Tropical Diseases

One of the most common tropical diseases is schistosomiasis, a disease caused by a worm called a blood fluke. The worm, which can be about 0.47 in (12 mm) long, can live in the human body from five to 20 years. About 200 million people are infected by the worm in developing countries, with as many as 500,000 deaths due to the disease each year.

Symptoms include abdominal pain, diarrhea, and weight loss. If untreated, the disease can cause enlargement of the liver, bleeding from blood vessels in the esophagus, and problems to the central nervous system.

The worm, which is usually hosted by Biomphalaria snails, depends on fresh water to survive. The disease is common in areas where bathing occurs in freshwater contaminated by human feces and where many individuals have the disease.

The life cycle of the blood fluke entails travel throughout the human body and through the body of its other host, the snail. The worm enters the human body in a larval stage in fresh water through contact with human skin. The larva enter a blood or lymph vessel and move to the heart and lungs, where they grow for several days. Then they move to the liver and the portal circulation, where they grow for several weeks.

Ultimately, the worms move to the intestinal wall, where they settle. The creatures lay eggs, which are released in feces and become new larva after contact with fresh water. These larva enter the host snail, where they eventually produce cercariae, the final larval stage. These are the creatures that enter the human body after contact with skin.

Drug therapy is an effective treatment for the disease. Prevention efforts depend on controlling infection through drug therapy and on convincing individuals to steer clear of fresh water which may be infected.