Parasitism

A parasite is an organism that lives at the expense of another organism, which is called the host. The word comes from the Greek term meaning one who eats at the table of another. While many biologists retain a stricter definition of parasite, Robert Poulin and Serge Morand claim in an article from the September 2000 edition of the Quarterly Review of Biology that almost 50 percent of all species may be considered parasites. The stricter definition requires that the organism in question must live in or on the host for a significant portion of the host’s life, in which case the percentage of total known species is probably much less than this. Under this definition, organisms such as the tarantula hawk—a spider wasp that lays its egg in a tarantula, which is then consumed by the larva upon hatching—would not count as a parasite, as the invariably fatal parasitic act comes at the end of the spider’s lifecycle and does not take up a significant portion of it. The study of parasites is important because they are a major cause of illness in humans, in livestock, and in the crops humans use for food and clothing.

Background

Some parasites, such as tapeworms, live inside the bodies of their hosts. These are called endoparasites. Others, such as bed bugs, ticks, and mosquitoes, live outside the bodies of their hosts and are called ectoparasites. Likewise, some parasites are dependent upon their hosts for their entire life cycle, while others are simply opportunistic in their parasitism. That is, some organisms have evolved such that their entire existence is dependent upon their hosts while others, such as hyenas—a parasite in the broad sense—will steal prey from other predators rather than expend more energy on hunting themselves. Hyenas would not be considered parasites under the stricter definition. Parasitic behavior is usually related to activities of consumption, but there are plenty of examples of parasitic behavior in reproduction, such as the dependence of viruses on their hosts to replicate their DNA, or cuckoos, which lay eggs in the nests of other birds for them to raise.

Endoparasites are classified according to two types: intercellular parasites, which live in spaces in the hosts’ body such as the digestive tract, and intracellular parasites, which live inside the cells of the host. Examples of the latter include bacteria, viruses, and protozoans. Intracellular parasites depend upon a third organism to deliver them to the host, which is called a carrier or a vector. Plasmodium organisms, for example, are transferred into humans and other animals when mosquitoes feed on blood to get proteins necessary for their egg-laying. These organisms are the cause of malaria, a serious and often fatal disease affecting millions of people.

Evidence suggests that symbiotic relationships can be mutual between two organisms to a third’s detriment. For example, mosquitoes with Plasmodium guests tend to feed on more blood and reproduce more than those without the parasite. In evolutionary terms, these two organisms benefit each other in some respects while both take advantage of a third: the bitten organism. Parasites often display more variance and diversity in their gene pools than nonparasites, which is an indicator of evolutionary fitness.

Overview

Parasites can have a huge effect on the ecosystem. There is a species of snail that originated in the northeastern Atlantic but colonized the ecosystem of the northwestern Atlantic coast in the nineteenth century. The snail became a dominant member of the ecosystem, consuming a vast amount of algae, an important food source for a wide variety of organisms. A study published in the May 2007 issue of the Proceedings of the National Academy of Sciences found that parasites of this snail sometimes caused their host to consume less algae by damaging its digestive system. This left more algae for organisms that would have otherwise been deprived of it, showing that parasites can be regulators of their eco-community. A study in 2019 found that the introduction of parasites reduced snail grazing in marshes in North Carolina, which better protected the area against drought.

Two important concepts in parasitology, the study of parasites, are virulence and transmission. Virulence is the tendency of the parasite to cause illness and/or death to its host. Transmission refers to the manner in which the parasite moves from organism to organism, often through reproductive mechanisms. For instance, many parasites that use insects as their hosts leave spores on the insect’s eggs, and when the larvae emerge, they eat the eggshells and the parasite. Virulence is sometimes associated with higher transmission rates, and this is called the trade-off model. However, a high rate of parasite exploitation of its host increases its host’s mortality rate, as well as the host’s rate of evacuating its parasite, which shortens the infectious period during which transmission occurs. Evidence suggests that parasites with an intermediate rate of virulence have a higher evolutionary fitness rate than hypervirulent parasites.

It is thought that a fast growth rate increases evolutionary fitness by reducing the age and increasing the size at which the parasite switches habitats. Broadly speaking, size is thought to improve survival rates by inhibiting predations. However, there are costs involved with a fast growth rate. A quickly-growing organism requires far more energy than its slower growing counterparts, and fast growth can reduce resistance to starvation in a stressful environment. A fast growth rate can also result in higher virulence, which can jeopardize the transmission process, although some studies have shown that a large transitional size helps the parasite adapt to its new habitat. A parasite that is ready for transmission at a younger age increases fitness, but a larger size (which fast growers tend to exhibit) at the time of transmission has a more ambiguous effect on fitness. Parasites such as tapeworms tend to be intermediately virulent even with a fast growth rate, and they do not appear to affect host behavior, which subsequently can affect their own survival. Further research into parasites is of interest to medicine, agricultural science, and wildlife conservation.

Bibliography

Bailly, Matthieu L., et al. “Identification of Taenia sp. in a Mummy from a Christian Necropolis in El-Deir, Oasis of Kharga, Ancient Egypt.” Jour. of Parasitology 96:1 (2010): 213–15. Print.

Benesh, Daniel. “What Are the Evolutionary Constraints on Larval Growth in a Trophically Transmitted Parasite?” Oecologia 162.3 (2010): 599–608. Print.

Brazier, Yvette. "What to Know About Parasite Infection in Humans." Medical News Today. 21 Mar. 2022, www.medicalnewstoday.com/articles/220302. Accessed 28 Dec. 2022.

Brown, J.K.M. “Little Else But Parasites.” Science 299.5613 (2003): 1680–681. Print.

Carlton, Jane M. Malaria Parasites: Comparative Genomics, Evolution and Molecular Biology. Norfolk: Caister Academic, 2013. Print.

Damiani, Claudia, et al. “Mosquito-Bacteria Symbiosis: The Case of Anopheles gambiae and Asaia.” Microbial Ecology 60.3 (2010): 644–54. Print.

Despommier, Dickson. People, Parasites, and Plowshares: Learning from our Body’s Most Terrifying Invaders. New York: Columbia UP, 2013. Print.

Juarez-Estrada, Marco A., et al. "Parasitism: The Good, the Bad, and the Ugly." Frontiers in Veterinary Science, vol. 1, 16 Oct. 2023, doi.org/10.3389/fvets.2023.1304206. Accessed 19 Nov. 2024.

MacLean, Allyson M., et al. “Phytoplasma Effector SAP54 Induces Indeterminate Leaf-Like Flower Development in Arabidopsis Plants.” Plant Physiology 157.2 (2011): 831–41. Print.

Poulin, Robert, and Serge Morand. “The Diversity of Parasites.” Quarterly Rev. of Biology 75.3 (2000): 277–93. Print.

"Powerful Parasites: Tiny Regulators of Ecosystem Health." Fishbio, 27 Jan. 2020, fishbio.com/powerful-parasites-tiny-regulators-ecosystem-health/. Accessed 28 Dec. 2022.

Wood, Chelsea L., et al. “Parasites Alter Community Structure.” Proceedings of the National Academy of Sciences 104.22 (2007): 9335–339. Print.