Behavior-altering parasites are parasites with two or more hosts, capable of causing changes in the behavior of one of their hosts to facilitate 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 at a higher trophic level which becomes the definitive host 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, protozoa, viruses, and animals.
Among the behavioral changes caused by parasites is carelessness, making their hosts easier prey. The protozoan Toxoplasma gondii, for example, infects small rodents and causes them to become careless and attracted to the smell of feline urine, which increases their risk of predation and the parasite's chance of infecting a cat, its definitive host.
- 1 Behavioral change
- 2 Evolutionary perspective
- 3 Mechanisms
- 4 References
Parasite manipulations can be either direct or indirect. Indirect manipulation is the most frequent method used by behavioral 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. 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, development 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.
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 from the family Baculoviridae 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.
The protozoan Toxoplasma gondii infects animals from the Felidae family (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. Other parasites that increase their host's risk of predation include Euhaplorchis californiensis, Dicrocoelium dendriticum, Myrmeconema neotropicum and Diplostomum pseudospathaceum.
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, only to increase it when it is ready to transmit to a human host.
Making the host careless increases the risk of its being eaten by a non-host predator, interrupting the parasite's life-cycle. Some parasites manipulate their intermediate host to reduce this risk. For example, the parasitic trematode Microphallus sp., uses the snail Potamopyrgus antipodarum as an intermediate host. The parasite manipulates 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-hosts for the parasitic trematode).
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.
The parasitic nematode Myrmeconema neotropicum infects the intermediate ant host Cephalotes atratus. 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 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.
Crickets infected by Horsehair worms 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.
The trematode Leucochloridium paradoxum 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.
Schistocephalus solidus 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 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 and therefore effectively reduce its own mortality rate, due to premature transmission.
The emerald cockroach wasp (Ampulex compressa) parasitises its host, the American cockroach (Periplaneta americana) as a food source and for its growing larvae. The wasp stings the cockroach twice: First in the thoracic ganglion, 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 – "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".
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.
The wasp Dinocampus coccinellae is both an endoparasite and ectoparasite of ladybugs. The wasp injects an egg into the bug's abdomen, where the larva feeds on its haemolymph. 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 bug would thrash its limbs, scaring the predator off. A week later the grown wasp emerges from its cocoon; most of the bugs die at this point.
Strepsiptera of the Myrmecolacidae family 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). A similar, but much more intricate behavior is exhibited by ants infected with the fungus Ophiocordyceps unilateralis: 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.
Several species of fly in the Phoridae family parasitise 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 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.
Addition of intermediate hosts
For complex life cycles to emerge in parasites, the addition of intermediate host species must be beneficial, e.g., result in a higher fitness. It is probable that most parasites with complex life cycles evolved from simple life cycles. The transfer from simple to complex life cycles has been analyzed theoretically, and it has been shown that trophically transmitted parasites 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 its 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 (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 leads to altered drifting behavior in its intermediate host, the amphipod Gammarus pulex; 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 g , and the host's immune response.
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. 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.
Infection of the central nervous system
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.
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 neurochemical communication
Parasites that induce behavioral changes in their hosts often exploit the regulation of social behavior in the brain. Social behavior is regulated by neurotransmitters, such as dopamine and serotonin, in the emotional centers of the brain – primarily the amygdala and the hypothalamus, 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, 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 alters behavior through the injection of venom directly into the host's brain, causing hypokinesia. 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 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 peptides into the host's bloodstream, influencing both its immune response and neural function. Other sources suggest a possible origin in molecular mimicry.
Mermithid nematodes infect arthropods, residing in their haemocoel (circulatory cavity) and manipulating their hemolymph osmolality to trigger water-seeking behavior. The exact means by which they do that are unknown.
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