absent or inadequate of public health facilities; and unsanitary conditions all contribute to an increased risk of infection in developing countries. Furthermore, transmission of the disease to different areas is facilitated by the movement of populations and refugees. The World Health Organization (WHO) has stated that schistosomiasis is the second most important tropical disease, in terms of public health, following malaria. It is estimated that 200 million people worldwide are infected with the schistosomiasis parasite, and that 20,000 deaths are associated with the severe consequences of infection. In both rural Central China and Egypt, it poses a major health risk to populations.
Schistosomiasis can result in symptomatic infections, as well as fatalities. However, the majority of infected people show no symptoms, or only mild infections. In some cases, this disease has been found to cause reduced productivity in infected adults and decreased growth and school performance in infected children. Treatment in infected regions has resulted in an increase in the health of the population, suggesting that the treatment methods are effective.
The WHO has reported dramatic improvements in certain regions as a result of an increase in treatment administration, along with increased efforts to control infection. Objectives of these infection control programs have been met within two years of implementation in some regions. However, the WHO also emphasizes the need to maintain this control for the programs to be fully effective. Schistosomiasis is also one of the infections targeted as part of the WHO's Initiative for Vaccine Research. A variety of vaccine candidates have been tested, but, so far, none have been able to provide more than a partial reduction in the worm burdens of those vaccinated relative to nonimmunized controls. Hopefully, better success can be achieved using mixture of recombinant antigens. Another approach to vaccination against schistosomiasis is to reduce egg secretion by targeting the fecundity of the female worm. Some success with this approach has been reported.
Another significant impact of schistosomiasis on human health is the likely link between urinary schistosomiasis infection and bladder cancer. In a number of infected regions, a significant correlation exists between the occurrence of bladder cancer in patients also showing urinary schistosomiasis. For example, the WHO reports that in some parts of Africa, schistosomiasis linked bladder cancer has an occurrence 32 times greater than bladder cancer in the United States.
See AlsoCancer and Infectious Disease; Economic Development and Disease; Immigration and Infectious Disease; Immune Response to Infection; Parasitic Diseases; Swimmer's Ear and Swimmer's Itch (Cercarial Dermatitis); Travel and Infectious Disease; Water-borne Disease; World Health Organization (WHO).
Arguin, P. M., P. E. Kozarsky, and A. W. Navin. Health Information for International Travel 2005–2006. Washington, DC: U.S. Department of Health and Human Services, 2005.
Centers for Disease Control and Prevention. “Schistosomiasis.” August 27, 2004. <http://www.cdc.gov/ncidod/dpd/parasites/schistosomiasis/factsht_schistosomiasis.htm> (accessed January 30, 2007).
WebMD. “Schistosomiasis.” March 31, 2005. <http://www.emedicine.com/emerg/topic857.htm> (accessed January 30, 2007).
World Health Organization. “Schistosomiasis.” <http://www.who.int/vaccine_research/diseases/soa_parasitic/en/index5.html#vaccine> (accessed January 30, 2007).