HOME

TheInfoList



OR:

In
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditar ...
and
medicine Medicine is the science and Praxis (process), practice of caring for a patient, managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, and Health promotion ...
, a host is a larger
organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...
that harbours a smaller
organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...
; whether a parasitic, a mutualistic, or a commensalist ''guest'' (
symbiont Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or paras ...
). The guest is typically provided with nourishment and shelter. Examples include
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 playing host to parasitic
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete worm ...
s (e.g. nematodes), cells harbouring
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...
ic (disease-causing)
virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es, a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in
botany Botany, also called plant science (or plant sciences), plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "bot ...
, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. The host range is the collection of hosts that an organism can use as a partner.


Symbiosis

Symbiosis spans a wide variety of possible relationships between organisms, differing in their permanence and their effects on the two parties. If one of the partners in an association is much larger than the other, it is generally known as the host. In parasitism, the parasite benefits at the host's expense. In commensalism, the two live together without harming each other, while in mutualism, both parties benefit. Most parasites are only parasitic for part of their life cycle. By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions. Some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate. A parasite may have a long-term relationship with its host, as is the case with all endoparasites. The guest seeks out the host and obtains food or another service from it, but does not usually kill it. In contrast, a parasitoid spends a large part of its life within or on a single host, ultimately causing the host's death, with some of the strategies involved verging on
predation Predation is a biological interaction In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or o ...
. Generally, the host is kept alive until the parasitoid is fully grown and ready to pass on to its next life stage. A guest's relationship with its host may be intermittent or temporary, perhaps associated with multiple hosts, making the relationship equivalent to the herbivory of a wild-living animal. Another possibility is that the host–guest relationship may have no permanent physical contact, as in the brood parasitism of the cuckoo.


Hosts to parasites

Parasites follow a wide variety of evolutionary strategies, placing their hosts in an equally wide range of relationships. Parasitism implies host–parasite coevolution, including the maintenance of gene polymorphisms in the host, where there is a trade-off between the advantage of resistance to a parasite and a cost such as disease caused by the gene.


Types of hosts

*Definitive or primary host – an organism in which the
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
reaches the adult stage and reproduces sexually, if possible. This is the final host. *Secondary or intermediate host – an organism that harbors the sexually immature parasite and is required by the parasite to undergo development and complete its life cycle. It often acts as a vector of the parasite to reach its definitive host. For example, '' Dirofilaria immitis'', the heartworm of dogs, uses the mosquito as its intermediate host until it matures into the infective L3 larval stage. It is not always easy or even possible to identify which host is definitive and which secondary. As the life cycles of many parasites are not well understood, sometimes the subjectively more important organism is arbitrarily labelled as definitive, and this designation may continue even after it is found to be incorrect. For example, sludge worms are sometimes considered "intermediate hosts" for salmonid whirling disease, even though the myxosporean parasite reproduces sexually inside them. In
trichinosis Trichinosis, also known as trichinellosis, is a parasitic disease caused by nematodes, roundworms of the ''Trichinella'' type. During the initial infection, invasion of the intestines can result in diarrhea, abdominal pain, and vomiting. Migrat ...
, a disease caused by roundworms, the host has reproductive adults in its digestive tract and immature juveniles in its
muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...
s, and is therefore both an intermediate and a definitive host. *Paratenic or transport host – an organism that harbors the sexually immature parasite but is not necessary for the parasite's development cycle to progress. Paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers. The trematode ''
Alaria americana ''Alaria americana'' is a species of a trematode in a family Diplostomidae. All of these species infect carnivorous mammals by living in their small intestines as mature worms. ''A. americana'' are most frequently found in temperate regions, pred ...
'' may serve as an example: the so-called mesocercarial stages of this parasite reside in
tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...
s, which are rarely eaten by the definitive canine host. The tadpoles are more frequently preyed on by
snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
, in which the mesocercariae may not undergo further development. However, the parasites may accumulate in the snake paratenic host and infect the definitive host once the snake is consumed by a canid. The nematode ''
Skrjabingylus nasicola ''Skrjabingylus nasicola'' is a species of parasitic nematode in the family Metastrongylidae. Its lifecycle includes an intermediary mollusc host and a paratenic host, a shrew or small rodent. Adult worms are found in the nasal sinuses of mustel ...
'' is another example, with slugs as the intermediate hosts, shrews and rodents as the paratenic hosts, and mustelids as the definitive hosts. *Dead-end, incidental, or accidental host – an organism that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development. For example, humans and horses are dead-end hosts for West Nile virus, whose life cycle is normally between culicine
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning "gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "lit ...
es and birds. People and horses can become infected, but the level of virus in their blood does not become high enough to pass on the infection to mosquitoes that bite them. * Reservoir host – an organism that harbors a
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...
but suffers no ill effects. However, it serves as a source of infection to other species that are susceptible, with important implications for
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
control. A single reservoir host may be reinfected several times.


Plant hosts of micropredators

Micropredation is an
evolutionarily stable strategy An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set o ...
within parasitism, in which a small predator lives parasitically on a much larger host plant, eating parts of it. The range of
plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s on which a herbivorous insect feeds is known as its host range. This can be wide or narrow, but it never includes all plants. A small number of insects are monophagous, feeding on a single plant. The silkworm larva is one of these, with mulberry leaves being the only food consumed. More often, an insect with a limited host range is oligophagous, being restricted to a few closely related species, usually in the same plant family. The diamondback moth is an example of this, feeding exclusively on brassicas, and the larva of the
potato tuber moth ''Phthorimaea operculella'', also known as the potato tuber moth or tobacco splitworm, is a moth of the family Gelechiidae. It is an oligophagous insect that feeds on the plant family Solanaceae and is especially known for being a major pest of ...
feeds on potatoes, tomatoes and tobacco, all members of the same plant family,
Solanaceae The Solanaceae , or nightshades, are a family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spices, weeds, and o ...
. Herbivorous insects with a wide range of hosts in various different plant families are known as polyphagous. One example is the buff ermine moth whose larvae feed on
alder Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
, mint, plantain, oak,
rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick r ...
, currant,
blackberry The blackberry is an edible fruit produced by many species in the genus ''Rubus'' in the family Rosaceae, hybrids among these species within the subgenus ''Rubus'', and hybrids between the subgenera ''Rubus'' and ''Idaeobatus''. The taxonomy of ...
, dock, ragwort, nettle and honeysuckle. Plants often produce toxic or unpalatable secondary metabolites to deter herbivores from feeding on them. Monophagous insects have developed specific adaptations to overcome those in their specialist hosts, giving them an advantage over polyphagous species. However, this puts them at greater risk of extinction if their chosen hosts suffer setbacks. Monophagous species are able to feed on the tender young foliage with high concentrations of damaging chemicals on which polyphagous species cannot feed, having to make do with older leaves. There is a trade off between offspring quality and quantity; the specialist maximises the chances of its young thriving by paying great attention to the choice of host, while the generalist produces larger numbers of eggs in sub-optimal conditions. Some insect micropredators migrate regularly from one host to another. The hawthorn-carrot aphid overwinters on its primary host, a hawthorn tree, and migrates during the summer to its secondary host, a plant in the
carrot family Apiaceae or Umbelliferae is a family of mostly aromatic flowering plants named after the type genus '' Apium'' and commonly known as the celery, carrot or parsley family, or simply as umbellifers. It is the 16th-largest family of flowering plan ...
.


Host range

The host range is the set of hosts that a parasite can use as a partner. In the case of human parasites, the host range influences the epidemiology of the parasitism or disease.


Host range of viruses

For instance, the production of
antigenic shift Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is ...
s in
Influenza A virus '' A virus'' (''IAV'') causes influenza in birds and some mammals, and is the only species of the genus ''Alphainfluenzavirus'' of the virus family '' Orthomyxoviridae''. Strains of all subtypes of influenza A virus have been isolated from wil ...
can result from pigs being infected with the virus from several different hosts (such as human and bird). This co-infection provides an opportunity for mixing of the viral genes between existing strains, thereby producing a new viral strain. An influenza vaccine produced against an existing viral strain might not be effective against this new strain, which then requires a new influenza vaccine to be prepared for the protection of the human population.


Non-parasitic associations


Mutualistic hosts

Some hosts participate in fully mutualistic interactions with both organisms being completely dependent on the other. For example,
termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
s are hosts to the protozoa that live in their gut and which digest
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
, and the human gut flora is essential for efficient
digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...
. Many corals and other marine invertebrates house zooxanthellae, single-celled algae, in their tissues. The host provides a protected environment in a well-lit position for the algae, while benefiting itself from the nutrients produced by
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
which supplement its diet. '' Lamellibrachia luymesi'', a deep sea giant tubeworm, has an obligate mutualistic association with internal, sulfide-oxidizing, bacterial symbionts. The tubeworm extracts the chemicals that the bacteria need from the sediment, and the bacteria supply the tubeworm, which has no mouth, with nutrients. Some hermit crabs place pieces of
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...
on the shell in which they are living. These grow over and eventually dissolve away the mollusc shell; the crab may not ever need to replace its abode again and is well-camouflaged by the overgrowth of sponge. An important hosting relationship is mycorrhiza, a symbiotic association between a fungus and the roots of a vascular host plant. The fungus receives carbohydrates, the products of photosynthesis, while the plant receives phosphates and nitrogenous compounds acquired by the fungus from the soil. Over 95% of plant families have been shown to have mycorrhizal associations. Another such relationship is between
leguminous plants A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock ...
and certain nitrogen-fixing bacteria called rhizobia that form nodules on the roots of the plant. The host supplies the bacteria with the energy needed for nitrogen fixation and the bacteria provide much of the nitrogen needed by the host. Such crops as beans, peas, chickpeas and alfalfa are able to fix nitrogen in this way, and mixing
clover Clover or trefoil are common names for plants of the genus ''Trifolium'' (from Latin ''tres'' 'three' + ''folium'' 'leaf'), consisting of about 300 species of flowering plants in the legume or pea family Fabaceae originating in Europe. The genus h ...
with
grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in law ...
es increases the yield of pastures. Neurotransmitter tyramine produced by commensal '' Providencia'' bacteria, which colonize the gut of the nematode ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'', bypasses the requirement for its host to biosynthesise tyramine. This product is then probably converted to octopamine by the host enzyme tyramine β-hydroxylase and manipulates a host sensory decision.


Hosts in cleaning symbiosis

Hosts of many species are involved in
cleaning symbiosis Cleaning symbiosis is a mutually beneficial association between individuals of two species, where one (the cleaner) removes and eats parasites and other materials from the surface of the other (the client). Cleaning symbiosis is well-known amon ...
, both in the sea and on land, making use of smaller animals to clean them of parasites. Cleaners include fish, shrimps and birds; hosts or clients include a much wider range of fish, marine reptiles including turtles and iguanas, octopus, whales, and terrestrial mammals. The host appears to benefit from the interaction, but biologists have disputed whether this is a truly mutualistic relationship or something closer to parasitism by the cleaner.


Commensal hosts

Remoras (also called suckerfish) can swim freely but have evolved suckers that enable them to adhere to smooth surfaces, gaining a free ride ( phoresis), and they spend most of their lives clinging to a host animal such as a whale, turtle or shark. However, the relationship may be mutualistic, as remoras, though not generally considered to be cleaner fish, often consume parasitic
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...
s: for example, these are found in the stomach contents of 70% of the common remora. Many molluscs, barnacles and polychaete worms attach themselves to the carapace of the Atlantic horseshoe crab; for some this is a convenient arrangement, but for others it is an obligate form of commensalism and they live nowhere else.


History

The first host to be noticed in ancient times was human: human parasites such as hookworm are recorded from ancient Egypt from 3000 BC onwards, while in
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...
, the
Hippocratic Corpus The Hippocratic Corpus (Latin: ''Corpus Hippocraticum''), or Hippocratic Collection, is a collection of around 60 early Ancient Greek medical works strongly associated with the physician Hippocrates and his teachings. The Hippocratic Text corpus ...
describes human
bladder worm ''Taenia solium'', the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans as its definitive host and pigs ...
. The
medieval In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire a ...
Persian physician
Avicenna Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the Islam ...
recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms. In Early Modern times,
Francesco Redi Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician, naturalist, biologist, and poet. He is referred to as the "founder of experimental biology", and as the "father of modern parasitology". He was the first person to c ...
recorded animal parasites, while the microscopist Antonie van Leeuwenhoek observed and illustrated the protozoan '' Giardia lamblia'' from "his own loose stools". Hosts to mutualistic symbionts were recognised more recently, when in 1877 Albert Bernhard Frank described the mutualistic relationship between a
fungus A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
and an
alga Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
in lichens.


See also

* PHI-base (Pathogen-Host Interaction database) * Generalist and specialist species * Host cell protein


References

{{DEFAULTSORT:Host (Biology) Biological interactions Parasitology Disease ecology