Parasitism is a symbiotic relationship between two species in which one organism benefits, while the other suffers harm. The parasite species gains the advantage, while the host species experiences the harm. For example, fleas are a parasite of dogs and cats (their host). Benefits gained by parasites include nutrition, shelter, and reproductive advantages. Most parasites are significantly smaller than their host. Usually, parasites don’t kill their host, although there are notable exceptions.
The word “parasite” comes from the Greek word parasitos, which means “one who eats at the table of another.” Parasitology is the study of parasites.
What Is a Parasite?
A parasite is a species that has adapted so that it has a symbiotic relationship with another species. Sometimes all members of a species participate in parasitism, but in other cases, only some organisms are parasites. In parasitism, the parasite benefits, while the host suffers harm. In other types of symbiotic relationships, either the host also benefits (mutualism) or else is unharmed (commensalism).
There are parasitic species in all of the kingdoms of life in biology. Most people are familiar with parasitic worms, fleas, and ticks (all animals), but there are also parasitic plants, fungi, protozoa, and bacteria.
Are Viruses Parasites?
Whether or not viruses are parasites depends on your definition of parasitism. Most parasitologists consider viruses as obligate intracellular parasites. Other scientists only include eukaryotes (animals, plants, fungi, algae) as potential parasites, and exclude prokaryotes (bacteria) and viruses.
Examples of Parasites
There are numerous examples of parasites. Here are a few that represent various strategies and organisms:
- Fleas and ticks are common external parasites of warm-blooded animals.
- The common cuckoo is a bird that lays eggs in the nests of other birds, getting the host species to raise its young.
- Aphids are insects that parasitize plants by sucking their sap.
- The single-celled protozoan Plasmodium is a parasite that causes malaria. The Anopheles mosquito is the vector that transmits the protozoan to its host.
- Head lice are human ectoparasites.
- Mistletoe is a hemiparasite or partial parasite that gets some of its nutrients from another plant.
- Armillaria are a type of honey fungi that grow in tree roots, eventually killing the tree.
- Trypanosomes are protozoa that parasitize humans, causing sleeping sickness.
- Some parasitic wasps sting other insects and paralyze them so that the host feeds the wasp’s young.
Types of Parasitism
There are several methods of classifying parasites:
- An obligate parasite require a host to complete its life cycle. For example, the bacterial species Chlamydia and Rickettsia are obligate parasites that can only grow within living host cells.
- A facultative parasite does not require a host in order to live and reproduce. For example, acanthamoeba is a protozoan that can live freely in fresh water or soil or can infect a host for their nutrition and other needs.
- A direct parasitic life cycle describes a life cycle of a parasite that only involves one host. For example, nematodes are worms that often complete their life cycle within a single host.
- An indirect parasitic life cycle has two or most host phases. It is a life cycle where the parasite has a definitive host, where it sexually reproduces, and one or more intermediate hosts. Plasmodium and Leishmania are parasites that have indirect life cycles.
- An endoparasite lives inside a host’s body. A tapeworm is an example of an endoparasite.
- An ectoparasite lives on or outside of a host’s body. Fleas and ticks are ectoparasites.
- A mesoparasite enters an opening in the host’s body and remains there (or partially there). For example, the isopod Cymothoa exigua enters a fish’s mouth and replaces its tongue.
There are six general parasitic strategies:
- Micropredator: A micropredator attacks more than one host. Examples of micropredators include leeches, fleas, ticks, lampreys, and vampire bats.
- Parasitic castrator: A parasitic castrator diminishes or destroys the host’s reproductive ability, diverting the energy for supporting the parasite. For example, the barnacle Sacculina parasitizes male crabs, giving them the broader abdomens of females.
- Directly-transmitted parasite: Directly-transmitted parasites do not need a vector to reach a host. Examples include parasitic mites, lice, fungi, bacteria, and viruses.
- Trophically-transmitted parasite: A trophically-transmitted parasite is eaten by its host. Many trematodes (flukes), cestodes (tapeworms), nematodes (roundworms), and protozoa are trophically-transmitted.
- Vector-transmitted parasite: A vector-transmitted parasite requires an intermediate host that carries it to a definitive host. Arthropods, such as mosquitoes, fleas, lice, and ticks, are common vectors for parasitic protozoa, bacteria, and viruses.
- Parasitoid: A parasitoid is a type of parasite that is an insect that eventually kills its host. There are parasitoid warms and dipterans (flies). Often, the parasitoid lays eggs on or inside its host to feed the parasite’s young.
Benefits of Parasites
Parasites harms their hosts, yet they serve a beneficial role in an ecosystem. At least half of all known species are parasitic, so their presence is an important indicator of ecosystem health. Parasites help transfer genetic information between species, aiding in diversity and adaptation. They keep top predators and dominant species in check. This creates healthy competition and preserves some species that might otherwise get wiped out.
- Cheng, Thomas C. (1973). General Parasitology. Academic Press. ISBN 978-0-12-170750-7.
- Combes, Claude (2005). The Art of Being a Parasite. The University of Chicago Press. ISBN 978-0-226-11438-5.
- Morand, Serge; Krasnov, Boris R.; Littlewood, D. Timothy J. (2015). Parasite Diversity and Diversification. Cambridge University Press. ISBN 978-1-107-03765-6.
- Poulin, Robert; Randhawa, Haseeb S. (2015). “Evolution of parasitism along convergent lines: from ecology to genomics”. Parasitology. 142 (Suppl 1): S6–S15. doi:10.1017/S0031182013001674
- Westwood, James H.; Yoder, John I.; Timko, Michael P.; dePamphilis, Claude W. (2010). “The evolution of parasitism in plants”. Trends in Plant Science. 15 (4): 227–235. doi:10.1016/j.tplants.2010.01.004