Parasitism

Parasitism is a type of ecological association between two different species, where one of the species benefits from the association at the expense of the other, usually without killing the organism. 

The organisms which get benefitted from the association are called parasites, and the others which have detrimental effects from the association are called the host. 

The parasites mostly rely on the host to derive nourishment while certain parasites must infect their specific host to survive, reproduce, and complete their life cycle; for example, the Rickettsia species, which is an intracellular bacterium that reproduces within the vacuoles of their host cells. Such parasites that cannot complete their life cycle in the absence of a host are called holoparasites or obligate parasites. 

General features of parasites

The following are the general features of parasites:

• Parasites are usually smaller than the host. 

• They can infect both vertebrate and invertebrate hosts. 

• They generally do not kill the host but can indirectly harm them by invading and spreading pathogens that affect the host’s metabolism. 

• Parasites have hooks or suckers to attach to their host to derive nourishment. 

• They can be protists, bacterias, viruses, plants, fungi, and animals. The majority of parasites belong to animal species. 

Types of parasites

Ectoparasites and endoparasites

Ectoparasites live over the surface of their host, while endoparasites live inside the host body. Examples of ectoparasites are lice and fleas that suck blood from their host. Endoparasites can be intracellular like Plasmodium, malaria-causing protozoan parasite, Trypanosoma (causes sleeping sickness), or Leishmania donovani (causing leishmaniasis). 

The fundamental differences between ectoparasites and endoparasites are as follows:

• Ectoparasites can be facultative or obligate parasites, while endoparasites are primarily obligate parasites.

• Ectoparasites exhibit aerobic respiration, whereas endoparasites often exhibit anaerobic respiration. 

• Ectoparasites can be temporary, intermittent, or permanent, whereas endoparasites are generally permanent parasites. 

• Both show specific specialisation; ectoparasites like fleas, bedbugs, and lice lack wings, while endoparasites like Taenia saginata (tapeworm) and Taenia solium (pork worm) lack digestive organs. 

Temporary and permanent parasites

Temporary parasites live in contact with the host for only a part of their cycle or occasionally at the time of feeding, for example, bedbugs and leeches. In contrast, permanent parasites live in contact with the host throughout their life, for example, Ascaris lumbricoides, Entamoeba, etc. 

Holoparasites and Hemiparasites

Parasites that are entirely dependent on the host for all their requirements and cannot complete their life cycle in the absence of a host are called holoparasites or obligate parasites; for example, Rafflesia and Cuscuta (plant parasites). Cuscuta or amarbel plants lack chlorophylls to carry out photosynthesis, thus, infecting the host plants to derive nourishment from them and receive the flower-inducing hormone or florigen from the host species. Hemiparasites, or facultative parasites, are capable of independent existence even in the absence of host species, for example, Loranthus and Viscum. 

Pathogenic and non-pathogenic parasites

Parasites that do not harm the host body are called non-pathogenic parasites, like Entamoeba coli, a protozoan protist found in the gastrointestinal tract of humans transmitted by faecal or oral matter. On the other hand, Entamoeba hystolytica, a  pathogenic parasite, causes amoebiasis (colon infection). Some other examples of pathogenic parasites are Vibrio comma (cholera), Corynebacterium diphtheriae (diphtheria), and Mycobacterium leprae (leprosy). 

Digenetic and monogenetic parasite

Digenetic parasites depend on two different host species to complete their life cycle – one of them is the primary host while the other is the secondary host. For example, Plasmodium, a malaria-causing protozoa protist, depends on two hosts, i.e., humans and female Anopheles mosquito, to complete its life cycle. Humans are the primary host within which the parasites divide asexually into their cells. The infected host’s cells then attack and rupture the RBCs. The ruptured red blood cells secrete a toxin called hemozoin, which gives rise to malarial symptoms like high recurring fever, chills, and shivering. 

Whereas the female Anopheles mosquito acts as a vector and a secondary host, within which the protozoan parasites undergo asexual reproduction. 

Digenetic parasites exhibit heteroxenous or complex life cycles. Monogenetic parasites infect a single host species and exhibit a monoxenous life cycle. 

Brood Parasitism or Spatial Parasitism

Cuckoo birds show brood parasitism during the breeding season. With time, the parasitic birds evolved to resemble the eggs of the host in size, shape, and colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest. The cuckoo bird lays eggs in the crow’s nest. The eggs of cuckoo look similar to the eggs of crows. Thus, the host bird incubates the eggs laid by the parasitic bird, assuming it as their own. 

Mode of infection

The parasites may infect the new host in the following ways:

• Boring through the skin (hookworm)

• Direct contact with an infected person (louse)

• Introduction into the body through insect bite; for example, tsetse fly acting as a vector for Trypanosoma cruzi which causes sleeping sickness; sandfly bite carrying Leishmania parasites that cause leishmaniasis (Kala-azar or black fever); or mosquito bite. 

• By swallowing food or consuming water contaminated with cysts of Entamoeba, eggs of Ascaris, or larvae of tapeworms. 

Adaptations for parasitic life

The following are the various adaptations for parasitic life:

• Anaerobic respiration in internal parasites

• Adhesive organs, like suckers in leeches and tapeworms, adhere to the skin or surface of the host. 

• Well-developed and complicated reproductive organs that lead to high reproductive efficiency

• Lack of unnecessary organs like wings in bedbugs and fleas, and the digestive system in Taenia. 

Conclusion

Parasitism, predation, competition, commensalism, amensalism, and mutualism are six types of symbiotic relationships. Parasitism is a type of symbiotic association between two organisms of different species in which one organism gets benefited at the expense of the other. The ones who benefit are called the parasites, which are usually smaller than the host on which they invade to derive nourishment or undergo reproduction and complete their life cycle.

Parasites have adapted themselves according to their site of infection in host species. In commensalism, one species gets benefited from the symbiotic association, while the other organism of different species is neither benefited nor affected. In mutualism, both species benefit from the association; for example, lichens, a composite entity jointly formed by an alga (phycobiont) and a fungus (mycobiont). The fungus provides fixation, water, minerals, and shelter to an alga, and in return, algae manufacturers food for the fungus. In predation, one species get benefitted, i.e., the predator, while the other species gets killed, i.e., the prey. In amensalism, one species is harmed while the other is neither benefited or harmed; for example, penicillium produces a toxin called penicillin that kills other microorganisms but is not affected itself. Competition is an interaction between two similar or different species, in which both of them suffer. Competition occurs due to the limited availability of resources between closely related species.