Parasitism (also known as parasitism ) describes the phenomenon within the animal and plant world that an organism or virus ( parasite ) misuses a usually much larger organism of a different species as a host . The host serves the parasite for food supply (mostly in the form of blood or other body secretions ) and in some cases also as a long-term living space . The host's health is damaged by the parasite, but usually remains alive. The consequences of parasitic infestation range from sting or bite injuries from massive physical complaints to serious infectious diseases and even death in extreme cases. The best-known and most widespread human parasites include human lice , mosquitoes and various types of ticks and worms .
Parasites whose parasitism usually leads to death are as parasitoids or predatory parasites called, such as parasitic wasps . Parasitism is a form of interspecific interrelationships . If, on the other hand, no harm is done to the host, this is known as probiosis , for example in commensalism .
In a broader sense, parasitism can be understood as an increase in the fitness of the parasite, which is sometimes associated with a decrease in the fitness of the host.
Parasite is etymologically derived from ancient Greek παράσιτος parásitos , German 'eating with someone else , parasite' , which goes back to ancient Greek παρά pará , German 'next to' as well as ancient Greek σῖτος sītos , German 'grain, [food made from grain]' . This originally meant the taster at sacrificial feasts, which was fed without being served. From there the meaning passed on to the drooling of the ancient comedy , who tries to get free meals through beautiful words. A change to a biological meaning in the sense of a living being that lives in or on others (see ecto- and endoparasites ) and deprives them of nutrients took place in the 18th century. From there, the meaning soon returned to the social field, for example in the Enlightenment polemics against the nobility or in the anti-Semitic stereotype of the Jewish parasite .
Parasites are highly specialized living things. Their habitat is usually restricted to a few host species, and it is not uncommon for there to be only one host species. Parasitism shows itself in very diverse forms. There are doubtful cases where parasitism is difficult to distinguish from other species-to-species interactions . Parasitism is by no means a rare phenomenon, because the vast majority of all living things parasitize. A ratio of up to 4: 1 is assumed, provided that no precise figures can be determined. The appearance of the next generation in the host is known as patents .
In general, a parasite is highly dependent on its host . Parasitization can relate to various host factors such as body substance, food supply, oxygen demand, osmotics, pH conditions or heat balance.
Parasitism is ubiquitous, so practically all living things have to deal with it. It is not uncommon to find dozens of different parasites on or in a single living being, whereby the microorganisms are not taken into account. In forest mice they found no less than 47 parasitic species.
Depending on the extent of the parasite infestation, the burden on the host varies. Even if parasite infestation does not cause life-threatening damage to the host, it always has a negative effect on its growth, well-being, susceptibility to infection, reproduction or lifespan. Toxic metabolic products of the parasite, remaining internal or external injuries or the deprivation of food can result in a shortening of life, especially in other unfavorable environmental conditions. Hosts, however, are by no means passive towards their parasites, but are usually able to limit the number and harmful effect through suitable defense mechanisms. In a joint development ( coevolution ), hosts and their parasites adapted to one another. As a result, an equilibrium developed at every stage of evolution , in which the parasite benefits without harming the host, which is its "livelihood", more than necessary or even completely destroying it (the same mechanism exists in infectious diseases between pathogen and Host regarding virulence , disease progression and immune defense).
Many parasites become parasites as they develop in different hosts. A distinction is made between intermediate hosts and the ultimate host . Sexual reproduction usually only takes place in the ultimate host.
Organisms that are attacked without the parasite being able to continue its development cycle are called false hosts . The parasite is often poorly adapted to its false host, so that the false host is more damaged by the parasite than the host.
Adaptation from parasites
Like all other living beings, parasites have also been adapted to their environment in a variety of ways in the course of evolution through mutation , recombination and selection, in particular to their respective host organisms:
- Adhesive and clamp organs use z. B. Lice (staple legs), which prevent the parasite from losing its host, which would usually result in its death.
- Degeneration of organs that are not necessary for a parasitic way of life. For example, lice lack wings , white-berry mistletoe has no roots, quendel silk has no leaves and endoparasitic worms have no digestive organs.
- Large egg numbers and complicated development and transmission paths ensure reproduction and the finding of a host. For example , tens of thousands of eggs are released with each tapeworm limb that escapes through feces . These can infect intermediate hosts and form asexual stages of reproduction in their liver ( Finns ). If the finnish intermediate host z. B. eaten by cats or foxes , a new infection is very likely.
- Behavior modification in the intermediate host : molecular signals from the parasite such as B. the little liver fluke cause unusual or conspicuous behaviors of the intermediate host, which lead to it being easy prey for the final host. In this way the parasite reaches the ultimate host, where its sexual reproduction takes place.
The evolution of all parasites and their hosts mutually influence each other, which is known as coevolution and results in a high degree of adaptation of the parasite and host. The habitat also has an impact on evolution, e.g. B. in marine parasites of humans .
Classification of parasites
Due to the very different adaptation, size and lifestyle of different parasites and the different forms of interaction between parasite and host, parasites are classified according to a variety of different criteria:
Micro and macro parasites
If one differentiates between parasites in terms of their size, the following two differentiation criteria result:
Microparasites are small, sometimes extremely small (and usually so numerous that it is impossible to tell the number of parasites in the host). It is therefore usually easier to study the number of infected hosts than the number of parasites. Microparasites are mostly protozoa that infect animals and plants as pathogens . Some plants have microparasitic lower fungi .
Macroparasites are usually so large that you can determine their number exactly or at least estimate their size. In animals, they are more likely to be found on the body or in body cavities (e.g. in the intestines ) than in tissue. The main macro-parasites of animals are worms (tape and flukes and nematodes ), as well as lice, ticks , mites and fleas , as well as some fungi. Macroparasites of plants generally live between the cells (intercellular) and belong to the higher fungi (e.g. powdery mildew ), to the insects (e.g. gall wasp ) or other plants (e.g. devil's twine or summer root ).
Ecto- and endoparasites
If one differentiates between the parasites in terms of their ability to penetrate the host's body, the following two classes result:
Ectoparasites, or external parasites, live on other organisms. They only penetrate their host organism with the supply organs and feed on skin substances or ingest blood or tissue fluid . Examples of ectoparasites are blood-sucking arthropods such as mosquitoes , lice or ticks . Ectoparasites are also often the carriers of diseases such as malaria or Lyme borreliosis . In malaria, heterozygous carriers of sickle cell anemia can have a selection advantage, since the parasites may be killed directly there (see Significance of sickle cell anemia for malaria ).
Endoparasites (also ento or internal parasites ) live inside their host. They include B .: warble flies , tapeworms and some fungi . They colonize cavities, epithelia , blood or even the tissue of various organs. The diseases they cause are called endoparasitoses . Furthermore, the endoparasites can be divided into two groups according to their characteristics when infesting cells. Extracellular endoparasites live outside of cells (e.g. Giardia on intestinal epithelium), Intracellular endoparasites, on the other hand, live predominantly inside host cells (e.g. malaria pathogens ). Many endoparasites can be found both extra- and intracellularly during their life cycle.
Facultative and obligatory parasites
Parasites can be differentiated based on the need for a host. Facultative parasites (or occasional parasites ) are free-living creatures that parasitize only occasionally. Their development can also take place without a parasitic phase.
Obligatory parasites are absolutely dependent on a host for their development.
Temporary and stationary parasites
Based on the length of the parasitic life phase, a distinction is made between temporary and stationary parasites.
Stationary parasites remain loyal to a host continuously throughout their life or at least during a developmental period. A host change only takes place in the event of close contact with another possible host animal or if the original host dies (e.g. pubic louse with a high level of attachment to the host, flea with limited attachment).
The stationary parasites can be divided into two groups:
- Periodic parasites are parasitic only in certain stages of development. A distinction is made between forms with a simple change between parasitic and non-parasitic stages and forms with multiple changes between the stages, such as those found in flukes, for example . In the simple exchange is called, depending on the parasite stage of Larvalparasitismus or Imaginal- or Adultparasitismus that common in nematodes can be observed.
- Permanent parasites do not have a free (non-parasitic) life stage. A distinction is made between forms in which all stages of development parasitize a single host, such as the real animal lice , to which the human lice belong, and forms that parasitize different hosts depending on the stage of development, such as the trichinae ( Trichinella ), a genus of tiny nematodes.
Temporary parasites visit a host for a limited time only. You are looking for him B. only briefly to eat (e.g. mosquito ).
Host specificity and host change
If parasites specialize in a single host species they are called monoxenic (or autoxenic ), if a few host species are specialized they are called oligoxenic , and parasites with many host species are called polyxenic (or pleioxene ). If parasites only need one host for their development, so that no host change takes place, they are referred to as homoxic (or monoxic ). The opposite are heteroxenic (or heteroic ) parasites that change host during their development. The term heteroic is also used in a more general sense for parasites that are not host-specific.
A host change can be observed , among other things, in malaria . Some types of pathogen, called plasmodia , use humans as intermediate hosts in order to ultimately promote their full development or further cell division in the Anopheles mosquito , which acts as the ultimate host.
As Kleptoparasitism (from ancient Greek κλέπτειν kléptein "steal") the exploitation referred to services of other animals, such as stealing food or taking advantage of nesting opportunities . In particular, several bird species are known to have a kleptoparasitic diet, at least occasionally.
Brood parasites or brood parasites are organisms that their own offspring by other brood care animal species can raise. Ultimately, it is a special form of kleptoparasitism. Brood parasitism is found in birds , fish and insects . Usually the host parents of another species are used to raise the young of the brood parasite. If, on the other hand, the host parents belong to their own species, it is no longer parasitism in the narrower sense, but is sometimes referred to as intraspecific brood parasitism .
Brood parasitism saves the parasitic parents from many investments, from building a nest to feeding the young animals to the possibility of further mating during the rearing phase. Finally, the risk of a complete clutch loss by nest predators also decreases if the own eggs are spread over numerous clutches. Since breeding parasites permanently reduce the fitness of the host parents, an intensive evolutionary adaptation (“evolutionary arms race”) between the parasite and the host can often be observed.
As Phytoparasiten is called parasitic plants , which some vital resources to purchase by means of a host plant. In the case of parasitic plants, a distinction is made between two groups, the parasitic flowering plants and the myco- heterotrophic plants. The parasitic flowering plants parasitize directly on other flowering plants with the help of special organs ( haustoria ).
There are chlorophyll-free (fully mycotrophic ) species such as the spruce asparagus , but also species such as the white forest bird , which are still leafy green and are only partially myco-heterotrophic or mixotrophic .
Parasitism in Ecology
The influence of parasites in ecosystems is immense and is often neglected. Their influence on neobiota can be clearly seen if the parasites were not introduced into the new habitat. In such cases, the neobiota can have an advantage in their fitness over native species and as a result multiply excessively. Examples of such disturbances of ecosystems by neobiota are the chestnut leaf miner or the so-called "killer alga" Caulerpa taxifolia .
Parasites of humans
Parasitic infections (infections by parasites) in humans are infections by protozoa or protista and worm infections , with the latter i. d. Usually it is an infestation, i.e. an infestation without multiplication. Infections lead to the full picture of the parasitosis even with the first attack, infestations only after accumulation of many individuals due to strong or long exposure. Some parasites transmit pathogens to humans, some of which cause fatal diseases (parasitoses). A list can be found under Parasites of humans. The definition of parasite does not apply to many bacteria and fungi; Due to their medical importance, they are treated in the fields of infectious diseases , bacteriology and mycology within microbiology .
Differentiation: viruses, transposons and prions
In addition to parasites, there are also pathogenic and sometimes infectious molecules and molecular complexes that do not meet the criteria for living beings such as metabolism, autonomous replication or compartmentalization, e.g. B. viruses , viroids , transposons , retro elements , self-serving DNA and the exclusively protein- based prions . They have some parasitic properties such as resource acquisition and a difference in size without being parasite. These pathogens are thematically treated by virology .
Viruses represent a special form here. Since they do not have their own metabolism, they do not belong to the class of parasites. They harm the sick person using a minimal genome that consists of only one type of nucleic acid (either DNA or RNA ). This genome, which is only used for reproduction, forces the infected cell to perform functions that lead to the virus not replicating independently. The cell death that often results from this can lead to considerable damage to the patient. If the infected organism is a bacterium, the virus is called a bacteriophage .
Examples of parasitism are also known from paleontology. In the Baltic amber, for example, inclusions have been handed down that prove parasitism (e.g. mite larvae on a long-legged fly, a stilted mosquito or a bark louse; nematode on a mosquito ).
- The science that deals with parasites is called parasitology and is part of both ecology and medicine ( infectiology ).
- The sequence of different, detaching parasites, which attack the individual developmental stages of their host, is called a parasite sequence .
- In insects , in which parasitism can occur in different stages of development, a distinction is made between egg, larva, pupa and imaginal parasites , in other living beings one speaks of youth and old age parasites .
- A disease or debilitation caused by parasites is called parasitosis .
- Zoonoses are infectious diseases that can be transmitted from animal to person and from person to animal (e.g. rabies ).
- A anthroponosis is limited solely to the human parasitosis.
- As Parasitozönose it means the combination of living in an organ or in a host parasitic organisms.
- If a parasite attacks another parasite, one speaks of hyperparasitism .
- Superparasitism denotes an occupation of the host organism by more parasitic individuals of a species than is normally the case, e.g. B. by random simultaneous multiple occupancy.
- Of opportunism is when normally harmless parasite in certain circumstances (eg. As in a weakened host immune system ) to serious illness or even death of the host lead.
- A neuroparasite controls the behavior of its host and turns it into a zombie, as it were . Example: toxoplasmosis .
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