Parasitism: Definition, Types, Facts & Examples

In ecosystems, organisms interact with each other and their environment. One method of interaction includes parasitism.

In fact, parasitism is the most common interaction between species. Parasitism spans across many forms of life, from the microscopic to macroscopic levels.

Parasitism is a relationship between organisms in which one organism lives at the expense of a host. The opposite situation would be symbiosis, in which there are mutual benefits to hosts and symbiotes.

In parasitism, a parasite can infect the circulatory systems, organs, surfaces and other body parts of an animal, or it can attack a plant's system. The host receives no benefit and suffers from infection and other morbidity, loss of production, lesions or even death. Parasites rely on their hosts to survive.

Obligate parasitism: An obligate parasite requires a particular species of host. Such a parasite species evolved for the specific relationship to the host. It will attach to the host and rely solely on it for survival.

However, the host is typically not overly harmed, thereby ensuring its availability for the obligate parasite to live upon. Head lice are an example of an obligate parasite, since they do not survive removal from their host.

Facultative parasitism: This is a rare form of parasitism. They are capable of surviving (free-living) without a host, and they can reproduce. Facultative parasites are not selective, but rather seek out any available host. Some roundworms (like Strongyloides stercoralis) and amoebae fall in this category.

Mesoparasitism: A mesoparasite lives partly, but not fully inside a host's body. It enters the body through an outer opening, like the ear.

Endoparasitism: Endoparasites live inside a host's body, in contrast to parasites living on the outside of a host. Some examples include parasitic copepods and tapeworms, as well as certain barnacles on shrimp and clams.

Ectoparasitism: Ectoparasites live outside a host's body. Some examples of ectoparasites include ticks and head lice.

Epiparasite: An epiparasite is a parasite that uses another type of parasite as its host species. An example would be protozoa that feds on a flea that feeds on a mammal.

Brood parasitism: Similar to the kleptoparasites (which are discussed later), brood parasites manipulate hosts into raising their young instead of the hosts'. The cuckoo is perhaps the most well known example of a species using brood parasitism. This results in energy and food being taken away from the intended offspring.

Often, the brood parasite's action kills the young of the host organism. Another example is a brown-headed cowbird, which takes over nests of other birds such as phoebes.

Social parasitism: Social parasites take advantage of the social colonies of certain types of insects, such as bees, ants and termites. Sometimes mimicry is used to enter a hive. Some similar animals even make other species raise their own young. One kind of ant, Tetramorium inquilinum, rides atop other ant species and gains food and transport in the process.

Kleptoparasitism: A kleptoparasite is an animal that steals the food or prey from another animal. An example would be the sharp-tailed bees whose larvae subsist on food meant for leaf-cutter bees. Or consider the seagull, perhaps the most notorious kleptoparasite of humans and their food at beaches worldwide.

Macroparasitism: A macroparasite is large enough to see with the naked eye; therefore it does not require a microscope to see it.

Microparasitism: Microparasites, in contrast to macroparasites, require a microscope for observing. They cannot be seen with the naked eye. Typically such parasites are unicellular. Protozoa are a type of microparasite.

Necrotrophic parasite: A necrotrophic parasite will consume part of a host until its eventual death. They keep the host alive long enough for the parasite's benefit. These kinds of parasites are also called parasitoids.

Biotrophic parasitism: Biotrophic parasitism describes the kind of parasites that do not kill their hosts, since they need the host to benefit.

Monogenic parasitism: A monogenic parasite needs only one host to complete its life cycle.

Digenic parasitism: A digenic parasite requires multiple hosts to complete its life cycle. An example for this is Plasmodium vivax, the protozoa that causes malaria. It needs a mosquito to carry it, which is the intermediate host. Then, the mosquito infects an additional host such as a human.

There are numerous modes of transmission for parasites to use with hosts. These include parasitoids, parasitic castrators, directly transmitted parasites, trophically transmitted parasites, vector-transmitted parasites and micropredators.

Directly transmitted parasites attach to a single host directly. Examples of directly transmitted parasites include lice, mites, copepods, several nematodes, fungi, protists, viruses and bacteria.

Vector transmission involves a parasite using two hosts of different species. Common examples of vector-transmitted parasites include protists (Plasmodium, Trypanosma and more), viruses and bacteria.

Trophically transmitted parasites also need two or more hosts. The main host, typically a vertebrate, eats another host. This transmission is used by all trematodes, cestodes, many nematodes and protists.

Parasitoids take over their host and grow to the point that it kills them, and then they emerge. This is common in insects that sting. Some nematodes and even fungi use this transmission.

Brood caterpillars use this strategy to hatch within a plant and lead to leaf death. And a plant example of a parasitoid is the strangler fig.

Micropredators use a method of transmission that entails association with several hosts in a generation. Most of them are blood-sucking organisms, such as leeches, mosquitos, fleas and vampire bats. There are also examples of sap-sucking parasites of plants, such as the leafhopper.

Parasitic castrators bring about a loss of reproductive ability in their hosts. The parasitic castrators use up the reproduction resources of their hosts. Some examples of these parasites include juvenile helminths and some kinds of barnacles.

There are numerous examples of parasites across many species. In humans, at least 100 kinds of parasitic organisms can lead to infections and morbidity. Insects, leeches, ticks, tapeworms, viruses, bacteria and helminths can parasitize humans.

Infectious diseases provide a constant example of the parasitic power of bacteria and viruses, such as with influenza. Enteric diseases inflict great suffering and are often callused by parasitic flagellates in the cases of giardiasis. Parasitic amoebae can lead to dysentery and other maladies.

Insects have their own parasites, including other insects. Often they will take advantage of young or larval insects. Some adult wasps will paralyze young cockroaches and then feed the roaches to their young.

Plants play the victim and the perpetrator of parasitism. Of the animals that inflict parasitism on plants, aphids are known for their sap consumption.

As for parasitic plants, over 4,000 flowering species exist. Some use modified root systems to siphon off water and nutrients from other plants' vascular systems. Others, which might not produce chlorophyll, attach to mycorrhizal fungi to get energy nutrients.

Fish suffer parasitism as well. Some nematodes, leeches and small crustaceans attach to fish gills. Some invade fish mouths. The parasites that invade fish can lead to illness in humans as well, if they are improperly cooked. This is also true in mollusks such as oysters that fall victim to Ascetosporea.

Understanding parasites helps public health experts find treatments of infection and prevention of invasion. Scientists are also teasing out not only the ecological aspects of similar evolutionary traits across parasitic species, but also the genetic turning points for organisms that lead to this destructive form of life.