Parasitism

A Lithognathus fish with a parasitic isopod, Cymothoa exigua, one of many fish parasites

In brood parasitism, the host raises the young of another species, here the egg of a cowbird, that has been laid in its nest.

In biology, parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the expense of the other, the host. Traditionally parasite primarily meant an organism visible to the naked eye, or a macroparasite (such as a helminth). Microparasites are typically far smaller, such as protozoa,^[1][2] viruses, and bacteria.^[3] Examples of parasites include the plants mistletoe and cuscuta, and animals such as hookworms.

Unlike predators, parasites typically do not kill their host, are generally much smaller than their host, and often live in or on their host for an extended period. Both are special cases of consumer-resource interactions.^[4] Parasites show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the Plasmodium species, and fleas. Parasitoidy is an evolutionary strategy within parasitism in which the parasite eventually kills its host.^[5]

Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics, to the modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission. Although parasitism often applies unambiguously, it is part of a continuum of types of interactions between species, grading via parasitoidy into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.

People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described endo- and ectoparasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In fiction, Bram Stoker's 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott's 1979 film Alien was one of many works of science fiction to feature a terrifying^[6] parasitic alien species.

Etymology[edit]

First used in English 1539, the word parasite comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek παράσιτος (parasitos), "one who eats at the table of another"^[7] and that from παρά (para), "beside, by"^[8] + σῖτος (sitos), "wheat", hence "food".^[9] The related term parasitism appears in English from 1611.^[10]

Types[edit]

The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".^[11] Within that scope are many possible ways of life. Parasites are classified in a variety of different but overlapping schemes, based on their interactions with their hosts and on their life cycles. An obligate parasite is totally dependent on the host to complete its life cycle, while a facultative parasite is not. A direct parasite has only one host while an indirect parasite has multiple hosts. For indirect parasites, there will always be a definitive host and an intermediate host.^[12][13]

Evolutionary strategies[edit]

Micropredator, typical parasite, parasitoid, and predator strategies compared. Their interactions with their hosts form a continuum. Micropredation and parasitoidy are now considered to be evolutionary strategies within parasitism.^[5]

Acrodactyla quadrisculpta is a parasitoid wasp, its host a spider. Parasitoids are parasites that eventually kill their hosts.^[5]

There are six major evolutionary strategies within parasitism. These apply to parasites whose hosts are plants as well as animals:^[5][14]

1. Parasitic castrators feed on their host's reproductive tissues, leaving other bodily processes largely intact, and therefore ensuring the host's survival and the freedom of the parasite to remain in the host body for as long as the host continues to live.^[5]
2. Directly transmitted parasites rely on happenstance encounters with members of their host species to feed and reproduce. They may spread from one host to another through skin-to-skin contact, or lie dormant until a host steps on or brushes against them.^[5]
3. Trophically transmitted parasites have a life cycle involving two or more hosts. In their juvenile stage, they infect and often encyst in an animal; this is the intermediate host. When this animal is eaten by a predator, the parasite survives the digestion process and matures into an adult. This predator thus become the definitive host for the parasite. Some parasites are capable of modifying the behavior of their intermediate hosts in order to increase the chances of being eaten by a predator.^[5]
4. Vector-transmitted parasites rely on a third party to carry them from one host to another. These are often microscopic non-animal parasites, namely protozoa, bacteria, or viruses, and their vectors are often parasitic arthropods such as fleas, lice, ticks and mosquitoes.^[5]
5. Parasitoids kill their hosts and thus migrate between hosts frequently.^[5]
6. Micropredators actively hunt for hosts in the manner of traditional predators. Micropredators choose hosts that are large but helpless or ineffective in resisting the attack. For example, a mosquito attacks animals too slow to protect themselves from the bite.^[5] Similarly, phytophagous scale insects, aphids, and caterpillars attack much larger plants, and serve as vectors of bacteria, fungi and viruses causing plant diseases, and plants defoliated by caterpillars may die, as in parasitoidy. Female scale insects are unable to move, so they are obligate parasites, permanently attached to their hosts.^[14]

These strategies for successful parasitism are adaptive peaks; many intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable.^[5]

Classification[edit]

Human head lice (Pediculus humanus capitis) are obligate ectoparasites.

Ectoparasites[edit]

Parasites that live on the outside of the host, either on the skin or the outgrowths of the skin, are called ectoparasites. They are directly transmitted between hosts. Examples include lice, fleas, and some mites.^[15]

Endoparasites[edit]

Schistosoma mansoni is an obligate endoparasite of human blood vessels, causing schistosomiasis (bilharzia).

Those that live inside the host, including all parasitic worms (helminths), are called endoparasites. Endoparasites can exist in one of two forms: intercellular parasites (inhabiting spaces in the host's body) or intracellular parasites (inhabiting cells in the host's body). Coinfection by multiple parasites is common.^[16]

Intracellular parasites, such as pathogenic (disease-causing) protozoa, bacteria or viruses, tend to rely on a third organism, the carrier or vector, to transmit them to a host.^[17]

Autoinfection is the infection of a primary host with a parasite, particularly a helminth, in such a way that the complete life cycle of the parasite happens in a single organism, without the involvement of another host. This can occur with the intestinal parasite Strongyloides stercoralis. Strongyloidiasis involves premature transformation of noninfective larvae into infective larvae, which can penetrate the intestinal mucosa (internal autoinfection) or the skin of the perineal area (external autoinfection).^[18]

Mesoparasites[edit]

Those parasites living in an intermediate position, being half-ectoparasites and half-endoparasites, are called mesoparasites. For example, the cod worm Lernaeocera branchialis invades the gill tissue of a host fish, but quickly grows a tubelike structure that cuts through the body tissues until it reaches the fish's heart, through which it robs the host of its blood. The rear end of the parasite remains outside, so that it can scatter eggs into the water.^[19]

A parasitoidal wasp ovipositing into the body of a spotted alfalfa aphid

Parasitoids[edit]

A parasitoid sooner or later kills its prey, so this form of parasitism is close to predation. Idiobiont parasitoid wasps sting their prey on capture, either killing them outright or paralyzing them immediately. The prey is then carried to a nest, an egg is laid on or in it, and the parasitoid develops rapidly. Koinobiont parasitoid wasps lay their eggs in young hosts, usually larvae, which are allowed to go on growing, so the host and parasitoid develop together for an extended period. Some koinobionts regulate their host's development hormonally, for example preventing it from pupating or making it moult whenever the parasitoid is ready to moult.^[20]

Hyperparasites[edit]

A hyperparasitoid chalcid wasp (Pteromalidae) on the cocoons of its host, a braconid wasp (Microgastrinae), itself a koinobiont parasitoid of Lepidoptera

A hyperparasite or epiparasite feeds on another parasite, as exemplified by a protozoan living in a helminth parasite.^[21] The term is used slightly more loosely to refer also to parasitoids whose hosts are either parasites or parasitoids. Hyperparasitoids may be facultative or obligate, and the young may develop inside or outside the host's body, usually a larva.^[5][20]

Social parasites[edit]

The large blue butterfly is a mimic and social parasite of ants.

Social parasites take advantage of interactions between members of social organisms such as ants, termites, and bumblebees. Examples include the large blue butterfly, Phengaris arion. Its larvae employ mimicry to parasitize certain species of ants,^[22] Bombus bohemicus, a bumblebee which invades the hives of other species of bee and takes over reproduction, their young raised by host workers, and Melipona scutellaris, a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen.^[23] An extreme example of social parasitism is the ant species Tetramorium inquilinum of the Alps, which lives exclusively on the backs of other species of Tetramorium host ants. With tiny and weakened bodies, they have evolved for a single task: holding on to their host, since if they fall off, they will die.^[24]

In kleptoparasitism (from Greek κλέπτης (kleptes), thief), parasites appropriate food gathered by the host. An example is the brood parasitism practiced by cowbirds, whydahs, cuckoos, and black-headed ducks which do not build nests of their own and leave their eggs in nests of other species. The host behaves as a "babysitter" as they raise the young as their own. If the host removes the cuckoo's eggs, some cuckoos return and attack the nest to compel host birds to remain subject to this parasitism.^[25]

Intraspecific social parasitism may also occur, as in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation. The high ranking females benefit at the expense of the low ranking females.^[26]

Parasitism can take the form of isolated cheating or exploitation among more generalized mutualistic interactions. For example, broad classes of plants and fungi exchange carbon and nutrients in common mutualistic mycorrhizal relationships; however, some plant species known as myco-heterotrophs cheat by taking carbon from a fungus rather than donating it.^[27]

Adelpho-parasites[edit]

The male anglerfish Ceratias holboelli lives as a tiny sexual parasite permanently attached below the female's body.

An adelpho-parasite (from Greek αδελφός (adelphos), brother) is a parasite in which the host species is closely related to the parasite, often being a member of the same family or genus. An example of this is the citrus blackfly parasitoid, Encarsia perplexa, unmated females of which may lay haploid eggs in the fully developed larvae of their own species. These result in the production of male offspring.^[28] The marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic.^[29]

Sexual parasites[edit]

Cuscuta (a dodder), a stem holoparasite, on an acacia tree

In many animals, males are much smaller than females. In some species of anglerfish, such as Ceratias holboelli, the males are so small they have become sexual parasites, wholly dependent on females of their own species for survival, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.^[30]

Parasitic plants[edit]

A parasitic plant derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are in almost every biome in the world.^[31] All parasitic plants have modified roots, named haustoria (singular: haustorium), which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. Parasitic plants are classified depending on where the parasitic plant latches onto the host and the amount of nutrients it requires.^[31] Some parasitic plants are able to locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plant in approximately 20 families of flowering plants are known.^[32][31]

Species within Orobanchaceae (broomrapes) are some of the most economically destructive species on Earth. Species of Striga (witchweeds) are estimated to cost billions of dollars a year in crop yield loss annually, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. Striga infects both grasses and grains, including corn, rice and Sorghum, undoubtedly some of the most important food crops. Orobanche also threatens a wide range of important crops, including peas, chickpeas, tomatoes, carrots, and varieties of the genus Brassica (cabbages). Yield loss from Orobanche can reach 100%; despite extensive research, no method of control has been entirely successful.^[33]

The honey fungus, Armillaria mellea, is a parasite of trees, and a saprophyte feeding on the trees it has killed.

Parasitic fungi[edit]

Parasitic fungi derive some or all of their nutritional requirements from plants, other fungi, or animals, and unlike mycorrhizal fungi which have a mutualistic relationship with their host plants, they are pathogenic. For example, the honey fungi in the genus Armillaria grow in the roots of a wide variety of trees, and eventually kill them. They then continue to live in the dead wood, feeding saprophytically.^[34]

Borrelia burgdorferi, the bacterium that causes Lyme disease, is transmitted by Ixodes ticks.

Parasitic bacteria[edit]

Many bacteria are parasitic, though since the result is infection and disease, sometimes leading to death, they are generally thought of as pathogens instead.^[35] Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, Bacillus anthracis, the cause of anthrax, is spread by contact with infected domestic animals; the bacillus's spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. Borrelia, the cause of Lyme disease and relapsing fever, is transmitted by a vector, ticks of the genus Ixodes, from the diseases' reservoirs in animals such as deer. Campylobacter jejuni, a cause of severe enteritis (gut inflammation), is spread by the fecal-oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. Haemophilus influenzae, an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. Treponema pallidum, the cause of syphilis, is spread by sexual intercourse.^[36]

Enterobacteria phage T4 is a bacteriophage virus. It infects its host, Escherichia coli, by injecting its DNA through its tail, which attaches to the bacterium's surface.

Viruses[edit]

Viruses are obligate intracellular parasites, characterized by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and archaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. Viruses consist of a strip of genetic material (DNA or RNA), covered in a protein coat and sometimes a lipid envelope. They thus lack all the usual machinery of the cell such as enzymes, relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages, infecting bacteria, and it is possible that viruses are both extremely ancient, being at least as old as the first cells, and polyphyletic, having evolved from several entirely unrelated ancestors.^{[37][38][39][40]}

Transmission[edit]

Life cycle of Entamoeba histolytica, an anaerobic parasitic protozoan transmitted by the fecal-oral route

Parasites use a variety of methods to infect their hosts, including physical contact, the fecal-oral route, free-living infectious stages, and insect vectors, suiting their differing hosts.^[41]

Among protozoan endoparasites, such as the malarial parasites in the genus Plasmodium and sleeping sickness parasites in the genus Trypanosoma, infective stages in the host's blood are transported to new hosts by biting blood-drinking (hematophagous) insects acting as vectors.^[42]

Host defenses[edit]

In vertebrates[edit]

The dry skin of vertebrates such as the short-horned lizard prevents entry of many parasites.

The first line of defense against invading parasites in vertebrates is the physical barrier of the tough and often dry and waterproof skin, as in reptiles, birds and mammals. Most microorganisms need a moist environment to survive. By keeping the skin dry, it prevents invading organisms from colonizing. Human skin also secretes sebum, which is toxic to most microorganisms.^[43] On the other hand, larger parasites such as trematodes detect chemicals produced by the skin to locate their hosts when they enter the water. Saliva in the vertebrate mouth prevents foreign organisms from getting into the body orally. The mouth also contains lysozyme, an enzyme found in tears and the saliva. This enzyme breaks down cell walls of invading microorganisms.^[43] Should the organism pass the mouth, the stomach with its hydrochloric acid, toxic to most microorganisms, is the next line of defense.^[43] Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Parasites can also invade the body through the eyes. The lashes on the eyelids of mammals help to prevent microorganisms from entering the eye. Tears contain the enzyme lysozyme, which kills most invading microorganisms.^[43] Once inside the body, parasites must overcome the immune system's serum proteins and pattern recognition receptors, intracellular and cellular, that trigger the adaptive immune system's lymphocytes such as T cells and antibody-producing B cells. These have receptors that recognize parasites.^[44]

In insects[edit]

Insects often adapt their nests to aid in parasite defense. For example, one of the key reasons why the wasp Polistes canadensis nests across multiple combs, rather than building a single comb like much of the rest of its genus, is as a defense against the infestation of tineid moths. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.^[45]

In plants[edit]

Leaf spot on oak. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.

In response to parasitic attack, plants undergo a series of metabolic and biochemical reaction pathways that enact defensive responses. For example, parasitic invasion causes an increase in the jasmonic acid-insensitive (JA) and NahG (SA) pathway.^[46] These pathways produce chemicals that induce defensive responses, such as the production of chemicals or defensive molecules to fight off the attack. Different biochemical pathways are activated by different parasites.^[47] In general, there are two types of responses that can be activated by the pathways. Plants can either initiate a specific or non-specific response.^[48] Specific responses involve gene-gene recognition of the plant and parasite. This can be mediated by the ability of the plant’s cell receptors recognizing and binding molecules that are located on the cell surface of parasites. Once the plant’s receptors recognizes the parasite, the plant localizes the defensive compounds to that area creating a hypersensitive response. This form of defense mechanism localizes the area of attack and keeps the parasite from spreading. Furthermore, a specific response against parasitic attack prevents the plants from wasting its energy by increasing defenses where it is not needed. However, specific defensive responses only target specific parasites. If the plant lacks the ability to recognize a parasite, specific defense responses are not activated. Nonspecific defensive responses work against all parasites. These responses are active over time and are systematic, meaning that the responses are not confined to an area of the plant, but rather spread throughout the entirety of the organism. However, nonspecific responses are energy costly, since the plant has to ensure that the genes producing the nonspecific responses are always expressed.^[48]

Evolutionary ecology[edit]

Restoration of a Tyrannosaurus with holes possibly caused by a Trichomonas-like parasite

Parasitism has arisen independently many times. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles,^[49] and it is frequent in plants and fungi. Almost all free-living animals are host to one or more parasitic taxa.^[49] This is harder to demonstrate from the fossil record, but for example holes in the skulls of several specimens of Tyrannosaurus may have been caused by Trichomonas-like parasites.^[50]

Coevolution and cospeciation[edit]

A parasite sometimes undergoes co-speciation with its host. An example is between the simian foamy virus (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial cytochrome oxidase subunit II from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses co-speciated with Old World primates for at least 30 million years.^[51]

The presumption of a shared evolutionary history between parasites and hosts can sometimes elucidate how host taxa are related. For instance, there has been a dispute about whether flamingos are more closely related to the storks or to the ducks. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication or extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of grebes. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then switched hosts to ducks, creating the situation which had confused biologists.^[52]

Coevolution favoring mutualism[edit]

The gram-negative bacterium Wolbachia within an insect cell

Long-term coevolution sometimes leads to a relatively stable relationship tending to commensalism or mutualism, as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals infected with parasitic worms are often clearly harmed, and therefore parasitized, such infections may also reduce the prevalence and effects of autoimmune disorders in animal hosts, including humans.^[53] In a more extreme example, some nematode worms cannot reproduce, or even survive, without infection by Wolbachia bacteria.^[54]

Lynn Margulis and others have argued, following Peter Kropotkin's 1902 Mutual Aid: A Factor of Evolution, that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the symbiogenesis which formed the eukaryotes from an intracellular relationship between archaea and bacteria, though the sequence of events remains largely undefined.^[55][56]

Competition favoring virulence[edit]

Competition between parasites can be expected to favor faster reproducing and therefore more virulent parasites, by natural selection.^[57] Parasites whose life cycle involves the death of the host, to exit the present host and sometimes to enter the next, evolve to be more virulent, and may alter the behavior or other properties of the host to make it more vulnerable to predators.^[3]

However, among competing parasitic insect-killing bacteria of the genera Photorhabdus and Xenorhabdus, virulence depended on the relative potency of the antimicrobial toxins (bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when caterpillars were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.^[57]

Conversely, parasites whose reproduction is largely tied to their host's reproductive success tend to become less virulent or mutualist, so that their hosts reproduce more effectively.^[3]

The protozoan Toxoplasma gondii facilitates its transmission by inducing behavioral changes in rats through infection of neurons in their central nervous system.

Modifying host behavior[edit]

Some parasites modify host behavior in order to increase the transmission between hosts, often in relation to predator and prey (parasite increased trophic transmission). For example, in California salt marshes, the fluke Euhaplorchis californiensis reduces the ability of its killifish host to avoid predators.^[58] This parasite matures in egrets, which are more likely to feed on infected killifish than on uninfected fish. Another example is the protozoan Toxoplasma gondii, a parasite that matures in cats but can be carried by many other mammals. Uninfected rats avoid cat odors, but rats infected with T. gondii are drawn to this scent, which may increase transmission to feline hosts.^[59]

Parasite-stress theory[edit]

Parasites infect hosts within their same geographical area (sympatric) more effectively. This phenomenon supports the Red Queen hypothesis, which states that interactions between species, such as host and parasites, lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to allopatric hosts, those from different geographical regions.^[60] When populations of lake snails were exposed to two pure parasites (digenetic trematode), whether sympatric parasites, allopatric parasites or a mixture, the parasites were more effective in infecting their sympatric snails than their allopatric snails. The parasites were apparently adapted to infect local populations of snails.^[60]

Trait loss[edit]

Bed bug, Cimex lectularius, is flightless, like many insect ectoparasites.

Parasites are able to exploit their hosts for a variety of functions. Many insect ectoparasites including bedbugs, batbugs, lice and fleas have lost their ability to fly, relying instead on their hosts for transport.^[61] Trait loss more generally is widespread among parasites.^[62]

Secondary sex characteristics[edit]

Parasitism has been suggested as part of the explanation of the evolution of secondary sex characteristics in breeding male animals, such as the plumage of peacocks and the manes of male lions. The argument is that female hosts select males for breeding based on such characteristics because they act as honest signals of costly handicaps. A possible mechanism for this, suggested by Ivar Folstad and Andrew Karter, is that the male hormone testosterone encourages the growth of secondary sex characteristics such as manes, but at the price of reducing an animal's immune defences.^[63]

The parasitic barnacle Sacculina carcini (highlighted) attached to a crab

Parasitic crustaceans such as Peltogaster carvatus and Sacculina specifically cause damage to the gonads of their host crabs. In the case of Sacculina, the testes of over two thirds of their crab hosts had degenerated sufficiently for these male crabs to have gained female secondary sex characteristics such as broader abdomens, smaller claws (chelae) and egg-grasping appendages.^[64] The trematode Zoogonus lasius causes parasitic castration of the intertidal snail Ilyanassa obsoleta; other trematodes directly or indirectly castrate other species of snail.^[64]

Parasite-host assemblages[edit]

Parasite ecology is complex, usually involving hosts that have multiple parasites (multi-parasite hosts), parasites that have multiple hosts (multi-host parasites), and competition within a host. These interactions affect parasite and host reproduction and therefore evolution, including of the virulence of parasites and of methods of transmission. Reviewing the field, T. Rigaud and colleagues noted in 2010 that among the outcomes demonstrated empirically are that multiple infection can affect virulence, and can trigger evolutionary change; that the parasites involved in the same host may have contrasting transmission modes; and that assemblages of multiple parasites can be more virulent than would be expected from each individual parasite. Rigaud and colleagues also consider trade-offs of virulence among hosts (that adaptation for higher virulence in one host means lower virulence in others), predicting that when there are many hosts, the outcome will be non-specialist parasites with relatively low virulence. Where hosts differ in quality, Rigaud and colleagues predict that parasites should become optimally virulent in their primary host. On the other hand, they also predict that the more diverse the host community, the lower the incidence of parasites should be, because infecting hosts that are unsuitable or resistant means the loss of those parasites (wasted transmission).^[65]

Extended phenotype[edit]

The evolutionary biologist Richard Dawkins argued in a 1989 book that organisms have an extended phenotype that consists not only of the expression of their own genes, but, among other things (such as artefacts), of those of their parasites. This, he suggested, could apply not only to endoparasites but to those such as cuckoos that only briefly come into contact with their hosts.^[66]

Value[edit]

Although parasites are generally considered to be harmful, the eradication of all parasites would not necessarily be beneficial. Parasites account for at least half of life's diversity; they perform an important ecological role (by weakening prey) that ecosystems would take some time to adapt to; and without parasites, organisms may eventually tend to asexual reproduction, diminishing the diversity of sexually dimorphic traits.^[67] Parasites provide an opportunity for the transfer of genetic material between species. On rare, but significant, occasions this may facilitate evolutionary changes that would not otherwise occur, or that would otherwise take even longer.^[3]

Although parasites are often omitted in depictions of food webs, they usually occupy the top position. Parasites can function like keystone species, reducing the dominance of superior competitors and allowing competing species to co-exist.^[68][69][70]

Many parasites require multiple hosts of the different species to complete their life cycles and rely on predator-prey or other stable ecological interactions to get from one host to another. In this sense, the parasites in an ecosystem reflect the health of that system.^[71]

Quantitative ecology[edit]

A single parasite species usually has an aggregated distribution across host individuals, which means that most hosts harbor few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders parametric statistics invalid. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.^[72]

History[edit]

Ancient[edit]

Human parasites including roundworms, the Guinea worm, threadworms and tapeworms are mentioned in Egyptian papyrus records from 3000 BC onwards; the Ebers papyrus describes hookworm. In ancient Greece, parasites including the bladder worm are described in the Hippocratic Corpus, while the comic playwright Aristophanes called tapeworms "hailstones". The Roman physicians Celsus and Galen documented the roundworms Ascaris lumbricoides and Enterobius vermicularis.^[73]

Medieval[edit]

The Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.^[73]

Early Modern[edit]

A plate from Francesco Redi's Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on living animals found inside living animals), 1684

Antonie van Leeuwenhoek observed and illustrated Giardia lamblia in 1681, and linked it to "his own loose stools". This was the first protozoan parasite of humans that he recorded, and the first to be seen under a microscope.^[73]

Francesco Redi described ecto- and endoparasites in his 1687 book Esperienze Intorno alla Generazione degl'Insetti, illustrating ticks, the larvae of nasal flies of deer, and sheep liver fluke. His 1684 book Osservazioni intorno agli animali viventi che si trovano negli animali viventi (Observations on Living Animals, that are in Living Animals) described and illustrated over 100 parasites including the human roundworm.^[74] He noted that parasites develop from eggs, contradicting the theory of spontaneous generation.^[75]

Birth of modern parasitology[edit]

Modern parasitology developed in the 19th century with accurate observations by several researchers and clinicians. In 1828, James Annersley described amoebiasis, protozoal infections of the intestines and the liver, though the pathogen, Entamoeba histolytica, was not discovered until 1873 by Friedrich Lösch. James Paget discovered the intestinal nematode Trichinella spiralis in humans in 1835. James McConnell described the human liver fluke in 1875. Patrick Manson discovered the life cycle of elephantiasis, caused by nematode worms transmitted by mosquitoes, in 1877. Manson further predicted that the malaria parasite, Plasmodium, had a mosquito vector, and persuaded Ronald Ross to investigate. Ross confirmed that the prediction was correct in 1897–1898. At the same time, Giovanni Battista Grassi and others described the malaria parasite's life cycle stages in Anopheles mosquitoes. Ross was controversially awarded the 1902 Nobel prize for his work, while Grassi was not.^[73]

Cultural significance[edit]

"An Old Parasite in a New Form": an 1881 Punch cartoon by Edward Linley Sambourne compares a crinoletta bustle to a parasitic insect's exoskeleton

In classical times[edit]

In the classical era, the concept of the parasite was not strictly pejorative:^[76] the parasitus was an accepted role in Roman society, in which a person could live off the hospitality of others, and in return provide "flattery, simple services, and a willingness to endure humiliation".^[77][78]

In society[edit]

Parasitism has a derogatory sense in popular usage. According to the immunologist John Playfair,^[79]

In everyday speech, the term 'parasite' is loaded with derogatory meaning. A parasite is a sponger, a lazy profiteer, a drain on society.^[79]

The satirical cleric Jonathan Swift refers to hyperparasitism in his 1733 poem "On Poetry: A Rhapsody", comparing poets to "vermin" who "teaze and pinch their foes":^[80]

Parasitic "facehugger" alien species in James Cameron's 1986 science fiction film Aliens

The vermin only teaze and pinch

Their foes superior by an inch.

So nat'ralists observe, a flea

Hath smaller fleas that on him prey;

And these have smaller fleas to bite 'em.

And so proceeds ad infinitum.

Thus every poet, in his kind,

Is bit by him that comes behind:

In fiction[edit]

In Bram Stoker's 1897 Gothic horror novel Dracula, and its many film adaptations, the eponymous Count Dracula is a blood-drinking parasite. The critic Laura Otis argues that as a "thief, seducer, creator, and mimic, Dracula is the ultimate parasite. The whole point of vampirism is sucking other people's blood—living at other people's expense."^[81]

Disgusting and terrifying parasitic alien species are widespread in science fiction, as for instance in Ridley Scott's 1979 film Alien.^[82] In one scene of that film, an alien bursts out of the chest of a dead man, with blood squirting out under high pressure assisted by explosive squibs. Animal viscera were used to reinforce the shock effect. The scene was filmed in a single take, and the startled reaction of the actors was genuine.^[6][83]

References[edit]

Further reading[edit]

• Combes, Claude (2005). The Art of Being a Parasite. The University of Chicago Press. ISBN 0-226-11438-4. 
• Desowitz, Robert (1998). Who Gave Pinta to the Santa Maria?. Harvest Books. ISBN 0-15-600585-9. 
• Zimmer, Carl (2001). Parasite Rex. Free Press. ISBN 0-7432-0011-X. 

External links[edit]

Wikimedia Commons has media related to Parasites.

• Parasitology Parasites Zoonoses—(Polish/English) over 50 movies (Filmoteka) and over 250 photos (Fotogaleria/Photogallery) with human and animal parasites.
• Aberystwyth University: Parasitology—class outline with links to full text articles on parasitism and parasitology.
• KSU: Parasitology Research—parasitology articles and links.
• Medical Parasitology—online textbook.
• Division of Parasitic Diseases, Centers for Disease Control and Prevention
• VCU Virtual Parasite Project—Virtual Parasite Project at Virginia Commonwealth University's Center for the Study of Biological Complexity
• Parasites World—Parasites articles and links.
• Toxoplasma gondii in the Subarctic and Arctic
• Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance Wikibooks