Parasitism describes a relationship between two species, a parasite and its host, in which the parasite benefits, while the host is harmed. Parasitism is one form of symbiosis , which more generally describes any situation involving a close relationship between organisms of different species.
Parasites are different from predators and parasitoids (which also derive benefits from certain interspecific interactions while harming the other participant) in that the host of a parasite is not necessarily killed. Instead, parasites derive benefits from their hosts, most often nutritional resources and shelter, over a longer period of time. It is in fact advantageous to parasites if they do not harm their hosts too badly, because that prolongs the period during which parasites can obtain benefits from hosts. However, in some cases, the impact of parasites on a host is great enough to cause disease, and in extreme cases, the death of the host may also occur.
Parasitism is a common survival strategy among biological organisms, and many species are characterized by parasitic lifestyles for all or part of their lives. All the major kingdoms of life include some parasitic species. In addition, there are very few biological species that are free of parasites altogether.
Categories of Parasites
Parasites may be grouped by any of several traits. Ectoparasites live outside the body of the host, usually on the body surface. Well-known ectoparasites include fleas, ticks, and leeches. Endoparasites live within the host's body. Endoparasites can further be divided into those that live within host cells (bacteria and viruses), and those that live in spaces in the host's body (all other, generally larger, endoparasites).
Parasites may also be grouped depending on whether they are obligatory parasites , which must have a host in order to survive, or facultative parasites , species for which a parasitic lifestyle is optional. Facultative parasites adopt parasitic lifestyles if the opportunity arises, but they are also able to live free of a host organism.
Parasites are also grouped based on their size. Microparasites include viruses, bacteria, and fungi. These reproduce within the host and are characterized by comparatively small size and short life cycles. Microparasites also induce an immune response in the host, so that the ability to exploit a certain individual may be temporary. Macroparasites , on the other hand, typically describe larger parasites such as insects, worms, or vertebrates . They are larger in size (usually visible to the eye) and do not reproduce in the body of the host. Instead they release offspring which then find and infect new hosts.
Parasites may derive any of a number of benefits from their interactions with host species. Some obtain only nutrients, while others also gain shelter and a site for reproduction. They also vary in the closeness of their relationship to their host. Mosquitoes, for example, visit vertebrate hosts only to feed. Certain mites, on the other hand, remain intimately associated with their hosts throughout their lives.
Parasitic Life Cycles
Life cycles of parasites may be simple or complex. Parasites that are characterized by a simple or direct life cycle have only one host and are described as monoxenous . The parasite generally spends most of its life in or on the host, and may reproduce within the host. Because offspring must be transmitted to other hosts, however, the parasite or its progeny must have some way of leaving the host, surviving in the external environment for some period, and locating and infecting a new host. Parasites with simple life cycles have both parasitic and free-living life stages. The proportion of the total life cycle spent in each stage varies according to the parasite.
Parasites with more complex life cycles involving multiple hosts are described as having indirect or heteroxenous life cycles. The primary host of a heteroxenous species is the one in which adult parasites live and reproduce. The secondary or intermediate host is used by immature life stages of the parasite and is also essential. In many cases, the parasite passes through critical developmental stages in the intermediate host. The intermediate host may also aid in transmitting parasites to their final host. Fleas, for example, are sometimes intermediate hosts for mammalian parasites such as tapeworms.
A well-studied parasite with a complex life cycle is the liver fluke. Parasitic flukes reach adulthood in the bile duct of a primary host species such as a sheep or a cow. Flukes can cause extensive damage to the liver. During reproduction, eggs are released by flukes into the host's digestive system, ultimately passing out of the host in fecal material. Once the eggs hatch, immature juveniles infect a snail as an intermediate host. In the intermediate host, development and asexual reproduction occurs. At a further developmental stage, the parasite leaves the intermediate host and encysts on local vegetation. When the parasites are ingested, along with the vegetation, by a sheep or cow, they enter the intestine and then migrate to the liver and bile duct, ready to begin a new generation.
Some parasites are transmitted directly from one host to another by species, often insects, described as vectors. One particularly effective vector for vertebrate parasites is the mosquito, which plays a role in the transmission of numerous parasites including heartworm, the viruses that cause yellow fever and encephalitis, and Plasmodium, the protozoan that causes malaria.
Examples of Parasites
Species in countless taxonomic groups have parasitic lifestyles. The protozoans include several well-known parasite groups, such as amoebas and the organisms responsible for malaria. Malaria is a serious disease that occurs in large portions of the world, particularly in tropical areas. The malaria protozoan has a complex life cycle that involves asexual reproduction in humans and other vertebrate species, and sexual reproduction in mosquitoes. Mosquitoes also act as vectors for malaria, transmitting the parasites from one vertebrate host to another.
There are several groups of parasitic worms. The flat, ribbonlike parasitic worms of the class Cestoda are known as tapeworms. Tapeworms reside in the small intestines of their hosts, where they live in a constant bath of well-processed nutrients. For this reason, tapeworms do not need and have lost several physiological systems such as the circulatory and digestive systems. Food absorption occurs directly across the entire body surface of tapeworms.
Nematodes, or roundworms, include many important parasitic species. Well-known nematode parasites include pinworm, the large human roundworm, hookworm, and heartworm, which affects dogs and cats. In addition, parasitic nematodes cause diseases such as river blindness and elephantiasis, which results in blocked lymph flow and causes swellings in the body. Trichinella spiralis, which causes the disease trichonosis following the ingestion of uncooked, infected pork, is also a nematode worm.
A third group of parasitic worms are the trematodes, or flukes. Aside from the liver flukes mentioned above, trematode species are responsible for schistosomiasis and other significant diseases in humans.
There are also vertebrate parasites. One example is the lamprey, a primitive fish species that feeds by attaching to other fishes with a circular toothfilled mouth and sucks blood and other bodily fluids. Lampreys are often ultimately fatal to their hosts.
Many plant species are parasitic. The most famous of these is probably mistletoe, which infests various species of trees. Its "roots" tap into the tree's phloem network in order to intercept resources. Mistletoe is spread by birds, which transport the sticky white seed-berries from tree to tree.
One special form of parasitism is brood parasitism. Brood parasites are species, most commonly birds, that lay their eggs in nests of another species. The host species devotes the considerable energy required to brood eggs till hatching and to feed the chicks.
Brood parasites include species such as wydahs, cuckoos, and the brown cowbird of North America. Often, brood parasites increase their chance of success by laying eggs that resemble those of the host. Some brood parasites raise their own nest in addition to leaving eggs in the nests of other individuals, while others are exclusively parasitic. Some brood parasites are very specific about the hosts they exploit, while others use a wide variety of hosts. The brown cowbird, for example, is known to leave eggs in the nests of more than 200 different songbird species. Brood parasites will sometimes eject an egg from the host nest as they deposit their own egg. In addition, newly hatched brood parasite nestlings may eject eggs or step-siblings as well.
Parasitic nestlings elicit automated feeding behaviors from their adoptive parents by calling and opening their beaks wide. Classic photos of brood parasitism often show smallish parents feeding chicks that are significantly larger than they are. In some instances of brood parasitism, the energy devoted to raising a parasite prevents parents from raising chicks of their own. In others, brood parasites manage to coexist with the host offspring.
Certain potential host species can detect the addition of a foreign egg by a brood parasite and will abandon the nest and begin again elsewhere. In some cases, seeing adult brood parasites near their nest is enough to trigger abandonment.
Social parasitism is a special form of parasitism unique to certain social insects, particularly ants. Socially parasitic ants derive some or all of their resources from other ant species. In some cases, this involves no more than the stealing of food resources from other ant colonies, either of the same species or of a different species. However, in more extreme cases, socially parasitic ant species do not build their own nests or raise their own offspring. Instead, their strategies involve killing the queen of another colony and then making use of the workers, or stealing and enslaving workers from other colonies. In the most extreme case, the socially parasitic species actually lives within the nest of a host species, using host workers to raise young and obtain resources.
The Importance of Parasitism in the Evolution of Species
Parasitism is hypothesized to affect the evolution of many biological species. For example, parasitism may play a role in group size—larger groups of conspecific organisms are known to be more vulnerable to infestation by parasites. The naked mole rat, a highly social rodent species in which individuals live in large colonies, is probably "naked" (that is, practically hairless) because hairlessness reduces opportunities for parasite invasion. On the other hand, parasitism may promote the evolution of sociality by encouraging such social behaviors as reciprocal grooming or cleaning. This occurs in numerous mammalian species, including many primates, where individuals can be seen picking lice and other parasites from each other's fur.
The evolution of parasites and their hosts is also one of the best examples of coevolution , a situation in which there are two species, each of whose evolution depends upon and responds to the evolution of the other. Other pairs of species that may coevolve are predators and their prey, and flowering plants and their insect pollinators.
Coevolution between parasites and their hosts is antagonistic, and is sometimes described as an "evolutionary arms race," because each species attempts to evolve in such a way as to foil the other. That is, hosts are constantly evolving to avoid parasites, while parasites are evolving so that they can continue to exploit their hosts.
Another situation in which parasitism is hypothesized to play an important role is in the mating behaviors of species. One theory of sexual selection , called the handicap hypothesis, depends on parasitism as the critical evolutionary factor. The handicap hypothesis attempts to explain the evolution of brightly colored males, or males with elaborate ornamentation, in many species of animals. The peacock is a classic example of this—think of the gaudy coloration and elaborate tail of male peacocks.
Why do males evolve these traits, which make them highly visible and hence vulnerable to predators? The explanation seems to be that colorful or ornamented males are preferred as mates by females, so that male reproduction depends on the evolution of these traits. Why do females prefer colorful males? The handicap hypothesis argues that only very healthy males would be able to develop and maintain bright colors or ornaments. It is believed that parasites would make males sickly, and prevent them from devoting the resources necessary to maintain their bright plumage.
Consequently, by mating with males who are brightly colored and in generally good shape, females are more likely to end up with mates that carry fewer parasites. These males may also be relatively parasite-free because they have "good genes," which might then be passed on to the female's offspring.
see also Interspecies Interactions.
Askew, Richard Robinson. Parasitic Insects. New York: American Elsevier Publishing Company, 1971.
Cameron, Thomas W. M. Parasites and Parasitism. New York: Wiley, 1956.
Crewe, W., and David R. W. Haddock. Parasites and Human Disease. New York: Wiley, 1985.
Despommier, Dickson D., and John W. Karpelou. Parasite Life-Cycles. New York: Springer Verlag, 1987.
Kreier, Julius P., and John R. Baker, eds. Parasitic Protozoa. San Diego: Academic Press, 1991.
Lapage, Geoffrey. Parasitic Animals. Cambridge, U.K.: Cambridge University Press, 1951.
Rothstein, Stephen I., and Scott. K. Robinson, eds. Parasitic Birds and Their Hosts: Studies in Coevolution. New York: Oxford University Press, 1998.
Zimmer, Carl. Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. New York: Free Press, 2000.