Among human diseases caused by parasites, schistosomiasis ranks second behind malaria in terms of its social, economic, and public health impact in tropical and subtropical regions of the world. One hundred twenty million people have symptoms, and 20 million suffer severe consequences from the disease.
Schistosomiasis was first recognized in the time of the Egyptian pharaohs. The worms that cause the disease were discovered in 1851 in a hospital in Cairo by Theodor Bilharz, a German pathologist. The disease was originally named bilharziasis after him.
Schistosomiasis is endemic in 76 tropical developing countries, and 600 million people are at risk for acquiring the disease. There are estimates that up to 200 million are already infected. Extreme poverty and poor sanitary conditions are major risk factors for the disease, along with inadequate public health infrastructure.
Between 1950 and 1990, the number of dams in the world increased from 5,000 to more than 36,000. There has been a corresponding increase in schistosomiasis, especially in sub-Saharan Africa.
The disease affects many children between the ages of 10 and 19. It also affects farmers and freshwater fishermen. Water resource development can also aggravate the disease.
The major forms of schistosomiasis are caused by five different species of waterborne flatworms called schistosomes. They cause several variations of the disease.
The worms enter the body through contact with infested water. This can be by washing hands, washing food, swimming, fishing, farming, and growing rice. Migration is introducing the disease into more urban areas in northeast Brazil and Africa, and refugees moving around are spreading it in Somalia and Cambodia. Tourists are also at risk for the disease.
Female long worms can lay 200 to 2,000 eggs per day over a five-year period!
The urinary form of the disease is characterized by the presence of blood in the urine, which can lead to bladder cancer or kidney problems. The intestinal form of the disease is characterized by intermittent, bloody diarrhea, which can lead to serious complications of the liver and spleen. Those who get the disease are very weakened by it and are often unable to work.
Within days of infection, people develop a rash or itchy skin. Fever, chills, cough, and muscle aches can start within a month or two of infection. Most people have no symptoms during the early stages of infection.
Infected individuals can contaminate their environment. The worm eggs in human excrement hatch on contact with water and release larvae. The tiny larvae must find a freshwater snail to survive. Once inside the snail, the larvae divide several times and produces thousands of new parasites. The new parasites are excreted by the snails into the surrounding water. It only takes a few seconds for these worms to penetrate human skin.
From the skin, the parasites get into the bloodstream. Within 30 to 45 days, they grow into long worms (12 to 16 millimeters in length). In intestinal disease, the worms live in the blood vessels that line the intestine. In urinary disease, they live in the blood vessels of the bladder. Only half of the eggs are excreted in feces or urine. The rest are trapped in the body tissues and do damage to vital organs. It is the eggs, not the worm, that damage the intestines, bladder, and other organs.
The disease is diagnosed by analysis of a stool or urine sample to see if the parasite is present. A blood test is also available. It takes six to eight weeks after exposure to contaminated water for the blood test results to be accurate.
Schistosomiasis is endemic in the following areas: Southern Africa, sub-Saharan Africa, Lake Malawi, the Nile River Valley, Brazil, Suriname, Venezuela, Antigua, Dominican Republic, Guadeloupe, Martinique, Montserrat, Saint Lucia, Iran, Iraq, Saudi Arabia, Syria, Yemen, Southern China, Philippines, Laos, Cambodia, Japan, Central Indonesia, and the Mekong Delta.
There are now three safe and effective drugs to treat schistosomiasis. Patients have to take pills for only one or two days.
There are a number of ways to prevent the disease. According to the CDC, they include the following: