Schistosomiasis is a parasitic infection acquired through contact with freshwater throughout the tropics and subtropics. The major forms in humans are caused by water-borne flatworms or blood flukes called schistosomes. Schistosomiasis is an increasing concern for travelers, particularly those undertaking "off-the-beaten-track" adventure tours around freshwater areas that are endemic for the disease.
Schistosomes live in certain freshwater snails, which release large numbers of tiny free-swimming larvae (called cercariae) into the water. These cercariae can penetrate the unbroken skin of a person who comes in contact with infested water and migrate to veins around the liver or bladder where mature worms produce eggs. Adult worms can live up to 15 years and produce large numbers of eggs that exit the body through urine or feces.
The disease is endemic in many developing countries. Schistosomiasis is a particular problem in the Nile valley, eastern Africa (especially coastal regions), western Africa (especially the savanna), Lake Malawi, Lake Victoria, along the Euphrates and Tigris rivers in the Middle East, and in parts of Brazil.
Worldwide, it affects more than 200 million people in rural agricultural and peri-urban areas. Intestinal schistosomiasis occurs in 53 countries in Africa, the eastern Mediterranean, the Caribbean, and South America. Oriental or Asiatic intestinal schistosomiasis occurs in the Mekong River basin and is endemic in 7 countries in Southeast Asia and in the Western Pacific region. Another form of intestinal schistosomiasis has been reported from 10 central African countries. Urinary schistosomiasis is endemic in 54 countries in Africa and the eastern Mediterranean.
Infection in travelers is usually acquired by bathing, swimming, wading, boating, or rafting in cercariae-infested waters. Even brief water exposure can lead to infection since the cercaria can penetrate intact skin within 30 seconds to 10 minutes. Risk of infection is highest along lake shores, slow-moving streams, and irrigation ditches that contain the snail host, but transmission has occurred in swamps, dams, rivers, and flooded paddy fields as well. Rafting trips generally involve some exposure to fresh water before or after the whitewater experience and have been proven to be high-risk for schistosomiasis acquisition.
Early infection is usually not symptomatic in persons who live in endemic areas or in persons with light infections. Expatriates and travelers without previous exposure may become seriously ill in the early stages of acute infection. A few hours after contact with schistosomes, some persons may experience tingling of the skin with a rash where the cercaria enter the body ("swimmers itch"); however, fewer than 10% of persons have this initial rash/itching. When a rash does occur, it usually resolves within 24 hours. Weeks later, once the adult worms begin to produce eggs, the person may develop influenza-like symptoms, with high fever, malaise, respiratory symptoms (coughing, wheezing, shortness of breath), diarrhea, and hives. The liver and spleen can also be affected. Most of the harmful effects of the disease in humans are caused by the reaction to the schistosome eggs. Long-term consequences of infection include severe liver disease, kidney failure, and bladder cancer.
Never assume that freshwater is free of schistosomes in an endemic area; even deep lake water far offshore cannot be regarded as safe. If water exposure cannot be avoided, try to prevent cercariae from reaching the skin by wearing protective footwear or clothing. Cercariae die quickly when removed from water and cannot survive drying; therefore, quick drying of exposed skin and clothing provides some protection. Rubber boots and wetsuits are protective. Chlorinated water (such as a properly maintained swimming pool) or water stored in a snail-free container for 48 hours is safe.
Need for Medical Assistance
Individuals who experience fever, influenza-like illness with general tiredness, and hives should seek immediate assistance. Blood in the urine or bloody diarrhea are also indicators to seek medical assistance. An acute neurological event such as limb weakness or paralysis requires immediate medical evaluation. In general, self-treatment following possible exposure is not advocated. A medical evaluation may be prudent upon return from high-risk travel that includes freshwater exposure and should include urine and stool exams for schistosome eggs; however, eggs may not appear in the urine or stool for up to 3 months after exposure. A reliable laboratory test is available, and safe and effective drugs are available for treatment of schistosomiasis.