Parasitism
Parasitism is one version of symbiosis ("living together"), a phenomenon in which two organisms which are phylogenetically unrelated co-exist over a prolonged period of time, usually their entire life. The requirement for a prolonged interaction precludes predatory or episodic interactions (such as a mosquito feeding on a host), which are usually not seen as symbiotic relationships. Symbiosis encompasses commensalism ("eating at the same table", wherein two organisms co-exist in the same space, and one organism benefits while neither harming nor helping the other), through mutualism (wherein both species benefit from the interaction) to parasitism, wherein one organism, usually physically smaller of the two (the parasite) benefits and the other (the host) is harmed. (Various forms of "social parasitism", kleptoparasitism, and "cheating parasitism", as discussed below, are characterized by a less close association between parasite and host, however.) Especially in the field of medical parasitology, the term "parasite" has come to mean a eukaryotic pathogenic organism. Thus, protozoan and metazoan infectious agents are classified as parasites while bacteria and viruses are not. Interestingly, fungi are not discussed in textbooks of medical parasitology, even though they are eukaryotic.
Types of parasitism
Parasites that live inside the body of the host are called endoparasites (e.g., hookworms that live in the host gut) and those that live on the outside are called ectoparasites (e.g., some mites). An epiparasite is a parasite that feeds on another parasite. Many endoparasites acquire hosts by passive mechanisms, such as the nematode Ascaris lumbricoides, an endoparasite of the human intestine. A. lumbricoides produces large numbers of eggs which are passed from the host's digestive tract into the external environment, relying on other humans to inadvertently ingest them in places without good sanitation. Ectoparasites, on the other hand, often have elaborate mechanisms and strategies for finding hosts. Some aquatic leeches, for example, locate hosts by sensing movement and then confirm their identity through skin temperature and chemical cues before attaching.
Necrotrophs are parasites that use another organism's tissue for their own nutritional benefit until the host dies from loss of needed tissues or nutrients. Necrotrophs are also known as parasitoids. Biotrophic parasites cannot survive in a dead host and therefore keep their hosts alive. Many viruses, for example, are biotrophic because they use the host's genetic and cellular processes to multiply.
Some parasites are social parasites, taking advantage of interactions between members of a social host species such as ants or termites to their detriment. Kleptoparasitism involves the parasite stealing food that the host has caught or otherwise prepared. A specialized type of kleptoparasitism is brood parasitism, such as that engaged in by many species of cuckoo. Many cuckoos use other birds as lifetime "babysitters"; cuckoo young are raised and fed by adults of the host species, while adult cuckoos fend for themselves.
Cheating or exploitation types of parasitism are often found in situations where there are generalized non-specific mutualisms between broad classes of organisms, such as mycorrhizal relationships between plants and many types of fungi. Some myco-heterotrophic plants behave as "mycorrhizal cheaters", establishing mycorrhiza-like interactions with a fungal symbiont, but taking carbon from the fungus (which the fungus, in turn, gets from other plants) rather than donating carbon.
Evolutionary aspects
Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa.
The hosts of parasites often evolve elaborate defensive mechanisms as well. Plants often produce toxins, for example, which deter both parasitic fungi and bacteria as well as herbivores. Vertebrate immune systems can target most parasites through contact with bodily fluids. Many parasites, particularly microorganisms, evolve adaptations to a particular host species; in such specific interactions the two species generally coevolve into a relatively stable relationship that does not kill the host quickly or at all (since this would be detrimental for the parasite as well; but see parasitoid).
Sometimes, the study of parasite taxonomy can elucidate how their hosts are similar or related. For instance, there has been a dispute about whether Phoenicopteriformes (flamingos) are more closely related to Ciconiiformes (storks and related groups) or to Anseriformes (waterfowl and allies). Flamingos share parasites with ducks and geese, so these groups are thought to be more closely related to one another than either is to storks. Modern DNA methods, however, have shown that flamingos are not closely related to either.
It is important to note that "benefit" and "harm" in the definition of parasitism apply to lineages, not individuals. Thus, if an organism becomes physically stronger as a result of infection but loses reproductive capabilities (as results from some flatworm infections of snails), that organism is harmed in an evolutionary sense and is thus parasitized. The harm caused to a host by a parasite can take many forms, from direct pathology, including various specialized types of tissue damage, such as castration, to more subtle effects such as modification of host behaviour.
Parasites in fiction
Parasites living in or off humans are a favorite theme in the science fiction and horror genres, particularly in the subgenre of body horror. The fear of the human mind and body being used in such a way by another being is an understandably disturbing idea. In such work, the parasite often is responsible for a metamorphosis of the host, or takes control of the host mentally. Examples of works with this theme would be several of the films of David Cronenberg (particularly Shivers), the Alien series of films, the Yeerks of the Animorphs book series, and the Goa'uld of the television series Stargate. For a list of fictional parasites, see List of parasitic organisms.
See also
- List of parasitic organisms
- Intestinal parasite
- Macroparasite
- Malarial parasite
- Myco-heterotrophy
- Parasitic plant
- Parasitic wasp
- Parasitoid
- Pinworm
- Superparasitism
- Teratology
- Toxoplasmosis
Further reading
- Zimmer, Carl 2001. Parasite Rex. Free Press. ISBN 0-7432-0011-X
External links
- Toxoplasmosis
- Parasites and Parasitological Resources
- Aberystwyth University: Parasitology – class outline with links to full text articles on parasitism and parasitology.
- KSU: Parasitology Research - parasitology articles and links.
- Medical Parasitology – online textbook.
- Division of Parasitic Diseases, Centers for Disease Control and Prevention
- VCU Virtual Parasite Project - Virtual Parasite Project at Virginia Commonwealth University's Center for the Study of Biologicial Complexity