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Biology LibreTexts Parasitism

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    Parasitism occurs when one organism (the parasite) takes nutrients from another (the host). The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. Parasites do not necessarily kill their hosts. When they do, it is often a slow process, allowing the parasite time to complete its reproductive cycle before it or its offspring are able to spread to another host. A parasite may remain attached to the same host for its full lifespan, but some parasites have complex life cycles involving multiple host species. For example, a tapeworm causes disease in humans when contaminated and under-cooked meat such as pork, fish, or beef is consumed. Figure \(\PageIndex{a}\) illustrates the life cycle of the pork tapeworm. The tapeworm can live inside the intestine of the host for several years, benefiting from the host’s food, and it may grow to be over 50 feet long by adding segments. The parasite moves from one host species to a second host species in order to complete its life cycle.

    The life cycle of a tapeworm. Eggs develop into larvae in pigs. If humans eat undercooked pork, the larvae mature into adults in the intestine.
    Figure \(\PageIndex{a}\): This diagram shows the life cycle of the tapeworm, a human worm parasite. In the first step, eggs or tapeworm segments from human feces contaminate the environment. Second, pigs ingest these eggs or segments. Third, the eggs mature into larvae, which encyst in the muscle tissue of the pig. It is also possible for humans to develop cysts if they ingest eggs. Fourth, humans become infected when they eat pork that is undercooked. Fifth, the larvae mature into adults, attaching themselves to the intestine wall. Sixth, adult tapeworms can grow many segments and become quite long. They produce the eggs or segments that continue the life cycle.  (credit: modification of work by CDC)

    Parasites infect many types of organisms, including other animals and plants. For example, fleas and roundworms are common dog parasites. Plants can be infected by fungi, bacteria, and viruses; there are also plants that parasitize other plants (figure \(\PageIndex{b}\)). Even bacteria can be parasitized by viruses called bacteriophages. 

    Dodder on its host plant, white hibiscus. Dodder has light yellow-green, spindly stems and wraps around the host.
    Figure \(\PageIndex{b}\): Instead of conducting its own photosynthesis, the parasitic plant dodder siphons nutrients from its host plant. Image by Scot Nelson (public domain).


    Modified by Melissa Ha from Community Ecology from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)

    This page titled Parasitism is shared under a CC BY-NC license and was authored, remixed, and/or curated by Melissa Ha and Rachel Schleiger (ASCCC Open Educational Resources Initiative) .

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