An obligate parasite or holoparasite is a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...

that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to

reproduce Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...

. This is opposed to a

facultative parasite A facultative parasite is an organism that may resort to parasitic activity, but does not absolutely rely on any host for completion of its life cycle. Examples of facultative parasitism occur among many species of fungi, such as family members ...

, which can act as a parasite but does not rely on its host to continue its life-cycle. Obligate parasites have

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

a variety of parasitic strategies to exploit their hosts. Holoparasites and some

hemiparasite A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the ...

s are obligate. It is advantageous for the parasite to preserve the health of their host when this is compatible with their nutritional and reproductive requirements, except when the death of the host is necessary for transmission.Combes, C. (1997) Fitness of Parasites: Pathology and Selection ''International Journal for Parasitology'' 27 (1): 1–10.

Species

Obligate parasitism is exhibited in a range of organisms, with examples in

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

es,

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

, and

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s.Balashov, Yu.S. (2011) Parasitism and Ecological Parasitology. ''Entomological Review'' 91 (9): 1216–1223. They are unable to complete their development without passing through at least one parasitic stage which is necessary to their life-cycle. Whether one regards

es as living organisms or not, they cannot reproduce except by means of resources within living cells. Accordingly, it is convenient and customary to regard them as obligate intracellular parasites. Among the

Vespidae The Vespidae are a large (nearly 5000 species), diverse, cosmopolitan family of wasps, including nearly all the known eusocial wasps (such as ''Polistes fuscatus'', ''Vespa orientalis'', and ''Vespula germanica'') and many solitary wasps. Each ...

family, '' Vespula austriaca'' is an example of an obligate reproductive parasite; its common host is '' Vespula acadica''. In the genus ''

Bombus A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related gener ...

'', '' B. bohemicus'' is an obligate parasite of ''B. locurum'', ''B. cryptarum'', and ''B. terrestris.''

Host-parasite interaction

Life-cycle

Parasitic life cycles involve the exploitation of at least one host. Parasites that infect a single species are said to have direct life-cycles.May, R. M. & Anderson, R. M. (1979) Population biology of infectious diseases. ''Nature'' 280: 455–461. For example, the hookworm species ''

Necator americanus ''Necator americanus'' is a species of hookworm (a type of helminth) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum Nematoda. It is an obligatory parasitic nematode that lives in the small intestine ...

''. Parasites that infect more than one host are said to have a complex or indirect life-cycle. For example, the

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

plasmodium.

Intermediate or final host

An intermediate or secondary host is exploited by the parasite only for a short transition period. A final or primary host is exploited by the parasite and is the only location in which the parasite is able to reach maturity and if possible, reproduce sexually. For example, ''

Ribeiroia ondatrae ''Ribeiroia ondatrae'', or the frog-mutating flatworm is a parasite in the genus ''Ribeiroia'' which is believed to be responsible for many of the recent increases in amphibian limb malformations, particularly missing, malformed, and additiona ...

'' uses Ramshorn snail as its first intermediate host, amphibians and fish as second intermediate hosts and birds as definitive hosts.

Parasitic permanence

Obligate parasites may not necessarily spend all of their time behaving as parasites. When a parasite is permanent, a number of generations occur in or on the host of an infested individual.

Head lice The head louse (''Pediculus humanus capitis'') is an obligate ectoparasite of humans. Head lice are wingless insects that spend their entire lives on the human scalp and feeding exclusively on human blood. Humans are the only known hosts of thi ...

are an example of this. Temporary parasites are organisms whose parasitic mode of life is limited to a few or even one stage of development. An example of this is the larval stage of harvest mites, while the adult stage is non-parasitic.

Location on host

The parasite may live outside of the host ectoparasite; for example, a

tick Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...

. Alternatively, the parasite may live within the host

endoparasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

; for example, the fluke. An obligate parasite that does not live directly in or on the host, but rather acts at a distance for example, a

cuckoo Cuckoos are birds in the Cuculidae family, the sole taxon in the order Cuculiformes . The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals and anis. The coucals and anis are sometimes separ ...

which hatches and is raised by non-relatives is known as a brood parasite.

Invasion strategies

In order to establish infestation in a susceptible host, obligate parasites must evade defences before, during and after entry into the host. Due to the wide range of obligate parasite types, it is impossible to identify a general invasion strategy. Intracellular parasites use various strategies to invade cells and subvert cellular signalling pathways. Most bacteria and viruses undergo passive uptake, where they rely on the host cell for uptake. However,

apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...

ns engage in active entry. One obligate wasp parasite,'' Polistes atrimandibularis'', infiltrates their hosts' colony by modifying their chemical signature to match that of the hosts'. This tricks the host wasps into thinking the parasite is one of their own.

Evasion of host defences

A number of obligate intracellular parasites have evolved mechanisms for evading their hosts' cellular defences, including the ability to survive in distinct

cellular compartment Cellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not always, defined as membrane-bo ...

s. One of the mechanisms that hosts employ in their attempt to reduce the replication and spread of pathogens is apoptosis (programmed cell death). Some obligate parasites have developed ways to suppress this phenomenon, for example ''

Toxoplasma gondii ''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but felids, such as d ...

'', although the mechanism is not yet fully understood.

Manipulation of host behaviour

Changes in a host’s behaviour following infection with obligate parasites are extremely common. Unusual behaviour observed in infected individuals is noted, and if its complexity suggests that this behaviour will benefit the transmission of the parasite, then this is said to be an example of

adaptive Adaptation, in biology, is the process or trait by which organisms or population better match their environment Adaptation may also refer to: Arts * Adaptation (arts), a transfer of a work of art from one medium to another ** Film adaptation, a ...

manipulation.Hughes, D. (2013) Pathways to understanding the extended phenotype of parasites in their hosts. ''The Journal of Experimental Biology'' 216: 142–147. However, there is a difficulty in demonstrating changes in behaviour are the result of a selective process favouring transmission of the parasite. It has been suggested that these changes may merely be a side-effect of infection. Most behaviour changes have not been demonstrated to lead to fitness gains in either the host or the parasite. An example of this behaviour is the attraction of rats to cat urine after infection with ''

''. However, the "scientific metaphors, including anthropomorphisms" sometimes used in "popular media and the scientific literature" to describe the manipulation of host behavior have been described as "catchy, yet misleading".

Extended phenotype

In some cases the behaviour we observe in an organism is not due to the expression of their genes, but rather to the genes of parasites infecting them. This behaviour is an

extended phenotype ''The Extended Phenotype'' is a 1982 book by the evolutionary biologist Richard Dawkins, in which the author introduced a biological concept of the same name. The main idea is that phenotype should not be ''limited'' to biological processes suc ...

Evolution of host behaviour manipulation

Three main evolutionary routes have been suggested for the appearance of host behaviour manipulation by parasites. The first is a ''parasite driven'' scenario of manipulation, while the second and third are ''host driven'' scenarios of manipulation. #Manipulation ''sensu stricto'' (extended phenotype- abhorrent behaviour displayed by parasitised hosts results from the expression of the parasites genes) this capacity could have been the product of natural selection in an ancestral parasite with the trait.Adamo, S. A. (2012) The strings of the puppet master: How parasites change host behaviour in Hughes, D.P., Brodeur, J. & Thomas, F. (Eds.), ''Host Manipulation by Parasites'' (pp. 36–51).Oxford, UK: Oxford University Press. #The

mafia "Mafia" is an informal term that is used to describe criminal organizations that bear a strong similarity to the original “Mafia”, the Sicilian Mafia and Italian Mafia. The central activity of such an organization would be the arbitration of d ...

-like strategy- retaliation for non-compliance (eg. great spotted cuckoo and magpie) magpies that eject the cuckoos eggs from their nest suffer a much greater rate of cuckoo predation. #The exploitation of compensatory responses induce host compensatory responses since these may at least partially match with the transmission routes of parasites. E.g. the sexually transmitted ectoparasite ''Chrysomelobia labidomerae'', parasitizing the leaf beetle host '' Labidomera clivicollis''~ infected males exhibit increased sexual behaviour and as a result enhance inter- and intra- sexual contacts (copulation and competition) which provide more opportunities for parasite transmission.Abbot, P. & Dll, L. M. (2001). Sexually transmitted parasites and sexual selection in the milkweed leaf beetle, ''Labidomera clivicollis''. ''Oikos'' 92: 91–100 It has been suggested that

behaviours are not adaptive, but are Exaptative. While they may have a benefit for the parasitic organism, they did not arise with the intention of this benefit.

Parasitic mimicry in brood parasites

The

cowbird Cowbirds are birds belonging to the genus ''Molothrus'' in the family Icteridae. They are of New World origin, and are obligate brood parasites, laying their eggs in the nests of other species. The genus was introduced by English naturalist Will ...

and

require the nests and parental care of other

passerines A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped'), which includes more than half of all bird species. Sometimes known as perching birds, passerines are distinguished from other orders of birds by th ...

in order for their young to fledge. These are known as brood parasites. The parasitic bird species mimics egg patterns and colours of the host species, which reduces egg rejection. The chicks of some species are able to manipulate host behaviour by making rapid calls that mimic the sound made by up to four of the host chicks. Mimicry of the host species also occurs in the paper wasp species ''

Polistes semenowi ''Polistes semenowi'' is a species of paper wasp in the genus ''Polistes ''that is found in southeastern and southern central Europe, as well as central Asia, and was until 2017 erroneously known by the name ''Polistes sulcifer'', while a differe ...

'' and '' Polistes sulcifer'' and the bumblebee species ''

Bombus bohemicus ''Bombus bohemicus'', also known as the gypsy's cuckoo bumblebee, is a species of socially parasitic cuckoo bumblebee found in most of Europe with the exception of the southern Iberian Peninsula and Iceland. ''B. bohemicus'' practices inquilini ...

'', with the parasite changing its proportions of cuticular hydrocarbons, species- and colony-specific identifying chemicals, to match that of the usurped host species.Kreuter, Kirsten; Elfi Bunk (November 23, 2011). "How the social parasitic bumblebee Bombus bohemicus sneaks into power of reproduction". ''Behavioral Ecology and Sociobiology'' 66 (3): 475–486. doi:10.1007/s00265-011-1294-z. Retrieved 21 September 2015. Several butterfly species will also exhibit brood parasitic behavior. An example is ''

Niphanda fusca ''Niphanda fusca'' is a parasitic butterfly primarily found in East Asian countries such as Japan and Korea. It is a "cuckoo-type" parasite of the ant '' Camponotus japonicus''. It utilizes chemical mimicry to trick the host worker ants into ad ...

,'' a butterfly that will release cuticular hydrocarbons (CHCs) to trick the host ant, ''C. japonicus,'' into adopting the larva as their own in their own nest. The ant will then raise the larva of the butterfly, feeding it directly from mouth-to-mouth, until it pupates. It is proposed that this mimicry has evolved through two processes: either as coevolutionary responses to host defences against brood parasites or modifying pre-existing host provisioning strategies. Competition between the parasite and host young for parental resources might lead to exaggeration of the aspects of the signal that most effectively exploit host parents. The parasitic young are likely to experience stronger selection for exaggerated signals than host young, because they are unrelated to the other chicks in the nest and therefore under selection to behave more selfishly.

Evolution of obligate parasitism

Current theory in

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...

indicates that host-parasite relationships may evolve towards equilibrial states of severe disease.Ewald, P.W. (1983). Host-parasite relations, vectors, and the evolution of disease severity. ''Annual Review of Ecology and Systematics'' 14:465–485. This differs from the conventional belief that

commensalism Commensalism is a long-term biological interaction ( symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...

is the ideal equilibrium for both the host and parasite.

References

{{reflist, 28em Ecology Parasites Parasitism