What is schistosomiasis?

Causes and Symptoms

Schistosomiasis is a parasitic disease
of humans that is prevalent in tropical and subtropical regions of the world. Also called bilharziasis, the disease is caused by blood flukes of the family Schistosomatidae. Five species of blood flukes have been identified as causing schistosomiasis, including Schistosoma mansoni, which is widespread across central Africa but also occurs in New World tropics; S. japanicum and S. mekongi of the Pacific region and a number of African countries; S. intercalatum, which occurs widely in Africa; and S. haematobium in Portugal, Turkey, and across North Africa. All schistosomes are trematode flukes that belong to the phylum of flatworms
called the Platyhelminthes.

Unlike many internal parasites of humans, schistosomes exist as separate male and female individuals. The male S. mansoni is about 6 to 10 millimeters in length and has a ventral groove within which the somewhat longer but considerably thinner female resides. The two remain together permanently, paired throughout life.

The life cycle of blood flukes involves two human hosts and an intermediate host, which is a species of freshwater snail. Reproduction occurs in the first human host. The female deposits its eggs in small veins of the intestinal wall or the wall of the urinary bladder, depending on the Schistosoma species. The eggs work through the intestinal wall and into the lumen, to be carried away and eliminated with the host’s feces or in the urine.

If human waste is deposited on land, then the Schistosoma eggs dry and decompose. If it is dropped in freshwater, however, then the eggs hatch, releasing ciliated larvae called miracidium that swim about in search of certain species of freshwater snails. The miracidium burrows into the snail and undergoes asexual reproduction, producing one or two generations of immobile sporocysts. The second generation of sporocysts transform into tailed larvae called cercaria that burrow out of the snail and into the water. The cercaria can survive about forty-eight hours in the water as they swim about in search of a second human host.

Human infection occurs if the swimming cercaria encounter someone wading, bathing, swimming, or walking in shallow water. The cercaria attach to the skin by oral and ventral suckers, discard their tails, and use a combination of muscles and enzymes to bore through the skin and enter the dermal blood vessels as larval forms called schistosomules. Within the blood vessels, schistosomules are carried throughout the circulatory system, eventually arriving in the pulmonary
capillaries of the lungs, where they transfer to systemic vessels that carry them to the portal veins. There, they mature and pair, a female lodging in the gynecophoric canal of a male. Together for life, the pair migrates through the circulatory system to the mesenteric veins of the lining mucosa or to the vesicular veins and begin to produce eggs. A noted departure from this life cycle is seen in S. haematobium, which lodges in the walls of the urinary bladder instead of the intestinal veins and voids its eggs into the urine.

The first symptoms of schistosomiasis infection are typically a rash or itchiness that occurs at and around the entry site of the cercaria. These symptoms gradually disappear, but within one to two months additional flulike symptoms such as fever, chills, and muscle aches become evident. These symptoms are typically produced by immunological reaction to the eggs rather than infestation by the blood flukes. Heavy infestations may result in enlargement of the lymph nodes, spleen, and liver; dysuria and hematuria; abdominal pain; and diarrhea.

Pathophysiological injury resulting from schistosomiasis infection may include inflammation, necrosis, and fibrosis produced by the lodging and movement of the eggs in the walls of the intestines and bladder. Movement of the schistosomules in the lungs may also cause respiratory problems.

Some flukes of the Schistosomatidae family cause a condition called swimmer's itch. The blood flukes that cause swimmer's itch are actually parasites of birds and mammals other than humans. The intense “itching” irritation occurs when aquatic cercaria of these species mistakenly try to penetrate into the skin of people who are wading or swimming. The cercaria lodge within the skin and die, producing the intense itching sensations characteristic of swimmer's itch.

Treatment and Therapy

Schistosoma infections are diagnosed by blood tests that reveal the presence of eggs or chemicals released by the blood parasites. Infections are treated with a battery of safe and effective drugs, including praziquantel, effective against all types of schistosomiasis; oxamniquine, effective against Schistosoma mansoni; and metrifonate, an effective treatment for urinary schistosomiasis caused by S. haematobium.

Perspective and Prospects

Schistosomiasis is an ancient parasitic disease that has plagued humans for centuries. Schistosome eggs have been found in mummies five thousand years old from the upper kingdom of ancient Egypt. Today, schistosomiasis continues to be a debilitating and sometimes fatal disease of humans. It is endemic in the warmer pantropics of the world and in regions where bathing and other sanitation facilities are lacking, including sub-Saharan Africa, the Nile River Valley region, central and northern South America, a number of Caribbean islands, Southeast Asia, the Philippines, Japan, and much of the Middle East. Schistosomiasis also poses a threat to tourists who visit these areas. Schistosomiasis blood flukes do not occur naturally in the United States, but recent immigrants may be infected.

Worldwide, estimates of infection rates suggest a minimum of two hundred million to three hundred million or more humans infected by one of the Schistosoma species of blood flukes, which translates into roughly one in every twenty-five to thirty persons. At this rate of infection, schistosomiasis, along with malaria and hookworm, remains one of the three worst parasitic diseases of humans.

Bibliography

Dugdale, David C. III, Jatin M. Vyas, and David Zieve. "Schistosomiasis." MedlinePlus, Oct. 6, 2012.

Mahmoud, Adel A. F., ed. Schistosomiasis. River Edge, N.J.: World Scientific, 2001.

"Parasites – Schistosomiasis." Centers for Disease Control and Prevention, Nov. 7, 2012.

Parsonnet, Julie, ed. Microbes and Malignancy: Infection as a Cause of Human Cancers. New York: Oxford University Press, 1999.

Rollinson, David, and Andrew J. G. Simpson, eds. The Biology of Schistosomes: From Genes to Latrines. London: Academic Press, 1987.

"Schistosomiasis." World Health Organization, Mar. 2013.

Secor, W. Evan, and Daniel G. Colley, eds. Schistosomiasis. New York: Springer, 2005.

Stephenson, Lani S., ed. Schistosomiasis and Malnutrition. Ithaca, N.Y.: Cornell University Press, 1986.

"Understanding Schistosomiasis." National Institute of Allergy and Infectious Diseases, Aug. 10, 2010.

World Health Organization. The Control of Schistosomiasis: Second Report of the WHO Expert Committee. Geneva: Author, 1993.