Parasitism – Definition, Types, Examples

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What is Parasitism?

Parasitism is described as a connection between two species in which one organism lives on or in the other and benefits by causing harm.

  • The word parasite comes from the Latin form of the Greek word, which means “one who feeds at another’s table.”
  • Generally speaking, parasitism is a connection between two organisms in which one organism benefits at the expense of the other.
  • The parasite is the creature that benefits, while the host is the organism that is hurt.
  • Tapeworms, fleas, and barnacles are all types of parasites. Tapeworms are flatworms that inhabit the intestines of animals including cows, pigs, etc. They consume the partially digested meal of the host, depriving it of nutrients.

Size of Parasites

  • The majority of parasites are considerably smaller than their hosts. Malaria parasites, for instance, are minute protozoa, while human pinworms measure less than one centimetre in length.
  • Nonetheless, some species attain extraordinary sizes. For instance, a didymozoid trematode infecting the sunfish, Mola mola, reaches a length of 12 metres while having a relatively small diameter and a much smaller volume than the host fish, which can weigh up to one tonne.
  • Similarly, the broad fish tapeworm, Diphyllobothrium latum, which resides in the intestines of numerous fish-eating mammals, including humans, attains a length larger than 10 metres, yet its volume is far smaller than that of its hosts.
  • Parasites are frequently much larger than their free-living relatives, which may seem startling at first appearance. This phenomena is seen in the phylum Platyhelminthes, which contains flatworms.
  • Most free-living flatworms are relatively small, measuring less than 1 mm to a few millimetres in length, however parasitic flatworms such as flukes and tapeworms are often much larger, measuring up to several centimetres for flukes and several metres for tapeworms.
  • This may have two possible causes. First, parasitic flatworms in the organs and tissues of their hosts have a far more abundant and consistent food supply than do free-living species; consequently, food supply is not a size constraint.
  • Second, selection may have promoted multiple and large gonads and, consequently, a large body size because parasites must produce a large number of offspring to overcome the risks associated with infecting other hosts.
  • But selection may also favour a smaller body size because parasites depend on living hosts and it is essential that hosts survive at least until the parasite’s children may infect other hosts. The reduced size of parasites in comparison to their hosts is consequently advantageous.

Mechanisms of Dispersal

  • Dispersal is essential for all species, regardless of whether they are free-living or parasitic, because a population confined to a small area runs the risk of extinction if environmental conditions become unfavourable, and because dispersal reduces inbreeding and the loss of evolutionary adaptability.
  • A third point is significant for parasites: dispersal may lessen the likelihood of hosts being overinfected. Important features of dispersal include dispersal over short distances away from a single host, dispersal in space and range extension over greater distances, and dispersal over time.
  • Trematode larvae demonstrate that a single stage can be responsible for all three components of dispersal. Larvae (cercariae) are frequently discharged against their will into the respiratory currents of snails in which they have matured, resulting in their dispersal away from the host.
  • They actively swim and use their tails to stay afloat, allowing them to be disseminated across great distances by water currents. In several species, the tail is equipped with specific flotation structures that extend the period of floating.
  • Adult flukes generate eggs, and larvae in their snail hosts are produced over extended periods, months, or even years, resulting in the dispersion of the parasite over time.

Types of Parasitism

Types of Parasitism according to where they live

  1. Ectoparasites – Ectoparasites refer to parasites that reside on the surface of the host’s body. Ectoparasites include fleas, ticks, and others.
  2. Endoparasites– Endoparasites are parasites that reside inside the body of the host. Examples of endoparasites include roundworms and blood-borne protozoa.
  3. Meso Parasites– Meso parasites are parasites that enter the host body through a hole and implant themselves. Copepods are an example of mesa-parasites.

Types of Parasitism on the Basis of the Life Cycle

Based on their life cycles, parasites can be divided into two categories:

  1. Obligate Parasite– This type of parasitism is characterised by the parasite’s total dependence on the host to complete its life cycle. Without the host, obligatory parasites perish. Thus, they do not cause substantial damage to the host. Fungi, bacteria, and viruses are parasites by necessity. When head lice are removed from the human scalp, for example, they die.
  2. Facultative Parasite– In this type of parasitism, the parasite is able to complete its life cycle without the host and can survive without the host. The free-living nematode species Strongyloides stercoralis produces the disease strongyloidiasis when it infects people.
  3. Monogenic – Monogenic parasites only require a single host to complete their life cycle. 
  4. Digenetic – Parasites with a digestive life cycle require multiple hosts to complete their life cycle. Plasmodium vivax, the parasite that causes malaria, is a digenetic organism. To complete its life cycle, the parasite must parasitize both humans and mosquitoes.

Types of Parasitism on the Basis of their Strategies

On the basis of their strategies, parasites are divided into three categories:

  1. Directly Transmitted Parasites– Directly transmitted parasites are those parasites that reach their hosts’ bodies on their own. These include fleas and mites.
  2. Trophically Transmitted Parasites- Trophically transmitted parasites are parasites that are ingested by hosts when they consume their prey. Roundworms and trematodes are examples.
  3. Vector Transmitted Parasites– Parasites that rely on an intermediate host to transport them to their definitive host are referred to as vector-transmitted parasites. Protozoan that causes sleeping sickness and is transmitted by bug bites.

Types of Parasites based on effect on its host

  1. Parasitic castrators: Parasitic castrators reduce a host’s reproductive capacity in whole or in part, but do not kill it. The energy that the host would have expended on reproduction is instead used to support the parasite. The barnacle Sacculina, for instance, degenerates the gonads of crabs such that males take on the look of females.
  2. Parasitoids: Eventually, parasitoids kill their hosts, making them virtually carnivores. All parasitoids are insects that lay their eggs within or on the host. When an egg hatches, the developing adolescent provides nourishment and shelter.
  3. Micropredator: A micropredator assaults multiple hosts, allowing the majority of hosts to survive. Micropredators consist of vampire bats, lampreys, fleas, leeches, and ticks, among others.
  4. Necrotrophic: Necrotrophic parasites, also known as parasitoids, consume a portion of the host’s tissue until the host dies from tissue or nutrient loss. 
  5. Biotrophic: Biotrophic parasites do not inflict enough damage to their hosts to kill them; they must maintain their hosts alive in order to exist.

Types of Parasites Based on Size

  1. Macroparasitism: Macroparasites are parasites that may be observed with the naked eye.
  2. Microparasitism: Microparasites are too small to be viewed with the naked eye and require a microscope. Typically, they are unicellular, such as protozoa.

Other Types of Parasites

1. Brood Parasitism

  • Brood parasitism is parasitism in which young parasites are nurtured by their hosts.
  • Parasitism of the brood involves the rearing of young. Birds that practise brood parasitism, such as cowbirds and cuckoos, lay their eggs in the nest of another species rather than building their own.
  • This is a sort of parasitism since the species that lay their eggs in other species’ nests profit (they don’t have to spend energy raising young) while the other species are hurt (they must expend energy raising young) (they do have to use energy to raise young, and it is not their genetic material).
  • Occasionally, the parasite species will even remove the eggs of the other species, forcing the host to raise just the parasite’s offspring.
  • Parasitism of the brood can also develop in fish. It is a form of kleptoparasitism, which includes directly or indirectly stealing food from the host; in this situation, food that would normally go to the host species is instead consumed by the parasite species.
  • Example- Cuckoo.

2. Klepto Parasitism

  • The parasitism in which the parasite steals the host’s food is known as kleptoparasitism.
  • Example of Skuas stealing food

3. Sexual Parasitism

  • Sexual parasitism is the sort of parasitism in which males are dependent on females for survival.
  • Example- anglerfish

4. Epiparasitism

  • A parasite that parasitizes another parasite is called an epiparasite. They are also called secondary parasites or hyperparasites.
  • A protozoan that lives in a flea that lives on a dog is one example.

5. Social

  • Social insects like ants, bees, and termites are easy prey for social parasites. They might do this to get into the hive.
  • Some bumblebees get into the hives of other types of bees and raise their young there.
  • Tetramorium inquilinum is a parasitic species of ant that lives its whole life on the backs of other species of ants, making them its slaves.
  • This parasite species has gotten food and a way to get around, but the ants have become so weak from this extreme form of parasitism that if they fall off their host, they won’t be able to crawl back on and will die.

6. Hyperparasitism

  • Hyperparasites eat another parasite, like protozoa living in helminth parasites or facultative or obligate parasitoids whose hosts are either normal parasites or parasitoids.
  • There are also levels of parasitism beyond secondary, especially in parasitoids that can do both. There can be up to five stages of parasitism in oak gall systems.
  • Hyperparasites can control the number of their hosts, and this is why they are used in farming and, to a lesser extent, in medicine.
  • The controlling effects can be seen in how the CHV1 virus helps stop the damage that chestnut blight, Cryphonectria parasitica, does to American chestnut trees and how bacteriophages can stop bacterial infections.
  • Even though not much research has been done on the subject, it is likely that most pathogenic microparasites have hyperparasites that could be very useful in both farming and medicine.

7. Adelphoparasitism

  • Adelphoparasitism, which comes from the Greek word for “brother” (), is also called “sibling-parasitism.” It happens when the host species and the parasite are closely related, often in the same family or genus.
  • In the parasitoid Encarsia perplexa that lives on citrus blackflies, unmated females can lay haploid eggs in fully developed larvae of their own species to make male offspring. The marine worm Bonellia viridis has a similar way of reproducing, but its larvae live in the water.

Facts of Parasitism

  • Parasitism, like predation, is a type of interaction between a consumer and a resource. However, unlike predators, parasites are usually much smaller than their hosts, do not kill them, and often stay in or on them for a long time.
  • Animal parasites are very specialised and reproduce faster than the animals they live on. Flukes, some species of tapeworms, and fleas that live on vertebrate hosts are common examples.
  • Parasites lower the fitness of their hosts by causing general or specific parasitic diseases, such as parasite castration or a change in the host’s behaviour. Parasites get stronger by using their hosts for resources they need to stay alive, like food, and by using intermediate hosts to help them move from one definitive host tissue to another.

Examples of Parasitism

Parasites are a type of interaction between two different species, so there are many examples. Here are some examples of parasitism that happen often.

Parasitism in Humans

  • Over 100 different kinds of organisms, such as fungi, leeches, lice, ticks, mites, tapeworms, protozoa, viruses, and helminths, can live on humans and make them sick.
  • Helminths are long worms that can live in the intestines and grow to be up to a metre long. They can lead to problems like malnutrition, jaundice, diarrhoea, and even death in the worst cases.
  • But they can be treated with drugs that kill parasites. Organisms that live on people, like viruses and bacteria, cause all infectious diseases, including the common cold.
  • Many parasites that live on people can also live on other mammals and birds.

Parasitism in Plants

  • Aphids are tiny, parasitic insects that feed on the sap of plants. Numerous varieties of fungi are also capable of attacking plants and destroying wheat, fruits, and vegetables.
  • Some plants are themselves parasitic. Parasitism has developed at least 12 times in angiosperms (flowering plants), and 4100 species (approximately 1%) of angiosperms are parasitic.
  • Parasitic plants develop haustoria, which are modified roots that drain water and nutrients from the xylem and/or phloem of the host plant. Some plants are mycorrhizal fungus parasites.
  • This frequently occurs when a species of plant has evolved to no longer generate chlorophyll. Since it is no longer able to photosynthesize, it must get nutrients for energy through alternative means.

Parasitism In Insects

  • Entomophagous parasites are parasitic insects that feed on other insects. Typically, these parasites prey on insect larvae or young. Some insects lay their eggs within the bodies of the larvae of other species; when the eggs hatch, the parasitic young kill and consume the host larva, obtaining nourishment from it.
  • Occasionally, the parent parasite paralyses a host, which is then consumed by its offspring. This occurs commonly in wasps such as Ampulex compressa, whose young eat paralyzed cockroaches that have been stung by the parent.
  • Other wasps, such as Ropalidia romandi, burrow into their host’s abdomen and reside there. They do not kill their hosts, but they can alter their appearance and behaviour, as well as render them sterile.
  • Parasitism is widespread among insects. Almost all insect species are parasitized by at least one form of insect parasite.

Parasitism In Fish

  • There are several creatures that are capable of parasitizing fish, and sometimes different populations of the same species of fish living in the same region can be distinguished based on their parasites.
  • Some parasites, including copepods (small crustaceans), worms, and leeches, inhabit the gills of fish. Cymothoa exigua is a parasitic isopod (another kind of tiny crustacean). It penetrates the mouth of a fish and finally severs its tongue.
  • The isopod then inhabits the former tongue’s location and becomes the new tongue. The host fish can still eat and will survive with an isopod in its mouth, but the isopod will devour a little portion of its blood and mucus.
  • Cleaner fish, such as bluestreak cleaner wrasses, remove parasites and dead skin from other fish, even large predatory fish that might otherwise consume them.
  • Fish parasites in dishes containing raw fish, such as sushi, can pose a threat to human health, as the parasites in these fish can also infect humans.
  • In the developed world, infection from eating raw fish is uncommon, and some raw seafood is frozen overnight to prevent contamination.

Why Do We Need Parasites?

It’s tempting to believe that parasites should be removed since they hurt their hosts. At least fifty percent of all known species are parasitic. Parasites play an essential role within an ecosystem. They aid in controlling dominant species, hence promoting competition and diversity. Parasites provide an evolutionary purpose by transferring genetic material between species. In general, the presence of parasites is indicative of a healthy ecology.


  • Rohde, K. (2013). Parasitism. Encyclopedia of Biodiversity, 656–673. doi:10.1016/b978-0-12-384719-5.00103-9 
  • Olano JP, Weller PF, Guerrant RL, Walker DH. Principles of Parasitism: Host–Parasite Interactions. Tropical Infectious Diseases: Principles, Pathogens and Practice. 2011:1–7. doi: 10.1016/B978-0-7020-3935-5.00001-X. Epub 2011 Apr 29. PMCID: PMC7149714.
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