In everyday life, and in science, a parasite is a creature that gets everything it needs from another creature. Socially, the human perception of parasites is that they contribute nothing, while living off the hard work and resources of the more industrious. Biologically, a parasite is something that lives in, or on, another living thing. Parasites get all their food and shelter from the organism - the host – that they infest. As one might expect however, it's not always so straightforward.
Endoparasites: parasites that live inside
Most parasites of humans live inside the host; any organism that lives inside another organism is an endoparasite (endo- means something internal). In humans these are helminths (worms of various types), protozoa, and sometimes larval stages of arthropods (insects, mites etc.).
A great many endoparasites live in the intestines, or at least pass through the intestines, having been swallowed in food or water. Virtually any organ can be occupied however: Trichinella sp. and Toxoplasma gondii live in muscles, Echinococcus sp. and liver flukes occupy the liver, Schistosoma hematobium targets the bladder, and a whole group of protozoans circulate in the blood.
Ectoparasites: parasites that live outside
Most people know about fleas and lice, even if they’ve never seen one. These are ectoparasites (ecto- means outside of). Human ectoparasites include fleas, lice, bedbugs, mites, ticks etc. In general, ectoparasites attach to the skin to feed but may not remain on the host for their entire lives.
Some of these organisms lie in a grey area between endoparasites and ectoparasites: scabies mites, for example, are generally considered ectoparasites though the female scabies mite does burrow into the skin. Fly larvae may feed on dead tissue in a wound, but some species never invade healthy tissue.
Intermittent parasites
Many parasites live on a host for only part of their life cycles. At other times, they exist as nonparasitic organisms in the environment or infest other hosts. The beef tapeworm, for example, spends part of its life as a tapeworm in a human intestine, part as an egg in the environment, and part as a larval form in the muscles of a cow. The Chinese liver fluke is an adult in the liver of a human, but before it completes its life cycle, it also lives in a snail and a fish, with short periods of swimming free in fresh water.
Is a mosquito a parasite?
Mosquitoes remind people of parasites because they suck blood. Organisms like this are sometimes called micropredators: they’re too small to be a typical predator, but they don’t depend on the host for shelter, only food.
Usually, arthropods that only feed on people are not considered parasites, with the exception of the bedbug. Many of them, however are vectors of parasites: they infect the host with a parasite when they bite. Mosquitoes carry malaria; black flies carry filarial diseases, kissing bugs carry Chaga’s disease, ticks carry Babesia.
Virtually every species has parasites that live on or in it, and parasitic organisms themselves comprise a vast variety of species. It's difficult to find even a simple parasite definition that would apply to all of them.
Sources
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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