A parasite-infected snail


The life cycle of the tiny schistosome parasite takes it through snails and back to humans to cause deadly disease.


A small snail, removed from its shell, which is infected with schistosome parasites.
A small snail, removed from its shell, which is infected with schistosome parasites.
Bo Wang and Phillip Newmark

This confocal microscope image shows a small snail, removed from its shell, which is infected with schistosome parasites. The snail plays a key role in the life cycle of the parasite that causes schistosomiasis, a neglected tropical disease that sickens hundreds of millions of people.

More than 250 million people, mostly in Africa and Asia, have schistosomiasis. The World Health Organization classifies it as the deadliest neglected tropical disease, killing an estimated 280,000 people each year. Children with the disease are often ravaged by anemia, malnutrition and pervasive learning disabilities.

Schistosomes have a complicated life cycle, switching through many different body forms as they move from snails to water to humans. The cycle begins in tainted freshwater lakes and ponds, where parasite eggs released from human waste hatch into tiny creatures whose sole task is to infect a specific type of snail.

Within the snail host, the parasite produces massive numbers of offspring called cercariae. These fast-swimming, fork-tailed organisms are released into the water, from which they burrow through human skin and cause infection.

After penetrating host skin, the parasites must migrate into the blood vessels and find their way to the major vein that supplies the liver. During this journey, the parasites reorganize their tissues, and upon reaching the liver, begin developing reproductive organs, pair with a mate, and grow into mature adults.

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