Schistosomiasis (bilharziasis or bilharzia) is caused by a blood fluke (a tiny worm also called a flatworm) that goes through a complicated life cycle utilizing a species of freshwater snail. After maturing inside a human host, adult flukes pair for life and produce thousands of eggs that damage organs and are discharged in urine and feces. The larvae that hatch from the eggs work their way into the snails that in turn produce large number of larvae capable of penetrating human skin. The flukes lives in the veins, bladder and large intestine of their human hosts and borrow molecules form their hosts to wear on their surfaces so the hosts’ immune system can't recognize them as alien.

Schistosomiasis is found in rural and suburban areas in 71 countries in Asia, Africa and Latin America (where it first arrived in the bodies of slaves). Globally, 200 million , or one in about 30 people, have the schistosomiasis parasites in their bodies. The WHO estimates that a quarter of million people died from it every year in sub-Saharan Africa alone.

Schistosomiasis was known to the ancient Egyptians. Schistosome eggs have been found in royal mummies thousands of years old. X-ray examinations of mummies have revealed pathological calcification associated with schistosomiasis and schistosomiasis worm tissues have been found in rehydrated ancient tissue. The name bilharzia is derived from Theaodor Bilharz, a German physician who discovered the parasites in Cairo in 1851. [Source: Patrick Skelly, Natural History, June 2008]

Schistosomiasis is painful. Infection often occurs two to three weeks after exposure to the contaminated water. Symptoms include fever, lack of appetite, weight loss, abdominal pain, weakness, headaches, joint and muscle pain, diarrhea, nausea and coughing. Rarely, the central nervous system is involved. Chronic infections can cause disease of the lungs, liver, intestines and/or bladder and enlargements of the spleen and liver and bloody diarrhea and urine.

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schistosomiasis worms and snails

Schistosomiasis Prevention and Treatment

Since there is no practical way to distinguish infested water from non-infested water, fresh water swimming should be avoided except in chlorinated swimming pools. The snails are usually more common in shallow, still or slow-moving water near shorelines and places with reeds, grasses or where human excrement has been discharged. The disease can be carried by insects and fish and can be contacted by drinking contaminated water (purification with iodine or chlorine kills the flukes) There is no chance of acquiring schistosomiasis in salt water.

The schistosomiasis can be treated with effective oral drugs (usually Nirdazole). Treatment involves one or two pills that cost about $1. Routine checks for the disease are advised if you have been some place where you make have contacted the disease. Diagnosis of infection is usually confirmed by microscope examinations of stools and urine.

In the 1950s and 60s schistosomiasis was one of the diseases targeted in China by the legions of "barefoot doctors" were sent out into the countryside by Mao Tse-tung. One of their most successful herbal medicines, pumpkin seeds, was used to rid victims of worms. Today the treatment is also used in Africa to combat schistosomiasis.

An algae fund off the Caribbean island of Curacao contains a poison that kills the snail that caused schistosomiasis.

Schistosomiasis Worms

The worms that cause the disease are a kind of flatworm called schistosome. There are 21 known species of schistosome. Five infect humans. Others attack antelope and zebra. One “Schistosome hippopotami” specializes in hippos.

Schistosomes live for years maybe even decades in the host’s body. A German medical journal described a woman who picked up the parasites while on a trip to Mwanza, Tanzania and had no idea they were in her body until she sought help from a doctor for stomach pain 15 year later.

Flatworms are regarded as the most primitive organism with a head. Under the “head” of the schistosome is large suction cup which the worm uses to anchor himself against the blood flow and shimmy through the veins.

Most of what is known about schistosomes is the result of research by Vaughan R. Southage of the Natural History Museum in London and the Louis-Albert Techum Tchuente of the Center of Schistosomiasis and Parasitology at the University of Yaounde in Cameroon.

Schistosomiasis Worm Sex

Most flatworms are hermaphroditic. Schistosomes are not. There are males and females, and they look quite different from one another. Males weigh three to four times more than the longer, slimmer females. Females don’t mature until they are paired with males; unmated females are only half as long as mated ones; and mated females lose their large size and stop laying eggs if their lose their partners. Males can be cut in half and even quarters and each piece will try to mate with a female. The urge to mate is so strong that if no females are present males will try to mate with other males. [Source: Patrick Skelly, Natural History, June 2008]

Males and female schistosomes locate each other in the blood system and mate to continue the cycle. When schistosomes of the opposite sex meet, the male holds the female in a groove under his undersides in way that has been compared to a “hot dog in a bun.” The grove which the female attaches to makes the male look like it has been cut open like a fileted fish. The grove gives the parasite its name, schistosome, or “split body.” [Ibid]

It was long thought that males and females paired for life but work Patrick Skelly of Tufts University found that paired females do latch onto new partners if new males are introduced. Instances of a “tug of war” involving one female and two males have been observed in petri dish. [Ibid]

The initial rendevous between a male and female takes place in the liver. After that the couple make their way to the blood vessels around the intestines and bladder. There the female begins laying eggs, which degrade the intestinal walls and move through the gut. Many of the eggs are discharged in urine and feces but many remain in the host and circulate through the body. It is the eggs that make a person sick. [Ibid]

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schistosomiasis life cycle

Schistosomes in the Body

The schistosomiasis cycle begins when schistosomes eggs are expelled into water in the urine and feces of their human hosts. The eggs hatch into free-swimming microscopic “miracidia”, which penetrate the tissues of certain freshwater snails and reproduce into microscopic forms called “cercariae” that are capable of infecting human hosts.

The “cercariae” latch onto the skin of their hosts, burrow inside and take up residence in the body. They have powerful tails but no teeth or hooks. To get a good grip they vomit up sticky secretions from their mouths which they use to glue themselves on to their hosts. This makes them hard to wash off and give them time to probe the surface of the skin looking for places where the scales of skin overlap and they can burrow inside. Enzymes help them degrade the barrier and it takes only a couple minutes for them to get under the skin.

Patrick Skelly wrote in Natural History magazine, “Once inside the subdermal tissues, the parasites jettison their tails, and further outfit themselves for a new environment. No longer in freshwater, they alter their entire physiology and biochemistry. They shed their old surface coats and synthesize new ones, slipping into something more comfortable, more suited to the indoors. After a few days just under the skin the baby invaders push on to hit a blood vessel, that will carry them to the lungs and, after several days of further maturation there to the blood vessels of the liver, where males and females mature and mate.”

Image Source: Centers for Disease Control and Prevention

Text Sources: Centers for Disease Control and Prevention (CDC), U.S. Department of Health and Human Services; World Health Organization (WHO) fact sheets; National Geographic, New York Times, Washington Post, Los Angeles Times, The New Yorker, Time, Reuters, Associated Press, AFP, Lonely Planet Guides and various websites books and other publications.

Last updated May 2022

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