Parasites

Parasites occur among virtually all life forms on earth. Almost every plant or animal is subject to parasitism by one or more parasites. Parasitism occurs when the parasite plant or animal lives on or inside of the host plant or animal in order to feed on it. Heterotrophic nutrition is one form of parasitic nutrition. Autotrophic (self-nourishing) organisms synthesize their food either through photosynthesis, or in the case of some bacteria, by chemosynthesis. All animals and fungi and most bacteria feed themselves by some means of nutritional heterotrophy. In the case of animals, they are holozoic heterotrophs, that is, they eat solid particles. In the case of yeasts, molds, or most bacteria, the nutritional form is the saprophytic “eating” of decaying plant or animal matter. Parasites are beneficiaries of the parasitical relationship, usually at the expense of the host upon which it lives or feeds. Some parasites cause little or no harm to their hosts. However, most parasites damage their host. In some cases the parasite may also kill the host. If death occurs from the parasite, the parasitic relationship is [Page 1339]called parasitoidism. Parasitoids that kill their hosts include some species of ants, wasps, bees, flies, butterflies, and moths.

Parasites cause a great deal of morbidity among plants and animals. Among humans, parasites cause enormous losses in productive energy. There are millions of human beings, especially in the tropics, who are infected with parasites that prevent them from working at their full capacity. Some parasites such as the Trypanosoma brucei cause sleeping sickness in humans. Trypanosoma brucei is a parasite of cattle, mostly found in Africa, which is passed to humans by the tsetse fly (Glossina palpalis).

There are several other forms of parasitism. Social parasitism is practiced by ants, which herd aphids and exploit them for a secretion that is a food for the ants. Cowbirds and cuckoos practice brood parasitism. They lay their eggs in the nests of other birds that then raise the parasite chick. The chick of the cowbird or cuckoo will usually kill the hatchling(s) of the host bird parents by pushing them from the nest. If a parasite is itself parasitized, the relationship is called hyperparasitism. For example, a hyperparasitic virus may be living on bacteria that are living on a protozoan, which is living in the digestive track of a flea, which is living on a cat.

Parasites can be directly harmful to humans. The parasites that infect human beings begin with the tiniest of viruses. Other parasites in humans include bacteria, single-celled parasites, and multicelled parasites. Common human parasites include pin worms, hook worms, round worms, lice, fleas, ticks, chiggers, yeast infections, and the fungus that causes “athlete's foot.” In addition, parasites such as fleas may spread diseases like the Black Death (bubonic, pneumonic, and septicemic). Many parasites are spread by fouled water or by animal wastes. Millions of people in the world drink and bathe in streams, rivers, and wells that are sources of parasitic infection. Moreover, many animal and plant foods carry parasites. Usually these can be killed with proper cooking. The U.S. government's Center for Disease Control estimates that 80 percent of the pathogenic outbreaks in the United States are due to improper food preparation.

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