What do Snails have to do with Schistosomiasis

Schistosomiasis is one of a number of parasitic diseases that can affect human beings.  It is caused by an infection of trematode flatworms of the genus Schistosoma.  An infection of these parasites can linger in the body for years, causing mostly minor, but sometimes life threatening, symptoms.  The trematodes associated with Schistosomiasis come into contact with their victim hosts in bodies of freshwater, after the parasite has matured inside water-dwelling snails.  For this particular parasite, snails are an essential part of the maturation process.  Where the snails are absent, Schistosoma trematodes can not develop to become harmful to humans.

Although not common in the United States or other more developed areas of the world, Schistosomiasis is estimated to infect more than 200 million people worldwide.  Poor hygiene, lack of clean water supplies, and a lack of monetary funds to combat easily controllable diseases all contribute to the rampant rate of infection this disease enjoys in some parts of the world.

The Schistosoma trematode goes through several developmental stages before it can infect a human victim.  When eggs from adult Schistosoma worms enter the water supply by means of an already infected host’s urine or feces, the eggs will hatch and release miracidia, the beginning larval stage of this trematode.  The miracidia swim around in the water until they find and penetrate the foot of their intermediary snail hosts.  Once in the snails, the larvae continue to grow until they develop into cercariae, a larval stage with a forked tail capable of penetrating a vertebrate host.  The cercariae leave the snails, returning to the water, until they find an unsuspecting human host.  The cercariae will either penetrate the human’s skin or be swallowed if the host drinks from the infected water supply.  Once inside the human body, the developing trematodes migrate until they enter the veins.  Here, they grow to adulthood.  Adult worms are commonly found in the veins that drain from the large or small intestines, the bladder, or the rectum.

Symptoms of Schistosomiasis are caused by the body’s reactions to the eggs that the adult worms lay in the host body.  Eggs that are not passed out by urine or feces often become lodged in the intestine or bladder, and cause inflammation and scarring.  If left untreated, this infection can cause damage to the liver, intestine, lungs, bladder, and other internal organs, as well as anemia and malnutrition.  Severe cases have been found where eggs were laid in the brain or spinal cord, causing seizures and paralysis.  Even so, treatment for Schistosomiasis is a simple course of treatment with a prescription medication. 

Prevention of Schistosomiasis includes common disease control methods such as improved sanitation, and increased access to the medication prescribed for the disease.  In the case of this specific parasitic infection, however, there is one other control method that must be put in place, and that is the elimination of the snails that act as intermediary host in the parasite’s life cycle.  Without the presence of these snails, the Schistosoma trematodes can not develop and will die off.  Caution must be used when implementing this step in the process because the chemicals used to eliminate the snails can also be harmful to other species of animals that live in or near the same waters, including humans.  Promising research is being done into natural control methods for these snails, such as introducing crayfish into the environment as a natural predator.


Center for Disease Control

World Health Organization