Parasite increased trophic transmission
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Behavior-altering parasites are
parasite 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 ...
s with two or more hosts, capable of causing changes in the behavior of one of their hosts to enhance their transmission, sometimes directly affecting the hosts' decision-making and behavior control mechanisms. They do this by making the intermediate host, where they may reproduce asexually, more likely to be eaten by a
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
at a higher
trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...
which becomes the
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
where the parasite reproduces sexually; the mechanism is therefore sometimes called parasite increased trophic facilitation or parasite increased trophic transmission. Examples can be found in
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 ...
, protozoa,
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, 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. Parasites may also alter the host behaviour to increase protection of the parasites or their offspring; the term bodyguard manipulation is used for such mechanisms. Among the behavioral changes caused by parasites is carelessness, making their hosts easier prey.Moore, J. (2002). Parasites and the behavior of animals. Oxford: Oxford University Press. The protozoan ''
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 ...
'', for example, infects small
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s and causes them to become careless and may even cause them to become attracted to the smell of feline urine, both of which increase their risk of
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
and the parasite's chance of infecting a cat, its
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
. Parasites may alter the host's behavior by infecting the host's
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
, or by altering its
neurochemical A neurochemical is a small organic molecule or peptide that participates in neural activity. The science of neurochemistry studies the functions of neurochemicals. Prominent neurochemicals Neurotransmitters and neuromodulators *Glutamate is the ...
communication (studied in neuroparasitology).


Examples

Parasite manipulations can be either ''direct'' or ''indirect''. Indirect manipulation is the most frequent method used by behavior-altering parasites, while the direct approach is far less common. Direct manipulation is when the parasite itself affects the host and induces a behavioral response, for example by creating neuroactive compounds that stimulate a response in the host's central nervous system (CNS), a method mostly practiced by parasites that reside within the CNS. Affecting the host's neural system is complicated and manipulation includes initiating immune cascades in the host. However, determination of the causative factor is difficult, especially whether the behavioral change is the result of direct manipulation from the parasite, or an indirect response of the host's
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...
. A direct approach to behavioral manipulation is often very costly for the parasite, which results in a trade-off between the benefits of the manipulation (e.g., fitness increase) and the energy it costs. The more common approach for parasites is to indirectly induce behavioral responses by interacting with the host's immune system to create the necessary neuroactive compounds to induce a desired behavioral response. Parasites can also indirectly affect the behavior of their hosts by disturbing their
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
,
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...
, or immunity. Parasitic castrators drastically modify their hosts' metabolism and reproduction, sometimes by secreting castrating hormones, changing their behavior and physiology to benefit the parasite.
Parasite 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 ...
s may alter hosts' behaviors in ways that increase their likelihood of transmission (e.g. by the host being ingested by a predator); result in the parasite's release at appropriate sites (e.g. by changes in the host's preferences for habitats); increase parasite survival or increase the host's likelihood of being infected with more parasites.


Viruses

''Baculoviridae'' Viruses from the family ''
Baculoviridae ''Baculoviridae'' is a family of viruses. Arthropods, among the most studied being Lepidoptera, Hymenoptera and Diptera, serve as natural hosts. Currently, 85 species are placed in this family, assigned to four genera. Baculoviruses are kno ...
'' induce in their hosts changes to both feeding behavior and environment selection. They infect moth and butterfly caterpillars, who some time following infection begin to eat incessantly, providing nutrients for the virus's replication. When the virions (virus "units") are ready to leave the host, the caterpillar climbs higher and higher, until its cells are made to secrete enzymes that "dissolve the animal into goo", raining down clumps of tissue and viral material for ingestion by future hosts. ''Lyssavirus''
Rabies Rabies is a viral disease that causes encephalitis in humans and other mammals. Early symptoms can include fever and tingling at the site of exposure. These symptoms are followed by one or more of the following symptoms: nausea, vomiting, ...
(a disease caused by viruses of the ''
Lyssavirus ''Lyssavirus'' (from the Ancient Greek, Greek ''lyssa'' "rage, fury, rabies" and the Latin ''Virus#Etymology, vīrus'') is a genus of RNA viruses in the family ''Rhabdoviridae'', order ''Mononegavirales''. Mammals, including humans, can serve ...
'' genus) causes the host to become aggressive and prone to biting others. This, along with increased salivation (virus particles are released into and spread via saliva), increases the chances of it spreading to new hosts. At the same time, the disease causes hydrophobia (fear of water), and laryngeal spasms preventing the host from drinking so that the virus-laden saliva is not washed down into the stomach and made unavailable for infection by biting.


Protozoal parasites

''Plasmodium falciparum'' 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 ...
parasite '' Plasmodium falciparum'', carried by the '' Anopheles gambiae'' mosquito, changes its host's attraction to sources of nectar in order to increase its sugar intake and enhance the parasite's chance of survival. It also decreases the host's attraction to human blood while gestating, only to increase it when it is ready to be transmitted to a human host. ''Toxoplasma gondii'' The protozoan ''
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 ...
'' infects animals from the family '' Felidae'' (its
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
), and its
oocysts Apicomplexans, a group of intracellular parasites, have life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an apicomplexan organism i ...
are shed with the host's feces. When a rodent consumes the fecal matter it gets infected with the parasite (becoming its '' intermediate host''). The rodent subsequently becomes more extroverted and less fearful of cats, increasing its chance of predation and the parasite's chance of completing its lifecycle. There is some evidence that ''T. gondii'', when infecting humans, alters their behavior in similar ways to rodents; it has also been linked to cases of
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
.


Parasitic helminths

Multiple parasites increase their host's risk of predation to facilitate their transition from their intermediate host to their definitive host, including: '' Euhaplorchis californiensis'', '' Dicrocoelium dendriticum'', '' Diplostomum pseudospathaceum,'' '' Euhaplorchis californiensis,'' and ''
Myrmeconema neotropicum ''Myrmeconema neotropicum'' is a tetradonematid nematode parasite. It appears to induce fruit mimicry in the tropical ant. Presently the only known host species is ''Cephalotes atratus'', a South American ant with a black abdomen. Upon infectio ...
''. ''Dicrocoelium dendriticum'' The lancet liver fluke ('' Dicrocoelium dendriticum'') is a parasitic trematode with a complex life cycle. In its adult state it occurs in the liver of its definitive host (
ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s), where it reproduces. The parasite eggs are passed with the feces of the host, which then are eaten by a terrestrial snail (first intermediate host). The fluke matures into a juvenile stage in the snail, which in an attempt to protect itself excretes the parasites in "slime-balls". The "slime-balls" are then consumed by ants (second intermediate hosts). The fluke manipulates the ant to move up to the top of grass, where they have a higher chance of being eaten by grazing ruminants. ''Leucochloridium paradoxum'' The trematode ''
Leucochloridium paradoxum ''Leucochloridium paradoxum'', the green-banded broodsac, is a parasitic flatworm (or helminth). Its intermediate hosts are land snails, usually of the genus ''Succinea''. The pulsating, green broodsacs fill the eye stalks of the snail, thereby ...
'' matures inside snails of the genus '' Succinea''. When ready to switch to its definitive host, a bird, the parasite travels to the eye stalks of its host and begins to pulsate, attracting birds with its striking resemblance to an insect larva. It also influences the normally nocturnal snail to climb out into the open during the day for an increased chance of being consumed by a bird.''Microphallus'' The parasitic
trematodes Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...
of the genus ''
Microphallus ''Microphallus'' is a genus of parasitic trematodes (flukes) in the family Microphallidae. The Greek name means "tiny penis". Hosts They are parasitic on a variety of molluscs, crustaceans, birds, and mammals, some species having complex life ...
'' parasitise the snail '' Potamopyrgus antipodarum'' as an intermediate host. The parasites manipulate the snail's foraging behavior to increase the chance of it being preyed upon by the parasite's definitive hosts (
waterfowl Anseriformes is an order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which i ...
). The infected snail forages on the upper side of rocks during the period of the day when waterfowl feed most intensely. During the rest of the day, the snail forages at the bottom of rocks to reduce the risk of being eaten by fish (non-host predators). ''Myrmeconema neotropicum'' The parasitic nematode ''
Myrmeconema neotropicum ''Myrmeconema neotropicum'' is a tetradonematid nematode parasite. It appears to induce fruit mimicry in the tropical ant. Presently the only known host species is ''Cephalotes atratus'', a South American ant with a black abdomen. Upon infectio ...
'' infects the intermediate ant host ''
Cephalotes atratus ''Cephalotes atratus'' is a species of arboreal ant in the genus ''Cephalotes'', a genus characterized by its odd shaped head. These ants are known as gliding ants because of their ability to "parachute" by steering their fall if they lose their ...
''. The nematode then induces a morphological change in the ant, which turns the gaster color from black to red, making it resemble fruit. This color transition makes the ant susceptible to predation by
frugivorous A frugivore is an animal that thrives mostly on raw fruits or succulent fruit-like produce of plants such as roots, shoots, nuts and seeds. Approximately 20% of mammalian herbivores eat fruit. Frugivores are highly dependent on the abundance an ...
birds, which act as the parasite's definitive hosts. The parasitic eggs are deposited in the bird's feces and are eaten by ants, which complete the cycle. ''Nematomorpha'' Crickets infected by horsehair worms (''Nematomorpha'') exhibit light-seeking behavior and increased walking speed, leading them to open spaces and ponds (the surface of which reflects moonlight); the crickets will eventually find and enter a body of water, where the worm will wiggle out of the cricket's abdomen and swim away. While crickets often drown in the process, those who survive exhibit a partial recovery and return to normal activities in as little as 20 hours. ''Schistocephalus solidus'' ''
Schistocephalus solidus ''Schistocephalus solidus'' is a tapeworm of fish, fish-eating birds and rodents. This hermaphroditic parasite belongs to the Eucestoda subclass, of class Cestoda. This species has been used to demonstrate that cross-fertilization produces a high ...
'' is a parasitic tapeworm with three different hosts, two intermediate and one definitive. In its adult stage the tapeworm resides in the intestine of
piscivorous A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evoluti ...
birds, where they reproduce and release eggs through the bird's feces. Free-swimming larvae hatch from the eggs, which are in turn ingested by copepods (the first intermediate host). The parasite grows and develops in the crustacean into a stage that can infect the second intermediate host, the
three-spined stickleback The three-spined stickleback (''Gasterosteus aculeatus'') is a fish native to most inland and coastal waters north of 30°N. It has long been a subject of scientific study for many reasons. It shows great morphological variation throughout its ra ...
(''Gasterosteus aculeatus''). The parasite's definitive host, a bird, then consumes the infected three-spined stickleback and the cycle is complete. It has been observed that ''S. solidus'' alters the behavior of the fish in a manner that impedes its escape response when faced with a predatorial bird. This parasite-induced behavioral manipulation effectively increases the chance of it being consumed by its definitive bird host. It has also been observed that the parasite does not induce this behavior until it has reached a developed stage that can survive in the host bird and therefore effectively reduce its own mortality rate, due to premature transmission.


Parasitic insects

''Ampulex compressa'' The
emerald cockroach wasp The emerald cockroach wasp or jewel wasp (''Ampulex compressa'') is a solitary wasp of the family Ampulicidae. It is known for its unusual reproductive behavior, which involves stinging a cockroach and using it as a host for its larvae. It thus ...
('' Ampulex compressa'') parasitises its host, the American cockroach (''
Periplaneta americana The american cockroach (''Periplaneta americana'') is the largest species of common cockroach, and often considered a pest. In certain regions of the U.S. it is colloquially known as the waterbug, though it is not a true waterbug since it is not ...
'') as a food source and for its growing larvae. The wasp stings the cockroach twice: First in the
thoracic ganglion The thoracic ganglia are paravertebral ganglia. The thoracic portion of the sympathetic trunk typically has 12 thoracic ganglia. Emerging from the ganglia are thoracic splanchnic nerves (the cardiopulmonary, the greater, lesser, and least splanc ...
, paralyzing its front legs and enabling the wasp to deliver a second, more difficult sting, directly into the cockroach's brain; this second sting makes the cockroach groom itself excessively before sinking into a state of
hypokinesia Hypokinesia is one of the classifications of movement disorders, and refers to decreased bodily movement. Hypokinesia is characterized by a partial or complete loss of muscle movement due to a disruption in the basal ganglia. Hypokinesia is a sym ...
– "a... lethargy characterized by lack of spontaneous movement or response to external stimuli". The wasp then pulls the idle cockroach into its burrow, where it deposits an egg onto its abdomen and buries it for the growing larva to feed on. Keeping the cockroach in a hypokinetic state at this stage, rather than simply killing it, allows it to stay "fresh" for longer for the larva to feed on. The adult wasp emerges after 6 weeks, leaving behind nothing but an empty cockroach "shell". ''Dinocampus coccinellae'' The wasp '' Dinocampus coccinellae'' is both an endoparasite and ectoparasite of ladybugs. The wasp injects an egg into the beetle's abdomen, where the larva feeds on its
haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
. When grown and ready to pupate the larva exits its host, which remains immobile, and weaves a cocoon on its underside, where it pupates. Were a predator to approach, the beetle would twitch its limbs, scaring the predator off. This use of the host as a protection has been termed as bodyguard manipulation. Similarly, several parasitic wasps induce their spider hosts to build stronger webs to protect the growing parasites. ''Hymenoepimecis argyraphaga'' The parasitic wasp ''
Hymenoepimecis argyraphaga ''Hymenoepimecis argyraphaga'' is a Costa Rican parasitoid wasp whose host is the spider Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets t ...
'' grows its larvae on spiders of the species ''
Leucauge argyra ''Leucauge argyra'' is a spider and is known for being the host of the ''Hymenoepimecis argyraphaga'', a Costa Rican or Puerto Rican parasitoid wasp.Eberhard, W. (2001) Under the influence: Webs and building behavior of ''Plesiometa argyra'' ...
''. Shortly before killing its host the larva injects it with a chemical that changes its weaving behavior, causing it to weave a strong, cocoon-like structure. The larva then kills the spider and enters the cocoon to
pupa A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in thei ...
te. ''Ophiocordyceps unilateralis'' A similar, but much more intricate behavior is exhibited by ants infected with the fungus ''
Ophiocordyceps unilateralis ''Ophiocordyceps unilateralis'', commonly known as zombie-ant fungus, is an insect-pathogenic fungus, discovered by the British naturalist Alfred Russel Wallace in 1859, and currently found predominantly in tropical forest ecosystems. ''O. unil ...
'': irregularly-timed body convulsions cause the ant to drop to the forest floor, from which it climbs a plant up to a certain height before locking its jaws into the vein of one of its leaves answering certain criteria of direction, temperature and humidity. After several days the
fruiting body The sporocarp (also known as fruiting body, fruit body or fruitbody) of fungi is a multicellular structure on which spore-producing structures, such as basidia or asci, are borne. The fruitbody is part of the sexual phase of a fungal life cy ...
of the fungus grows from the ant's head and ruptures, releasing the fungus's spores. ''Phoridae''Several species of fly in the family
Phoridae The Phoridae are a family of small, hump-backed flies resembling fruit flies. Phorid flies can often be identified by their escape habit of running rapidly across a surface rather than taking to the wing. This behaviour is a source of one of thei ...
parasitise
fire ant Fire ants are several species of ants in the genus ''Solenopsis'', which includes over 200 species. ''Solenopsis'' are stinging ants, and most of their common names reflect this, for example, ginger ants and tropical fire ants. Many of the nam ...
s. The fly injects an egg into the ant's thorax; upon hatching, the larva migrates into the ant's head, where it feeds on the ant's
haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
, muscle and nerve tissue. During this period some larvae direct the ant up to 50 meters away from the nest and towards a moist, leafy place where they can hatch safely. Eventually the larva completely devours the ant's brain, which often falls off (hence the species nickname: "decapitating fly"). The larva then pupates in the empty head capsule, emerging as an adult fly after two weeks. ''Reclinervellus nielseni'' The parasitic wasp larvae '' Reclinervellus nielseni'' attach to the spider '' Cyclosa argenteoalba'', releasing substances that modify the spider's web-building behavior so that it weaves a cocoon-like structure for the larvae to pupate in. This manipulated behavior was longer lasting and more prominent the longer the larvae were attached to the spider. ''Strepsiptera'' ''
Strepsiptera The Strepsiptera are an order of insects with eleven extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never ...
'' of the family
Myrmecolacidae Myrmecolacidae is an insect family of the order Strepsiptera The Strepsiptera are an order of insects with eleven extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhopp ...
can cause their ant host to linger on the tips of grass leaves, increasing the chance of being found by the parasite's males (in case of females) and putting them in a good position for male emergence (in case of males).


Parasitic crustaceans

''Rhizocephala'' Members of the order ''
Rhizocephala Rhizocephala are derived barnacles that parasitise mostly decapod crustaceans, but can also infest Peracarida, mantis shrimps and thoracican barnacles, and are found from the deep ocean to freshwater. Together with their sister groups Thoracic ...
'' such as S. carcini alter male hosts' hormonal balance, to encourage nurturing behavior similar to that seen in females. The parasite usually spends its entire life within the host; however, if it is removed from the host in a laboratory setting, male hosts will subsequently grow partial or complete female gonads. in


Mechanisms

The way in which parasites induce behavioral changes in hosts has been compared to the way a neurobiologist would effect a similar change in a lab. A scientist may stimulate a certain pathway in order to produce a specific behavior, such as increased appetite or lowered anxiety; parasites also produce specific behavioral changes in their hosts, but rather than stimulate specific neurological pathways, they appear to target broader areas of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. While the proximate mechanisms underlying this broad targeting have not been fully characterized, two mechanisms used by parasites to alter behavior in
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
hosts have been identified: infection of the central nervous system and altered
neurochemical A neurochemical is a small organic molecule or peptide that participates in neural activity. The science of neurochemistry studies the functions of neurochemicals. Prominent neurochemicals Neurotransmitters and neuromodulators *Glutamate is the ...
communication.


Central nervous system infection

Some parasites alter host behavior by infecting
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
s in the host's central nervous system. The host's central nervous system responds to the parasite as it would to any other infection. The hallmarks of such response include local inflammation and the release of chemicals such as
cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...
s. The immune response itself is responsible for induced behavioral changes in many cases of parasitic infection. Parasites that are known to induce behavioral changes through central nervous system inflammation in their hosts include ''
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 ...
'' in rats, ''
Trypanosoma cruzi ''Trypanosoma cruzi'' is a species of parasitic euglenoids. Among the protozoa, the trypanosomes characteristically bore tissue in another organism and feed on blood (primarily) and also lymph. This behaviour causes disease or the likelihood o ...
'' in mice and ''
Plasmodium mexicanum ''Plasmodium mexicanum'' is a parasite of the genus ''Plasmodium'' subgenus '' Paraplasmodium''. Like all ''Plasmodium'' species ''P. mexicanum'' has both vertebrate and insect hosts. The vertebrate hosts for this parasite are reptiles. Taxono ...
'' in the Mexican lizard.


Immune response

While some parasites exploit their hosts' typical immune responses, others seem to alter the immune response itself. For example, the typical immune response in rodents is characterized by heightened anxiety. Infection with ''Toxoplasma gondii'' inhibits this response, increasing the risk of predation by ''T. gondii''s subsequent hosts. Research suggests that the inhibited anxiety-response could be the result of immunological damage to the limbic system.


Altered neurotransmission

Parasites that induce behavioral changes in their hosts often exploit the regulation of social behavior in the brain. Social behavior is regulated by
neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurot ...
, such as dopamine and serotonin, in the emotional centers of the brain – primarily the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verte ...
and the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
, and although parasites may be capable of stimulating specific neurochemical pathways to induce behavioral changes, evidence suggests that they alter neurochemical communication through broad rather than specific targeting. For example, ''Toxoplasma gondii'' attaches to the hypothalamus rather than target a specific cellular pathway; this broad targeting leads to a widespread increase in host dopamine levels, which may in turn account for the loss of aversion to cat odor. In some cases, ''T. gondii'' is believed to cause increases in dopamine levels by secreting another compound,
L-Dopa -DOPA, also known as levodopa and -3,4-dihydroxyphenylalanine, is an amino acid that is made and used as part of the normal biology of some plants and animals, including humans. Humans, as well as a portion of the other animals that utilize -DOPA ...
, which may trigger a rise in dopamine levels, though concrete evidence for this mechanism has not yet been demonstrated. This rise in dopamine levels induces a loss of aversion to cat odor in the rats, increasing the risk of predation by cats, ''T. gondii''s definitive host. The mechanistic details underlying the increase in dopamine levels and the way it affects the rat's behavioral change remain elusive. The
emerald cockroach wasp The emerald cockroach wasp or jewel wasp (''Ampulex compressa'') is a solitary wasp of the family Ampulicidae. It is known for its unusual reproductive behavior, which involves stinging a cockroach and using it as a host for its larvae. It thus ...
alters behavior through the injection of venom directly into the host's brain, causing
hypokinesia Hypokinesia is one of the classifications of movement disorders, and refers to decreased bodily movement. Hypokinesia is characterized by a partial or complete loss of muscle movement due to a disruption in the basal ganglia. Hypokinesia is a sym ...
. This is achieved by a reduction in dopamine and octopamine activity, which affects the transmission of interneurons involved in the escape response; so while the host's brain circuitry responsible for movement control is still functional – and indeed it will slog along when pulled by the wasp – the nervous system is in a depressed state. Put differently: the wasp's toxin affects not the host's ''ability'' to move, but its ''motivation'' to do so. The original function of such secretions may have been to suppress the immune system of the host, as described above. The trematode '' Schistosoma mansoni'' secretes
opioid Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioid use ...
peptides into the host's bloodstream, influencing both its immune response and neural function. Other sources suggest a possible origin in molecular mimicry.


Other mechanisms

Mermithid nematodes infect
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s, residing in their haemocoel (circulatory cavity) and manipulating their
hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
osmolality Molality is a measure of the number of moles of solute in a solution corresponding to 1 kg or 1000 g of solvent. This contrasts with the definition of molarity which is based on a specified volume of solution. A commonly used unit for molali ...
to trigger water-seeking behavior. The means by which they do so are unknown.


Evolutionary perspective


Addition of intermediate hosts

For complex life cycles to emerge in parasites, the addition of an intermediate host species must be beneficial, i.e. result in a higher fitness. It is probable that most parasites with complex life cycles evolved from simple life cycles; the evolution from simple to complex life cycles has been analyzed theoretically, and it has been shown that trophically transmitted parasites (parasites that transmit from a prey host to a predator host during
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
) can be favored by the addition of an intermediate prey host if the population density of the intermediate host is higher than that of the definitive host. Additional factors that catalyze this transfer are high predation rates, and a low natural mortality rate of the intermediate host. Parasites with a single host species are faced with the problem of not being able to survive in higher trophic levels and therefore dying with their prey host. The development of complex life cycles is most likely an adaptation of the parasite to survive in the predator. The development of parasite increased trophic transmission is a further adaptation in relation to a complex life cycle, where the parasite increases its transmission to a definitive host by manipulating its intermediate host.


Evolution of induced behaviors

The ''adaptive manipulation'' hypothesis posits that specific behavioral alterations induced in a host can be used by parasites to increase their fitness. Under this hypothesis, induced behaviors are the result of natural selection acting upon the parasite's
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 ...
(in this case its ''host's behavior''). Many behaviors induced by obligate parasites to complete their lifecycles are examples of adaptive manipulation because of their clear relationship to parasite fitness. For example, evidence has shown that infection by the parasitic worm ''
Pomphorhynchus laevis ''Pomphorhynchus laevis'' is an endo-parasitic acanthocephalan worm, with a complex life cycle, that can modify the behaviour of its intermediate host, the freshwater amphipod ''Gammarus pulex''. ''P. laevis'' does not contain a digestive tract ...
'' leads to altered drifting behavior in its intermediate host, the amphipod ''
Gammarus pulex ''Gammarus pulex'' is a species of amphipod crustacean found in fresh water across much of Europe. It is a greyish animal, growing to long. Description Adult males of ''Gammarus pulex'' may reach a total length of , while females only grow to . ...
''; this altered behavior increases its host's predation risk by fish which are ''P. laevis''s definitive hosts. The induced behavioral change in the host thus leads to the parasite's increased success in completing its life cycle. In general, whether a specific behavioral change serves an adaptive purpose for the parasite, the host, or both, depends on the entire "host-parasite system": The life cycle of the pathogen, its virulence and the host's immune response. Conversely, evolved behaviors of the host may be a result of adaptations to parasitism.


References

{{reflist, 30em Parasites of animals Abnormal behaviour in animals Mind-altering parasitism