Freshwater snails are the intermediate hosts of a variety of trematodes (flukes) and some nematodes (roundworms) that cause many human diseases. The most important of these diseases is schistosomiasis (sometimes referred to as Bilharzia). This disease is caused by parasitic trematodes (blood flukes) that must find and invade a particular species of snail to continue their life cycle. The ecological requirements of these snails are a key determinant in the distribution and prevalence of this disease.
Schistosomiasis is a major public health problem in the world and blood flukes in general are important parasites of cattle and other large animals that humans depend on for survival. In humans, the blood flukes causing schistosomiasis lie in small veins in the lower abdomen; here males and females copulate and the females lay millions of eggs for 10 years or more. The eggs are passed into water supplies from feces and urine of infected people. After hatching the parasite actively swims and invades the body of a snail, usually by penetrating through the snail's foot. The parasite requires the presence of certain species of snails (e.g., Oncomelania, Biompha-laria, Bulinus) to transform (and multiply) itself into an infective stage, called cercaria. This life stage actively swims to the human host and penetrates the skin, which leads to infection.
A human's susceptibility to schistosomiasis depends on the species and strain of the invading blood fluke, the person's age, number of other parasites carried, nutritional status, and previous exposure to the parasite. Infection may cause incapacitation but the main effect is long-term damage to the intestine, bladder, and liver. Consequently, death may result from other infective agents attacking an already debilitated body.
Although schistosomiasis commonly occurs in developing countries, an infection related to this disease that occurs in developed countries is cercarial dermatitis or 'swimmer's itch'. This disease occurs worldwide and affects people who are swimming, wading, or working in littoral areas of both marine and freshwater habitats. Infection results when cercaria meant for birds or small mammals such as muskrats penetrate human skin, eliciting an immune response. The resulting skin rash goes away in about a week, the parasites degenerate, and the disease does not progress because humans are not the appropriate host for these animal schistsomes. Prevention involves brisk toweling after leaving the water (because penetration increases with drying and exposure length) and snail control.
Snails are vectors to several other human diseases (although much less common than schistosomiasis) as well, such as clonorchiasis and fasciolopsiasis. Like with schistosomiasis, the most medically important members of the Phylum Platyhelminthes (the flat-worms) have definitive hosts that are vertebrates and intermediate hosts that are mollusks (mostly gastropods).
Another human disease that has been important in Africa and parts of the developing world is Guinea worm or dracunculiasis. The large nematode
(~1m long) causing this disease releases larvae from an adult female worm (usually embedded in a human leg) through a skin lesion, when an infected human comes in contact with a pond or well. A larva is then ingested by a water flea (Cyclops), in which it develops and becomes infective. When a person drinks this infected water, the Cyclops is dissolved by the acidity in their stomach. The nematode larva is then activated, migrates through the subcutaneous tissue, and stays within the new host for about a year. It then emerges and starts the life cycle again. The treatment for guinea worm that has been used for thousands of years is to slowly wind the emerging worm around a stick over a several day period. This treatment is immortalized in the staff of Asclepius wound with a serpent, which is also the symbol of the modern-day healer or physician. An extensive Guinea worm eradication program has been highly successful and has eliminated the disease in most parts of the world and reducing human cases from 3.5 million to -32 000.
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