HOST (BIOLOGY): CHARACTERISTICS, TYPES AND EXAMPLES - BIOLOGY - 2024

A host or host, in biology and medicine, is a living being, animal or plant, from which another organism, pathogen or parasitic, obtains shelter and food. In medicine it is also used to describe patients receiving organ or tissue transplants.

This term is used in other contexts, for example "hosts" are also considered all plants on which other plants grow that are epiphytes (that live on), but that do not necessarily provide food for the latter.

Photograph of an epiphytic plant and its host plant, on which it grows (Source: Ziegler175 via Wikimedia Commons)

The word host comes from the Latin hospitator-oris, which means “that hosts”. In the scientific literature, the terms "host" or "host" can also be found to refer to the same concept.

However, the term “host” actually refers to the parasite or “the one who is staying”, since it comes from the Latin hospes-itis, which can have a double meaning depending on the context, which is why many authors of the Spanish language they consider better to avoid the use of this word.

Guest characteristics

In theory, any living being can be a host for some parasitic species, that is, virtually all organisms on the biosphere participate in some ecological parasite-host relationship.

In this type of relationship, it has been determined that coevolutionary processes can occur, where the parasite specializes to parasitize the host and the host seeks a way to prevent the parasite from affecting it considerably.

The main characteristics of a host, then, will greatly depend on the type of organism that parasites or inhabits it, since it is unlikely that a phytoparasite (plant parasite) can take the same advantage of resources in an animal as in a plant, for example.

A host enters into intimate physiological, genetic, and behavioral relationships, to name a few, with the parasites that affect it.

Animals are hosts (Image by Pexels at www.pixabay, com)

The hosts can harbor parasitic species inside them, but this term also refers to those organisms inhabited by commensals or mutualists, which do not necessarily imply a negative ecological relationship.

As almost any species of animal or plant can serve as a host, then it can be said that these can have aquatic, terrestrial or aerial habits. Well, it is not the same to parasitize a plant or an aquatic animal than an animal or a terrestrial plant or a flying animal.

On some occasions, a living being can "become" a host for another when it modifies some characteristic of its feeding behavior, when it moves great distances (animals), when there is an excessive proliferation of a previously non-existent parasitic species, etc.

Host types

According to the part of the life cycle of the parasites that takes place inside, the hosts can be classified as primary, secondary or intermediate and reservoirs.

Primary or definitive host

Primary hosts, also described in some textbooks as "final" or definitive hosts, are those within which a parasite or symbiont matures and reproduces.

For those parasites with sexual reproduction, the definitive host is the one where they reach sexual maturity and reproduce, which implies that the fusion of their gametic cells occurs inside it.

Thus, for parasites that have more than one host (usually an invertebrate and a vertebrate, in the case of animal parasites), the primary host is where reproduction occurs, while for parasites that only parasitize one type of host, this is known as definitive.

Secondary or intermediate host

Secondary or intermediate hosts are usually defined as those in which the immature stages of the parasites are maintained, be they juvenile parasites, larvae, among others, which are "intermediate" forms in their life cycle.

These hosts can be classified as ordinary intermediaries, vectors or transmitters or "wait" hosts.

Ordinary intermediate hosts are "passive" hosts; parasites use these to sustain themselves for a time or to feed. Vectors or transmitters are those secondary hosts that "transport" parasites from one host to another and are essential for the life of the parasite.

Waiting hosts, on the other hand, are hosts that could be considered "accidental" that are somehow incorporated into the life cycle of the parasite between the last intermediate host and the definitive host.

Reservoir guests

These are organisms that support the life cycle of a parasite as a “replacement” for one of the other hosts when one of them is missing (in those parasites with complex life cycles that are distributed among several hosts).

They are also considered "alternative" hosts and the word "reservoir" is normally used to establish that, although the parasite cannot definitively complete its life cycle, it is kept in these hosts while its true hosts "appear" or become available.

From the point of view of those parasites that are anthropozoonotic, that is, that have a life cycle distributed between animal vectors and human beings, “reservoirs” are considered infected animals that maintain parasite populations while they manage to infect humans., which are usually their definitive hosts.

Host examples

Human beings

In nature there are multiple examples of hosts. Without going too far, human beings are hosts to a large number of organisms: parasites, symbiotics or mutualists.

As for the parasites that can be hosted by the human body, there is a great variety. There are those that affect different body systems such as the stomach and intestines, lungs, heart, brain, etc., protozoan parasites (T. cruzi, T. brucei…) being especially important.

Viruses are also considered parasites for humans, and can significantly affect the health of carriers (HIV, hepatitis virus, influenza virus, etc.).

Humans are potential hosts for many parasitic, mutualistic, or commensal species (Image by Sasin Tipchai at www.pixabay.com)

Vertebrated and invertebrated animals

Both vertebrate and invertebrate animals are hosts for a wide variety of parasites, symbionts, and mutualists. Cows, for example, symbiotically associate with many bacteria that inhabit their stomachs and allow them to digest the cellulose of grasses and other plants that they feed on.

Plants

Plants can also be hosts for insects, fungi, bacteria and parasitic viruses, as well as other plants that can be parasites or epiphytes, which grow on the structures of certain plant species, and may or may not feed on these.

References

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2. Durmuş, S., Çakır, T., Özgür, A., & Guthke, R. (2015). A review on computational systems biology of pathogen – host interactions. Frontiers in microbiology, 6, 235.
3. Fernández, AR, & Cordero del Campillo, M. (2002). Parasitism and other biological associations. Parasites and hosts. Cordero del Campillo, M., Vázquez, FA, Fernández, AR, Acedo, MC, Rodríguez, SH, Cozar, IN, Baños, PD, Romero, HQ & Varela, MC Veterinary Parasitology, 22-38 Fernández, AR, & Cordero del Campillo, M. (2002).
4. Hammond, TT, Hendrickson, CI, Maxwell, TL, Petrosky, AL, Palme, R., Pigage, JC, & Pigage, HK (2019). Host biology and environmental variables differentially predict flea abundances for two rodent hosts in a plague-relevant system. International Journal for Parasitology: Parasites and Wildlife, 9, 174-183.
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