PARASITISM: CONCEPT, TYPES AND EXAMPLES - BIOLOGY - 2025

The parasitism is a kind of ecological relationship unidirectional interspecific (between individuals of different species) in which one species benefits at the expense of another for varying periods of time, may or may not end the life of another. For example, a tick that feeds on a dog's blood is a parasite.

Parasitism is, together with predation, amensalism and competition, a negative interspecific ecological relationship, since the interaction of two species negatively affects one of them, which sometimes translates into a population decline of the affected species.

The tick is considered a parasite

All living organisms that inhabit the earth are thought to be involved in some kind of "parasite-host" interaction, either as a host (the affected species) or as a parasite (the affected species).

Despite the fact that the term "parasite" refers to an organism whose nutrition comes from another organism, the clear distinction between parasites, predators and symbionts is somewhat diffuse, which is why it is a subject of discussion among many researchers in the field.

Other authors also consider that obligate parasites are those that reproduce in close association with their hosts during their life cycle, which implies an additional dependence on nutrition.

Parasite concept

Parasitism is a type of interspecific ecological relationship, that is, it occurs between different species. It is a type of negative interaction, since one of the interacting species benefits from the interaction, but the other is harmed.

In parasitism, the species that benefits at the expense of another is known as a parasite, while the species affected by the parasite is called the host.

Tetragnatha montana parasitized by Acrodactyla quadrisculpta larvae. Source: Miller, JA; Belgers, JDM; Beentjes, KK; Zwakhals, K.; van Helsdingen, P.

In a parasite-host interaction, the parasite acquires its food from the host, either by taking food that it reserved for itself or by feeding on some internal part of its body (body tissues, blood, sap, etc.), which results in a negative effect on the host, since the resources consumed by the parasite are no longer available to it.

In the host, the parasitic species also find a place to reproduce and survive different adverse conditions.

These ecological relationships last over time, given that parasites are often unable to live independently of their hosts (many times they lack locomotor organs and many of their sensory organs have atrophied).

Thus, ecologists consider that parasites have evolved to exploit the resources provided by the host in the most efficient way, while the hosts have been selecting the characteristics that allow them to somehow "limit" the negative effects of the parasite.

One of the best known examples of parasitism is that of the cuckoo, known as laying parasitism:

Types of parasitism

There are various types of parasitism, which have been described mainly based on the type of relationship that each parasite has with its host, the size relationship between the parasitic species and its host, and the mode or characteristics of said relationship.

Thus, according to the type of relationship between parasites and hosts, obligate parasitism and facultative parasitism have been defined. According to the characteristics of these ecological associations, there are also ectoparasitism, endoparasitism and mesoparasitism.

With regard to the size relationship between the interacting species, macroparasitism and microparasitism have also been described; If the parasites are classified according to their life cycle, they can be monogenic or digestive.

Forced parasitism

One that characterizes organisms that can grow only from food and nutrients that ensure their permanent association with another living being. In other words, those parasites that cannot live independently of their host, whatever the reason.

Experimentally, obligate parasites cannot be isolated or cultured in vitro on inert or "non-living" substrates.

Facultative parasitism

Facultative parasites represent a potential “transition state” between obligate parasites and free-living organisms, since these are characterized by having the ability to live in association with a host or to be free-living, depending mainly on environmental conditions.

Some authors consider that facultative parasites present a kind of “pre-adaptations” that facilitate facultative parasitism and that, evolutionarily speaking, they are from which transitions between facultative parasitism and obligate parasitism can occur.

The specific definition of a facultative parasite could be, then, that organism that despite being able to associate with a host to use it as food and reproductive site, has the ability to live independently, as long as environmental conditions allow it..

Ectoparasitism, endoparasitism, meroparasitism

These types of parasitism refer to the "place" of the host organism that is parasitized by the "invasive" species. Ectoparasites are those that live on the host's body surfaces. Examples of these can be ticks on the skin of ruminants or canids, and leeches.

Photograph of a tick, an ectoparasite of many mammals (Image by Myriam Zilles at www.pixabay.com)

Endoparasites are parasites that inhabit some cavity or internal region of their hosts (it can be an organ or inside its cells). Nematodes and many protozoans are good examples.

Mesoparasites, on the other hand, are those that inhabit the external openings of their hosts (the external ear, the cloaca, etc.). These are often parasitic copepods found at the anterior end of the organisms they parasitize.

Some publications also use the term epiparasite to describe those parasites that parasitize species that, in turn, are also parasites of other organisms.

Macroparasitism and microparasitism

This "classification" is used to distinguish very small parasites, impossible to see with the naked eye (microparasites or unicellular parasites), from parasites that are large enough to see them without any type of optical instrument (the macroparasites).

Digenetic parasites and monogenic parasites

According to the characteristics of their life cycle, parasites can also be classified as monogenic and digenetic, depending on whether they complete their cycle in the same host (monogenic) or require two or more different hosts to reproduce and feed (digestive).

Brood, nest or laying parasitism

This type of parasitism, known in English as “Brood parasitism” is typical of many species of birds (“parasites) that lay their eggs in the nests of other species (“ hosts), which nest on them and take care of them. of the young once they hatch.

It is a reproductive strategy in which parasitic species evade the physiological and energetic cost of brooding their eggs and ensuring the feeding and development of their young, imposing these tasks on the host species.

This type of parasitism can also be facultative or forced. In the case of birds that optionally “parasitize” others, they may lay some eggs in the host's nest and care for others themselves; but obligate brood parasites are completely dependent on their hosts for reproduction, which is materialized in offspring.

Brood parasitism can occur between different species (interspecific) or between individuals of the same species (intraspecific).

Just as is true for other types of ecological interactions or relationships, there exists, between parasitic birds and the interacting hosts, a "coevolutionary arms race" through which special abilities or characteristics are developed that allow species to evade the environment. parasitism (hosts) or going unnoticed (parasites).

A good example of this is “egg mimicry”, which refers both to the ability of many birds to distinguish between the eggs of a parasitic bird and their own, and to the ability of some parasitic species to “mimic” them. eggs of the species they parasitize.

Examples of parasitism

Some examples of parasitism are intestinal worms in humans, fleas, ticks, mosquitoes or the cuckoo laying their eggs.

Multiple examples of parasitic species exist in nature, especially if one takes into account the fact that interspecific relationships can occur between plants of different species, between plants and animals, between animals of different species, or between microorganisms (eukaryotes and prokaryotes). and other "higher" organisms.

Let us use as an example parasites that affect human health such as T. cruzi and M. hirudinaceus.

Trypanosoma cruzi

Life cycle of Trypanosoma cruzi, an intracellular parasite that affects human health (Source: Basquetteur via Wikimedia Commons)

T. cruzi is the etiological agent of an endemic pathology in Latin America known as “Chagas disease”.

It is an endoparasite whose life cycle is digenetic, since it reproduces in two hosts: a mammal (which generally suffers the consequences of parasitism) and an invertebrate (the vector, which is “immune” to the presence of the parasite inside).

Usually T. cruzi affects human beings, among which it is transmitted through the bite of hematophagous insects (that feed on blood) reduvidae, which are hemiptera specifically belonging to the Triatomine subfamily.

This parasite enters the body of its mammalian host through the feces deposited by the insect in the vicinity of the sting site. From this site of entry, the cells of this protozoan go to the bloodstream, where they are distributed throughout the body, preferentially to the cardiac tissues.

Man serves as a multiplication site for T. cruzi cells, while the insect vector only functions in its transport from one host to another.

Macracanthorhynchus hirudinaceus

M. hirudinaceus is also an endoparasite that affects humans and other mammals such as pigs; It belongs to the group of helminths and is a species of the acantocephalus.

At the front end of its body it has a proboscis with a hook that allows it to adhere to its host, in addition, it lacks a digestive tract, so its nutrition depends entirely on its association with the intestinal tract of the different animals it parasitizes.

The parasites belonging to this species have a beetle that serves as a “vector” and an intermediary in their life cycle.

Parasitism in plants

Plant ectoparasites, aphids (Source: Pollinator at English Wikipedia via Wikimedia Commons)

Plants constantly suffer from parasitic invasions. Such invasions can be by ectoparasites or endoparasites, which undoubtedly affect the growth and development of plant organs and tissues.

Various species of aphids can be described in a large list where the main plant ectoparasites are pointed out. These small insects, in turn, can be vectors for other parasites, almost always endoparasites that affect the health of crops.

However, many important phytopathogenic fungi have characteristics of endo- and ectoparasites at the same time, since they colonize both the surface of plants and the interior of their tissues. The species of the genus Phytophthora are one of the parasitic fungi best known for affecting plants of the Solanaceae group.

Examples of endoparasites

Among the endoparasites that take humans as hosts, worms stand out. Most of these worms are found in the digestive system. However, a few species can live in other parts of the body, such as the liver and blood.

These parasites are characterized by having protective layers that defend them from gastric juices, resistance to pressure difference, resistance to low oxygen levels, and high reproduction rate. Some examples of pinworms in humans are:

-Ascaris (Ascaris lumbricoides). Slender parasite, it can measure up to 40 cm in length. It generates diarrhea, anemia and constipation.

- Hookworm (Ancylostoma duodenal). It is a small worm, about 12 millimeters, that bites the intestinal wall causing bleeding. It generates the disease called anchiostomiasis.

-Necator (Necator americanus). It is smaller in dimension than the ascaris, rarely exceeding 10 millimeters. Causa necatoriasis. Some symptoms of this disease are anemia, diarrhea and problems in the development of the person.

-Tricocephalus (Trichuris trichiura). It is a worm about 5 centimeters long. It produces diarrhea, anemia, vomiting and general pain.

-Tenia (Taenia solium). Also called tapeworm, tapeworms cause anemia and diarrhea. Sometimes it can reach the nervous system and cause serious problems.

-Oxyide (Enterobius vermicularis). This parasite causes itching in the anal region and can cause seizures in children under 10 years of age.

-Filaria (Filaria bancrofti). This parasite produces elephantiasis, which consists of the swelling of certain areas of the body.

Examples of ectoparasites

Some examples of ectoparasites are:

-Nits. They are usually found on the scalp of some human beings, from where they suck the blood that allows them to live.

-The fleas. Similar to nits, they sustain themselves on the blood of their hosts. They are common in cats and dogs. There are also some species that take humans as hosts.

-Ticks. Like fleas and nits, these animals survive on the blood they suck from their hosts. They are generally found in dogs and other mammals.

-Mites. These microscopic parasites attack the skin causing itching, redness and inflammation of the infected skin. They are the cause of scabies.

-Mosquitoes. These parasites feed on the blood of mammals. They are often carriers of other parasites that cause diseases such as dengue and malaria.

-Aphids. Aphids, commonly called aphids, are parasitic insects that attack acacias. These insects extract the sap of the plants by opening holes in the leaves of this.

-Mistletoe. This is an example of a parasitic plant. The mistletoe fixes its roots in another plant, from which it extracts the necessary nutrients to live.

Invaders and usurpers

Parasitism goes beyond a simple relationship in which the parasite feeds on the host. There are cases in which the individual takes advantage of the resources or abilities of another individual, which constitutes a type of parasitism. Some examples are:

-Cuckoo. The cuckoo is a bird that lays its eggs in the nests of other birds (mainly in the nests of the birds Prunella modularis and Anthus pratensis). The method of this bird is to leave the egg in another nest with eggs of similar colors. When the cuckoo hatches, it throws the host's other eggs from the nest and is raised as an only child.

-Formica sanguinea This species of ants invades the nest of other smaller ants. Once the invasion is complete, the invading queen kills the invaded queen and enslaves the workers and drones.

-Crows (kleptoparasitism). Crows are a case of kleptoparasistism, which consists of stealing food that another predator has hunted. In this case, the parasite takes advantage of the hunting abilities of other individuals.

References

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4. Sorci, G., & Garnier, S. (2008). Parasitism.
5. Sorci, G., & Garnier, S. (2019). Evolutionary Ecology: Evolution of Parasitism.
6. Yarwood, CE (1956). Obligate parasitism. Annual Review of Plant Physiology, 7 (1), 115-142.