Behavior-altering parasite

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. Parasites 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 induce in their hosts changes to both feeding behavior and environment selection. They infect moth and butterfly caterpillars, which, 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 is a disease caused by viruses of the Lyssavirus genus, which are released into a host's saliva and transmitted when it comes in contact with an other animal's mucous membranes or open wounds. The disease causes the host to become aggressive and prone to attacking or biting others; this, along with increased salivation, increases the chances of it spreading to new hosts. At the same time, the host experiences hydrophobia (fear of water) and laryngeal spasms, which prevent it from drinking, keeping the virus-laden saliva from being washed down into the stomach or out of the mouth. Protozoal parasites Plasmodium falciparum The malaria 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, Toxoplasma gondii The protozoan Toxoplasma gondii infects animals from the family Felidae (its definitive host), and its oocysts 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. Parasitic fungi Ophiocordyceps unilateralis Ants infected with the fungus Ophiocordyceps unilateralis exhibit intricate behaviors: 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 of the fungus grows from the ant's head and ruptures, releasing the fungus's spores. 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, and Myrmeconema neotropicum. 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 (ruminants), 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. 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 of the genus Microphallus 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). 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). shortly after emerging from its cricket host, now drowned.|left 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 is a parasitic flatworm with three different hosts: two intermediate and one definitive. In its adult stage the tapeworm resides in the intestine of piscivorous 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 (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 this second sting makes the cockroach groom itself excessively before sinking into a state of hypokinesia – "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". Similarly, several parasitic wasps induce their spider hosts to build stronger webs to protect the growing parasites. Hymenoepimecis argyraphaga The parasitic wasp Hymenoepimecis argyraphaga grows its larvae on spiders of the species Leucauge 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 pupate. Pseudacteon Several species of fly in the genus Pseudacteon parasitize fire ants. 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, muscle and nerve tissue. During this period some larvae direct the ant up to 50 meters away from the nest and toward 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. wasp larva parasitizing a Cyclosa argenteoalba'' spider in Japan 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 of the family Myrmecolacidae 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 such as Sacculina 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. == Mechanisms ==

The ways in which parasites induce behavioral changes in hosts have 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. While the proximate mechanisms underlying this broad targeting have not been fully characterized, two mechanisms used by parasites to alter behavior in vertebrate hosts have been identified: infection of the central nervous system and altered neurochemical communication. Central nervous system infection Some parasites alter host behavior by infecting neurons 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 cytokines. 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 in rats, Trypanosoma cruzi in mice and Plasmodium mexicanum in the Mexican lizard. Infection with Toxoplasma gondii inhibits this response, increasing the risk of predation by T. gondiis subsequent hosts. Research suggests that the inhibited anxiety-response could be the result of immunological damage to the limbic system. In some cases, T. gondii is believed to cause increases in dopamine levels by secreting another compound, L-DOPA, which may trigger a rise in dopamine levels, though concrete evidence for this mechanism has not yet been demonstrated. Other sources suggest a possible origin in molecular mimicry. Other mechanisms Mermithid nematodes infect arthropods, residing in their haemocoel (circulatory cavity) and manipulating their hemolymph osmolality 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) 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. 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": == See also ==