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The surprising story of parasites: Why do they survive in their hosts?
In biology and medicine, a host is a larger organism that carries a smaller organism. These smaller organisms often include parasites, mutualists, or symbionts. They survive on the nutrients and shelter provided by the host. There are numerous examples of animals becoming infected with parasites (e.g. nematodes) from their hosts, while cells may harbor pathogenic viruses. Taking plants as an example, host plants provide food for micropredators, which is an evolutionarily stable relationship between parasites and hosts.
Symbiosis and parasitismSymbiosis can be divided into several different types of relationships, depending on how long these relationships last and how much they affect both parties. If one organism is much larger than the other, that organism is often called the host. In parasitism, the parasite benefits at the expense of the host, while in symbiosis both parties are able to live together in harmony and safety. Studies have shown that most parasites only have parasitic characteristics at a certain stage in their life cycle.
Parasitic lifestyles evolved in at least 233 independent events between parasites and their closest free-living relatives.
Some organisms exist in close association with their hosts, transforming into parasites only when conditions become hostile. For example, some parasites hide inside their hosts and, after growing to a certain stage, may cause the host's death.
Type of host
In parasitology, the type of host has multiple definitions. The primary host is the organism in which the parasite reaches the mature stage and reproduces. Some hosts, called intermediate hosts, host immature parasites and are necessary for their development. Parasites or transport hosts are organisms that host parasites but do not affect their development. Furthermore, with the emergence of certain hosts, unexpected hosts often do not allow the parasite to complete its life cycle, which can eventually lead to an infectious barrier situation.
For example, humans and horses are considered dead-end hosts for West Nile virus; once infected, they do not have enough virus in their bodies to transmit to the mosquitoes that bite them.
Plant hosts and micropredators
In plants, micropredators parasitize host plants in an evolutionarily stable strategy that involves feeding on plant parts. The host ranges of micropredators reflect their unique evolutionary adaptations. Some insects are specialized and dependent on a single plant for survival.
Host range effectsLarvae such as silkworms feed exclusively on mulberry leaves, while more diverse insects such as the brusu moth feed on a variety of plants.
Host range refers to the set of hosts that a parasite can exploit. In the case of human parasites, changes in host range can affect parasitic behavior or the epidemiology of clinical disease.
For example, influenza viruses infecting different hosts in pigs can lead to antigenic drift and the emergence of new virus strains.
Non-parasitic interactions
In some cases, the host forms a completely mutualistic relationship with other organisms, such as the symbiosis between termites and the protozoa in their intestines. Both parties in these relationships are completely dependent on the other's existence, and this interaction, to some extent, ensures the survival of the species.
Historical BackgroundThe relationship between host and parasite has been recognized since early in human history. For example, human parasites such as hookworms were widely recognized as early as ancient Egypt. Over time, scientific awareness of the importance of various hosts grew, with Albert Bernhard Frank first describing symbiotic relationships in lichens in 1877.
These diverse host relationships remind us that every organism living on Earth is interconnected, forming a complex and intricate ecological network. In such an ecosystem, how do you think the relationship between host and parasite will affect future biological evolution?
