Parasitism: Definition, Types and Examples of Parasitism

The ecological interaction between two organisms in which one is benefited while the other either gets harmed or killed is called parasitism. The organism that gets benefits at the expense of others is called parasite and other organisms are called the host. Parasites can be classified into many groups based on their characteristics, size, life cycle and connection to the host life form.

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What is Parasitism?

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Parasitism is a symbiotic relationship between a variety of species, where one of the organisms, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life.

The word parasite is a latin word meaning “survival of one organism at the cost of other”. Single-celled protozoans that causes malaria, sleeping sickness, amoebic dysentry, etc. are some of the examples of parasites. The size of parasites are usually smaller than the hosts. 

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Types of Parasitism

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Parasitism can be classified into various groups based on their-

• Characteristics
• Relations with their hosts
• Size
• Life cycles

Now, we are going to observe the classification of various types of parasitism and aim to define them. 

• Obligate Parasitism

This type of parasitism occurs when the parasite is completely reliant on the host to complete its life cycle. Obligate parasites cannot survive in the absence of the host. As a result, they do not cause significant harm to the host. Obligate parasites are completely dependent on the host to complete their life cycle. 

Obligate parasitism is found in fungi, bacteria, and viruses. Head lice, for example, die when removed from the human scalp.

• Facultative Parasitism

Facultative parasites do not require the host to complete their life cycle; they can survive without the host and only occasionally engage in parasitic activities. Certain plants, fungi, animals, and microbes are capable of acting as facultative parasites. Strongyloides stercoralis of nematodes species is a specific example of Facultative Parasitism.

This is a type of roundworm that can infect humans and cause the disease strongyloidiasis, but it can also be found free-living.

• Ectoparasitism

Ectoparasites, such as lice and ticks, are parasites that live on the outside of the host's body.

• Endoparasitism

Endoparasites live within the body of the host life form, such as nematodes and hookworms.

• Mesoparasitism

Mesoparasites enter the body of the host through external openings like the outer ear of the cloaca.

• Epiparasitism

An epiparasite is a parasitic organism that feeds on another parasitic organism. Hyperparasites and secondary parasites are other names for epiparasites. A protozoan which lives in a flea, survives on a dog is one of the examples of epiparasite.

• Social Parasitism

In social insects, social parasitism is a widespread and fascinating phenomenon. It may happen within a single species (intraspecific) as well as between two species (interspecific).

Social parasites take advantage of social insects like ants, bees, and termites. They may use mimicry to invade the hive. Some bumblebees invade the hives of other species of bees, making that species raise the parasites young. 

Tetramorium inquilinum, for example, is a parasitic ant that spends its entire life on the backs of other ant species, effectively enslaving the hosts. These parasitic species have benefited from food and transportation, but the ants have evolved to be so frail as a result of this intense type of parasitism that if they fall off their host, they will not be able to crawl back on, and they will die.

• Brood parasitism

Brood parasitism entails the rearing of offspring. Some of the bird species, such as cowbirds and cuckoos, lay their eggs in the nest of another species rather than making their own nests.

This process is classified as Parasitism because the species of the bird who laid her egg in another bird's nest don’t have to spend energy raising her offspring and gaining benefit. While the other species are victimized as they do have to utilize energy to raise offspring, and it is not their hereditary entity.

The parasite species may also knock the eggs of other species out of the nest, forcing the host to raise only the parasite's offspring. In fish, brood parasitism may occur. It's a form of kleptoparasitism, which includes taking food from the host either directly or indirectly; in this case, food that should have gone to the host species goes to the parasite species.

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Examples of Parasitism

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As parasites are a form of interaction between two different species, there are several examples. Some of the most famous parasitic examples are described below. 

Parasitism in Human beings

Fungi, leeches, lice, bacteria, protozoa, tapeworms, and other 100 varieties of parasitic species infect humans. Helminths is an organism that lives within the host's intestine which induces a variety of infectious diseases such as jaundice, malnutrition, and diarrhea and sometimes even causes the death of the host. Many infectious diseases out there result from the organisms that parasitize human beings like common cold. Viruses and bacteria are to blame for both infections.

Parasitism in Insects

Insects are prone to parasitism. Parasites that feed on larvae and young insects are known as entomophagous parasites. A few insects lay their eggs in the bodies of other insect species' larvae. When the eggs hatch, the larva is eaten by the young one, who gets nutrients from it.

Parasitism in Plants

Aphids are small green insects that feed on plant sap and parasitize them. Several forms of fungi invade crops, causing them to rot and spoil fruits, vegetables, and grains. The parasitic plants have haustoria, which are adapted roots that bind to the xylem or phloem of the host plant and drain it of nutrients and water.

Parasitism in Fish

Fish are parasitized by a variety of organisms, and different populations of the same species of fish living in the same area can often be distinguished by their parasites. Copepods (small crustaceans), nematodes, and leeches are parasites that bind to the fish's gills and live there. Cymothoa exigua is a parasitic isopod that feeds on fish.