The Life Cycle of Helminths

The term "helminth" comes from a Greek word meaning "worm." Although members of this group are not actually worms (that is, they lack the segmentation found in annelids such as earthworms), they have wormlike characteristics. Helminths include flukes, tapeworms and roundworms. As parasites, helminths require a host as part of their life cycle. Most helminths are either hermaphrodites (meaning they have both male and female reproductive organs) or they are bisexual (meaning they can mate with either male or female worms).

Distribution

• It is estimated that there are more than 10,000 species of parasitic flukes. Tapeworms include about 5,000 species. Roundworms are an incredibly diverse group, with over 15,000 species. Flukes, tapeworms and roundworms are found in nearly every type of ecosystem.

Fluke Life Cycle

• Schistosoma is a fluke that infects humans. A fluke begins its life in an egg. After hatching, it infects an intermediate host, such as a snail. The fluke develops into larva within the snail's body. Human infection begins when a larva penetrates through its human host's bare skin and makes its way into a blood vessel. The fluke develops into an adult within its human host's intestine. Fluke eggs are returned to the local water supply after passing through the human's digestive system, and the cycle begins again.

Tapeworm Life Cycle

• The life cycle of a tapeworm often involves a number of different hosts. In the case of a tapeworm that infects a human, the tapeworm's life cycle begins when eggs exit the human's body via its feces. Outside its host's body, the egg may be eaten by a pig. Once inside the pig, the tapeworm hatches from the egg and develops into a larva. The human becomes infected by the tapeworm larva after eating undercooked pork contaminated with tapeworm larvae. Once inside the human, the tapeworm larva develops into an adult within the human's intestines, and the cycle continues.

Roundworm Life Cycle

• A human may become infected by a roundworm such as a Guinea worm after ingesting contaminated water. Contaminated water typically contains water fleas that are infected by Guinea worm larvae. Within the human body, the Guinea worms develop into adults within their host's intestine. After reaching adulthood, the worm makes its way to near the surface of its host's skin, causing painful blisters. These blisters typically occur in the host's lower limbs. Infected humans often walk into water sources such as streams to relieve the pain; once standing in water, the blisters often burst, releasing the worms back into the water. Once back in the water, the Guinea worms release their eggs, and the cycle begins again.

Preventing Helminth Infection

• One of the best ways to avoid a helminth infection is to avoid eating undercooked meat. Roundworm and fluke infections may be avoided by not standing in potentially contaminated waters and drinking only bottled water in areas where water quality is questionable. If you suspect you have been infected by a parasite, seek medical attention as soon as possible.