What is parasitism?
Parasitism is a type of symbiotic relationship between two living organisms in which one benefits at the expense of the other. It can also be defined as a long-term relationship, in which one member called the parasites lives off or benefits from the other organism called the host, sometimes without killing the host.
Parasitism definition in biology
Parasitism in biology can be defined as an unhealthy relationship because the parasite deprives the host of nutrients and causes discomfort. In most cases, the presence of the parasites causes illness to the host organism. However, there are many parasites that carry and transmit disease. Parasitism is entirely different from mutualism where the two species benefits or commensalism where one benefits but doesn’t harm the other.
In this symbiotic relationship, as the parasite and its host evolve together, the parasite adapts to the host as its environment. Also, the host at times adapt ways of protecting themselves from the parasite. Some will even build a symbiotic relationship with another species that will help them get rid of the parasite. A common example of this relationship is the ladybugs that live on plants. The ladybugs get food by eating the aphids that attack the plant. Hence the plant benefits from this relationship with the ladybird as it gets rid of the aphids that parasitize it.
What is a Parasite?
A Parasite is an organism that lives in or on another organism depriving it of nutrients and causing harm. The word parasite was derived from the Latin form of the Greek word parasitos which means one who eats at the table of another. In a parasitic relationship, a parasite lives on or in the body of the host, causing harm and possibly death. However, although these parasites harm their hosts, it is not intentional. This is because it depends on the body of the host and its functions, such as digestion or blood circulation to survive. As the parasite and its host evolve together, the parasite adapts to the host as its environment. However, many parasites are pathogenic to their host.
Types of parasites
- Necrotrophic parasites
- Biotrophic parasites
- Monogenic parasites
- Digenetic parasites
- Parasitic plants
- Flatworms- Trematodes, Cestodes, and Monogeneans
- Horsehair worm
- True flies
- Twisted wing insects
- Parasitoid wasps
Read my article on What is a parasite? Examples of Parasites in Humans and Plants for further and detailed explanations on parasites and their examples.
Examples of Parasitism Relationships
- Tapeworms (segmented flatworms) get their food by eating the host’s partly digested food. They deprive the host of nutrients by attaching themselves inside the intestines of animals such as cows, pigs, and humans.
- Fleas depend on and harm dogs or humans by biting their skin causing them to itch, and sucking their blood. Also, fleas get food and a warm home from their host.
- Fleas parasitize warm-blooded vertebrates like ferrets, birds, rabbits, cats, dogs, rats, mice, squirrels, and humans.
- Aphid insects eat the sap of the plants they live on.
- Fungi cause disease of a lumpy jaw in hogs and cattle.
- Henneguya ictaluri, a parasitic cnidarian infects catfish and causes proliferative gill disease.
- A parasitic fungus attacks wheat and causes wheat rust.
- Sphaerospora renicola is a cnidarian that causes infection in carp fish.
- The downy mildew fungus attacks and parasitize fruits and vegetables.
- Liver flukes like Fasciola hepatica and Fasciola gigantica parasitize wild and domestic ruminants. Also, these liver flukes cause fascioliasis in humans.
- Cuscuta (dodder), a parasitic plant infects an acacia tree.
- Tetracapsuloides bryosalmonae, a cnidarian infects and parasitizes salmonids, causing Proliferative Kidney Disease (PKD).
- Mistletoe is a parasitic plant that steals nutrients and water from host trees.
- Treponema pallidum is a bacterium that infects and causes the diseases yaws, syphilis, and bejel amongst humans.
- Broomrapes are parasitic plants that infect crops like peas, tomatoes, carrots, and varieties of cabbage.
- A parasitic cnidarian, Myxobolus cerebralis, parasitize salmonids and causes whirling disease in them.
- Witchweeds infect grains and grasses.
- Parasitic fungi like Puccinia graminis, Endothia parasitica, Puccinia sparganioides, and Ceratocystis ulmi parasites plants.
- The fungus Ceratocystis ulmi parasitize European elm bark beetles and causes a plant disease known as Dutch elm disease which is spread by the beetles.
- The fungi Candida albicans and the genus Aspergillus parasitize and infect humans.
- Naegleria fowleri (brain-eating amoeba) infect humans and cause a disease known as Primary amoebic meningoencephalitis (PAM).
- Entamoeba histolytica is an intestinal parasite that parasitizes primates and humans. It causes a disease known as amoebiasis.
- Giardia lamblia is an intestinal parasite that infects animals and humans causing giardiasis.
- Haemophilus influenzae is a bacterium that infects children and adults causing meningitis, bronchitis, and influenza.
- Trichomonas vaginalis is a parasitic protozoan that infects humans and causes an STI known as trichomoniasis.
- Trypanosoma cruzi parasitizes bugs and is transmitted to humans through the bugs causing Chagas disease in humans.
- Campylobacter jejuni is a bacterium that infects poultry birds and humans.
- Leishmania parasites live in and infect sand flies. Humans can get sick from a disease called Leishmaniasis when bitten by an infected sandfly.
- Toxoplasma gondii parasitizes and infects most warm-blooded animals and humans. The parasite eventually causes the disease called toxoplasmosis.
- Acanthamoeba is a parasite that infects the human eye and causes Acanthamoeba keratitis, a severe eye infection that could lead to blindness.
- A cnidarian, Enteromyxum leei causes Enteromyxosis, a disease in cultured marine sparids.
- A protozoan parasite in the genus Phytomonas parasitizes the coconut palm plant and causes heart rot disease.
- Phytomonas stahelli, a plant parasite infects oil palm and causes sudden wilt disease in the plant.
- Phytomonas leptovasorum is a parasite of coffee crops that causes phloem necrosis disease in coffee crops.
- Ophryocystis elektroscirrha, a protozoan parasitizes butterfly larvae.
- The bacterium, Bacillus anthracis infects livestock and humans causing Anthrax disease.
- Borrelia, a bacterium parasitizes lice and ticks and is transmitted to humans by these insect vectors. This parasite causes Lyme disease or relapsing fever.
- The blood flukes of the genus Schistosoma infect and parasitize snails, cattle, dogs, pigs, rodents, goats, horses, and humans.
- Bacteriophages are viruses that infect bacteria destroying their cells.
- Viruses parasitize humans and cause viral infections and diseases like chickenpox, smallpox, herpes, rubella, pneumonia, measles, hepatitis, flu (influenza), HIV/AIDS, mononucleosis, mumps, and many others.
- Lung fluke ( Paragonimus westermani) parasitize humans and infects their lungs causing paragonimiasis.
- The Chinese liver fluke, Clonorchis sinensis infects humans and fish-eating mammals. Feeding on bile, the Chinese liver fluke infects the bile duct and gall bladder of humans.
- Monogeneans are flatworms that infect the gills and skin of saltwater and freshwater fishes.
- Gordian worms parasitize and develop in the bodies of grasshoppers, crickets, cockroaches, and some beetles.
- Ascaris are roundworms that parasitize the intestine of host animals like humans, dogs, pigs, and cats.
- Hookworms affect the skin lungs and small intestines of humans, cats, and dogs causing severe infection.
- Trichina worms are parasitic nematodes that live and reproduce in the small intestines of humans, rats, hogs, cats, dogs, and a wide variety of mammals.
- Pinworms live and mature in the large intestine and cecum of humans.
- Filarial worms parasitize arthropods and vertebrates. They cause elephantiasis and river blindness in humans.
- The dog heartworm is carried by mosquitoes and can infect dogs, cats, sea lions, ferrets, and sometimes humans.
- Acanthocephalans parasitize invertebrates, amphibians, fish, birds, and mammals.
- Aquatic leeches feed on the blood of birds, fishes, amphibians, and mammals.
- Ticks parasitize and feed on the blood of amphibians, birds, reptiles, and mammals.
- Mites like the Varroa parasites and Acarapis woodi live in the tracheae of honey bees.
- Spider mites, gall mites, and thread-footed mites are plant parasites.
- Sarcoptic mange mites attack animals burrowing them under their skin.
- The Demodex mite lives near or in the hair follicles of mammals and humans.
- Copepods of the order Siphonostomatoida parasitizes fishes.
- Body lice, head lice, and pubic lice parasitize humans.
- Twisted wing insects are endoparasites of other insects.
- Plasmodium parasitizes an anopheline mosquito and gets transmitted to humans by the bite of the mosquito. This protozoan parasite causes malaria in humans.
However other examples of parasitism can be illustrated and seen in the following parasitism types.
Types of Parasitism
- Facultative parasitism
- Obligate Parasitism
- Social parasitism
- Brood parasitism
- Sexual parasitism
In facultative parasitism, the facultative parasite does not depend on the host to be able to complete its life cycle. These parasites can survive without their host, and perform parasitic activities sometimes.
Certain plants, microbes, fungi, and animals can be facultative parasites. However, a typical example is the Strongyloides stercoralis, a nematode species. This nematode can be found free-living and can also cause the disease strongyloidiasis when it infects humans. Also, an example of this type of parasitism in humans is the Naegleria fowleri that is free-living but occasionally infects humans causing an often fatal result.
Furthermore is the fungus, Armillaria species that parasitize living trees. However, once the tree dies, due to fungal infection or not, the fungus still continues to eat the wood. It continues without any further need for parasitic activities. Likewise, green plants like Rhinanthus and Colpoon can grow independently without a host but can also act as facultative root parasites of other green plants around them.
In obligate parasitism, obligate parasites depend completely on their host to be able to complete their life cycle. With time, these parasites have evolved in a way that they can no longer exist without the existence of the host. Hence they do not usually cause severe harm to the host because these parasites need the host alive for them to survive. They usually need the host alive except the death of the host is needed for the parasite transmission.
An obligate parasite fails to reproduce once it cannot obtain a host and they are not able to complete their development without passing through one parasitic stage at least. Obligate parasitism can be exhibited in many types of organisms, like plants, fungi, animals, viruses and, bacteria.
A typical example of obligate parasites is head lice that die as soon as they are removed from the human scalp. Viruses too can be regarded as obligate parasites because they cannot reproduce without living cells. Also is an obligate parasite, Vespula austriaca with Vespula acadica as its host. Additionally is Bombus bohemicus, an obligate parasite of Bombus locurum, Bombus terrestris, and Bombus crytarum.
In hyperparasitism, the hyperparasite feed on another parasite. A typical example is the protozoa living in the parasitic helminth. Hyperparasites are used in agriculture and medicine. This is because these parasites can control the population of their hosts. For instance, is the way the CHV1 virus helps control the damage done by Cryphonectria parasitica (chestnut blight) in American chestnut trees. Likewise, how bacteriophages limit bacterial infections.
In social parasitism, the social parasite takes advantage of social insects. Examples of social insects are bees, ants, and termites. These social parasites may use mimicry to invade the social insect’s hive.
For instance, some bumblebees invade other bee species’ hives, take over reproduction, and make the host workers raise their young bumblebees. Another illustration is the ant species (Tetramorium inquilinum), a parasite that ends up spending its whole life on the back of other ant species. This parasite will eventually gain benefits like food and transportation. However, from this type of parasitism, the parasitic Ant has evolved to be so weak that if it falls off the host ant, it will be unable to crawl back on, and eventually dies.
Another example is a eusocial bee, Melipona scutellaris whose virgin queens invade another colony that doesn’t have a queen after escaping killer workers. However, intraspecific social parasitism can be seen in parasitic nursing. In parasitic nursing, a young species takes milk from unrelated female species. In wedge-capped capuchins, it is seen where higher-ranking females at times take milk from low-ranking females without reciprocating.
Interestingly, a different kind of parasitism is seen in some bird species. In brood parasitism, the host organism act as parents, raising the young of another organism as their own. This is definitely a form of parasitism because the brood parasite that lays eggs in other nests gains by not spending energy to raise their young ones. Whereas the host brood is harmed because they end up using their energy to raise young ones that are not their genetic material.
Brood parasites examples include different birds like cowbirds, black-headed ducks, whydahs, and cuckoos. These birds do not build their own nests but leave their eggs in the nests of other birds. However, the eggs of some brood parasites mimic the eggs of their host. Whereas some have tough shells making it hard to be killed by the host piercing it, e.g cowbird eggs.
These brood parasites abandon their eggs in the nest of another hoping that the adult bird of the other species will cater and raise their young ones for them. The brood parasites don’t really harm the host brood directly. However, the brood parasite may remove one or more of the host eggs to ensure the host brood doesn’t suspect the presence of the brood parasite’s egg. It may heave the host brood’s eggs or nestling from the nest.
Also, brood parasitism can occur in fish in the case of kleptoparasitism. Here the brood parasites directly or indirectly take food from the host. It is parasitism because the food that could have gone toward the host brood goes to the brood parasite species instead.
In kleptoparasitism, parasites steal food that has been gathered by the host. This type of parasitism happens within close relatives. Kleptoparasitism is generally uncommon but very noticeable in birds. For instance, skuas are specialized in relentlessly chasing other seabirds down until they disgorge their catch, eventually robbing them of their food.
Sexual parasitism is conspicuous in some species of anglerfish, like Ceratias holboelli. In this type of parasitism among the species of angler fish, the males are reduced to tiny sexual parasites. Hence the tiny males depend wholly on females of their own species for survival. They are not able to fend for themselves and attach permanently below the female’s body. However, the female protects the male against predators and nourishes it. Whereas, except for its sperm, the male gives nothing back.
Adelphoparasitism is also known as sibling parasitism. This type of parasitism occurs when the parasites are closely related to the host. The parasites and host usually are in the same family or genus.
A typical example is seen in the citrus blackfly parasitoid, where the unmated females of Encarsia perplexa may lay haploid eggs in the larvae of their own species. Therefore, producing male offspring.
In epiparasitism, an epiparasite parasitizes another parasitic organism. They are also called secondary parasites or hyperparasites. A typical example is a protozoan that lives in a flea that parasitizes a dog.
Parasites In Humans
There are different types of organisms that can parasitize humans. Organisms like fungi, leeches, tapeworms, lice, protozoa, ticks, viruses, and helminths parasitize humans. Some of these parasites are endoparasitic, ectoparasitic, and hematophagous to humans.
Helminths, for instance, are worms that can live inside the intestines of humans and can grow in length. However, they can cause several problems like malnutrition, diarrhea, jaundice, and in severe cases, causes death. Although, they can be treated with anti-parasitic drugs and medication.
All infectious diseases in humans are a result of parasites like viruses, protozoa, and bacteria. Also, most of the organisms that parasitize and infect humans parasitize other animals too.
Parasitism In Plants
There are small green insects (Aphids) that parasitize plants by eating their sap. Also, many types of fungi attack plants, fruit, and vegetables. Some plants are actually parasitic themselves like dodder, broomrape, witchweed, and mistletoes. Such parasitic plants have modified roots called haustoria, that connect to the xylem or phloem of the host plant. These haustoria drain water and nutrients from the host plant. Broomrapes attack crops like peas, tomatoes, carrots, and varieties of cabbage. Witchweeds attack grains and grasses.
Also, fungi like the downy mildew fungus attack and parasitize fruits and vegetables. Some fungi species like Puccinia graminis, Endothia parasitica, Puccinia sparganioides, and Ceratocystis ulmi parasites plants. Moreso, the fungus Ceratocystis ulmi causes a plant disease known as Dutch elm disease.
Parasitism In Insects
Parasitism is very common in insects that almost all species of insects are parasitized by at least one type of parasite. There are entomophagous parasites which are insects that parasitize other insects. Such parasites attack young insects or larva of their insect host. Some insects actually lay their eggs in the body of the host insect larva. Once their eggs hatch, the parasitic young one eventually eats up the larva and kills it. The young parasites gain all their nutrients from it.
Oftentimes, the parasite paralyzes a host and then feeds it to its young one. This is very common in wasps. The Ampulex compressa adult stings and paralyzes cockroaches that are then eaten by its young ones. Also, wasps like Ropalidia romandi burrow into their host’s abdomen and live there. However, they don’t kill their host, but they change the host’s behavior and appearance. Plus they even make their host sterile.
Parasitism in Fish
Many organisms infect and parasitize fishes. Some common fish parasites like copepods (small crustaceans), leeches, and nematodes attach to the gills of fishes and live there. The isopod, Cymothoa exigua is a common parasite that parasitizes fish. This parasite enters the mouth of the fish and then severs the tongue of the fish. The isopod then lives where the tongue was becoming the new tongue. Even though the host fish can still eat and survives with this parasite in its mouth, the isopod still consumes a little amount of the fish’s mucus and blood as it lives there.
Interestingly, Cleaner fish such as the Bluestreak cleaner wrasse removes parasites and dead skin from other fish. They do it to even large predatory fish that could end up eating them.
Fish parasites are a concern to human health. This is because there are some fish parasites that can also infect humans. Especially when humans eat foods that have uncooked fish like sushi. However, infection from eating uncooked fish is kind of rare in our developed world today because some raw fishes are frozen to prevent infections.