Parasitism is a type of ecological association between species where one of the species benefits from the association at the expense of the other, usually without killing the organism.
- The organisms that obtain the benefit from the association is called a parasite, whereas the organisms that are harmed by the association is called the host.
- The term parasite is derived from the Greek word ‘parasitos’ meaning ‘one that eats at the table of another’ to indicate the feeding habit of the organism.
- Even though traditionally, parasitism was considered a discrete interaction, many researchers believe that the interaction merges into other interspecific interactions.
- All infectious agents causing illness in different groups of living beings are included as parasites.
- The parasites exist in this association to use the host as both habitat and a source of nourishment; however, the extent and duration of association might differ.
- Parasitism is a type of consumer-resource interaction like predation, but unlike predators, parasites do not kill their host and often live in or on them for a longer period of time.
- Some of these associations are highly specific, and thus, a parasite of one species might not be parasitic to other species.
- These parasites are smaller than their host with a highly specialized metabolism. These also have a high reproduction rate to ensure their survival.
- These organisms can also shape the behavior of individuals in the host population, which leads to the evolution of behavioral syndromes.
- Even though most parasites feed on host species for nourishment, some parasites might use some organisms as secondary hosts for the transmission of one primary host to another. In secondary hosts, the parasites do not cause as much harm in primary hosts.
- The pattern and extent of harm caused to the host are central to the definitions of parasitism as different parasites can impair the host’s health through a wide range of mechanisms.
Types of Parasitism
Parasitism is of different types depending on the size, characteristics, interaction with the host, and their life cycles. Some of the parasites can exist in multiple classifications depending on the basis of classification.
1. Obligate Parasitism
- Obligate parasitism is a type of parasitism where the parasite is dependent on the host for its survival, and thus the interaction is obligate.
- These parasites cannot complete their lifecycle without exploiting an appropriate host.
- The parasites adapt and evolve over time so that they can no longer exist without the association with a host. The parasites develop different strategies in order to escape the host defenses and exploit the host.
- The parasites involved in obligate parasitism usually do not cause the death of the host as they require a host for their survival. However, some parasites might kill the host for transmission.
- Most of the obligate parasites include prokaryotes like bacteria and viruses. Some insects like head lice are also considered obligate parasites as they die if removed from the human scalp.
- Rickettsia species are intracellular bacteria that reproduce within the vacuoles of host cells.
- These species cannot be grown on artificial media and require tissue or cell cultures since these are obligatory species.
- The transmission of Rickettsia from one host to another occurs through arthropods like lice and fleas.
- These bacteria have evolved mechanisms to evade the host immune defenses, mostly by surviving within the cellular components like vacuoles.
- The parasitic interaction of Rickettsia is necessary for the completion of their lifecycle, including reproduction.
2. Facultative Parasitism
- Facultative parasitism is a type of parasitism where the parasites do not depend on the host for survival or the completion of their life cycle.
- These organisms use facultative parasitism as a form of survival strategy due to the shortage of resources in the environment.
- Facultative parasitism is common in bacteria and fungi that can exist in multiple habitats and transmit between hosts.
- Some of these parasites feed on living organisms, but when the organism eventually dies, they continue to feed on the dead organisms without the need for parasitic activity. This process is common in many fungi infecting plant species.
- Facultative parasitism, can, however, develop into obligate parasitism over time or due to changes in its habitat and composition.
Example- Armillaria species
- Armillaria is a parasitic fungal species that parasitize living plants for nutrients.
- The fungi feed on the roots of trees, but the interaction is facultative as the fungi can exist both as parasitic as well as free-living organisms.
- However, the fungi can survive even after the trees dies as the fungi can feed on the woods without the need for any parasitic activity.
- The organisms are also important ecological agents involved in the recycling of nutrients through microbial decomposition.
- Ectoparasitism is a type of parasitism where the parasite is present outside or on the surface of the host’s body.
- These parasites are termed ectoparasites, and they usually inhabit the skin or outgrowth of the skin on the host.
- The vast majority of ectoparasites are invertebrates that result in lesions on the surface of the host body.
- Many of the known ectoparasites are known to be vectors of pathogens that transmit to the host while feeding or defaecating.
- Some of the ectoparasites can transform into endoparasites over time as a form of defensive mechanism and adaptation to a parasitic lifestyle.
Example- Head lice
- Lice are ectoparasites that colonize different parts of the host body ranging from head to lower part of the human body.
- Head lice mostly live on the hair of the head, but these can wander to other parts of the body.
- These can get transmitted from one host to another either by direct contact or by the use of personal belongings like combs and towels.
- The insects feed on the blood from the scalp of the host, and the scalp also provides a living space to the parasite.
- Lice lay eggs on the surface of the human head, which then develop into larvae and adults as a part of the life cycle.
- Endoparasitism is a type of parasitism where the parasites are present within the body of the host.
- Endoparasitism can either be intercellular where the parasites reside within the spaces in the host body or intracellular where they exist within the cells of the host.
- Most of the intercellular endoparasites are protozoans and invertebrates like worms, whereas the intracellular endoparasites include bacteria, viruses, and fungi.
- Endoparasites directly feed on the nutrients present in the host body and result in different forms of diseases.
- Intracellular parasites usually depend on a third organism, known as vector or carrier for transmission from one host to another.
- Plasmodium is an endoparasite that feeds on the red blood cells in mammals, birds, and reptiles.
- Like most endoparasites, Plasmodium depends on a third organism (female Anopheles mosquito) for transmission from one host to another.
- Plasmodium exists in two different hosts, both of which are essential for the completion of the life cycle of the protozoan.
- The protozoans complete their sexual lifecycle in mammals whereas the gametophytic lifecycle is completed in the female mosquito.
- The interaction between Plasmodium and humans is an obligate interaction that is essential for the survival of the parasite.
- Mesoparasitism is a type of parasitism where the parasite lives partly within the host’s body.
- These parasites enter the body of the host through openings like the ear and cloaca and live within the body for a particular period of time.
- The parasitism is mostly facultative as the species do not need to enter the body of the host to obtain nutrients or to complete their lifecycle.
- Most of the species existing in a mesoparasitism interaction include invertebrates like insects.
- Copepods are crustaceans that exist in freshwater and saltwater either as free-living or parasites.
- The organism parasitizes fish, sharks, and marine mammals that occasionally enter the body of the host through gills and cloaca.
- Copepods damage the gills by feeding on the delicate tissues of the gills. These also feed on the blood circulating through the gill lamellae.
- These species can also exist as ecto or endoparasites depending on the stage of their lifecycle.
- Epiparasitism is a type of interaction between two parasites where one parasite parasitizes the other.
- Epiparasites are often termed as hyperparasites or secondary parasites that share some characteristics with the primary parasites like habitat and nutritional requirements.
- Depending on the extent of parasitism, there are different degrees of epiparasitism. Some epiparasitism associations are facultative or incidental.
- The facultative or incidental epiparasitism occasionally occur without co-inhabitation, mostly due to the lack of food resources.
- Epiparasitism also occur between organisms of the same species as they share common habitats and nutritional requirement.
Example- In Plants
- Species of the family Viscaceae often exist in an epiparasitic association with each other that can either be facultative or obligate.
- In plants, the bark strands occasionally come in contact with the vascular cambium of the host and establish a position within the cambium.
- The continued interaction leads to the formation of sinkers that extend from the bark across the phloem to the xylem tissue.
- As a result, the parasitic species gain water and nutrients produced by the host species for their growth and survival.
- Some species even grow through the flowers resulting in persistent seeds and fruits. The parasitic plants thus, find better reproduction and dispersal through the interaction.
7. Brood Parasitism
- Brood parasitism is a type of parasitism where the parasites depend on the host to raise their young ones. This is a form of parasitism as the parasites conserve energy whereas the host has to spend extra energy.
- This form of parasitism usually exists in birds, insects, and fishes where the parasites manipulate the host to raise their young ones on their own.
- In some cases, the parasites even kick off the young ones of the host species forcing the host to raise the young ones of the parasite.
- The parasitic parents thus can use the energy conserved to spend on finding food and producing other offspring.
- Some of the young ones of parasites might exhibit mimicry in that they appear similar to that of the host young ones while others show no evolved trait.
- Brood parasitism is most common in birds as many avian brood parasites utilize the interaction to escape the process of development of their young ones.
- Some of the birds might be specialists that parasitize a single host species or a small group of species whereas some can parasitize a wide rade of host species.
- A common example is a common cuckoo where the male parasitizes a wide variety of hosts, but the female specializes in a single species.
- The female has genes that regulate egg coloration that allow them to lay mimetic eggs in the nest of the host species.
- The host species for female common cuckoo include reed warbler that feeds and raises the young ones of the parasite species.
8. Social Parasitism
- Social parasitism is a type of interaction between social animals like ants and bees where a parasitizing organism depends on the labor provided by the host species.
- The colony of organisms usually consists of one or more species of the organism consisting of a parasitic species and host species.
- The interaction can be obligate, facultative, temporary, or permanent, and the relationship can take many forms.
- The parasitic species usually gain benefits like food and transportation, but the species eventually evolve to be weak as they mostly cannot exist without the interaction over time.
- The nature of interaction might be different in different groups where some parasitic ants steal the food and eggs of host ants. Other species might live in the nests prepared by the host species.
- The most common form of social parasitism is observed in ants where the parasites are often slave-makers.
- The parasitic ants raid the colonies of other ants and steal their eggs and larvae. The larvae and pupae that are not consumed by the parasites are converted into worker slaves through chemical imprinting.
- The slave ants or host ants then collect more eggs, gather food, and feed the parasitic ants. When the colony moves to a new home, the slaves even carry the parasitic ends to the new location.
- The colony continues to grow as the association continues as long as the host ants are alive. It is quite easy to differentiate between host ants and parasitic ants as they are different in structure and are of different species.
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