Behavior-altering parasite
[10] 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.
[13] 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.
At the same time, the host experiences hydrophobia (fear of water)[14] and laryngeal spasms,[15] which prevent it from drinking, keeping the virus-laden saliva from being washed down into the stomach or out of the mouth.
[18] 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.
[19] Ants infected with the fungus Ophiocordyceps unilateralis exhibit intricate behaviors: irregularly timed body convulsions cause the ant to drop to the forest floor,[20] from which it climbs a plant up to a certain height[21] before locking its jaws into the vein of one of its leaves answering certain criteria of direction, temperature, and humidity.
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.
[25] 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.
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).
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 parasite grows and develops in the crustacean into a stage that can infect the second intermediate host, the three-spined stickleback (Gasterosteus aculeatus).
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.
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[30] and therefore effectively reduce its own mortality rate, due to premature transmission.
[32] 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.
[44] Members of the order Rhizocephala such as Sacculina carcini alter male hosts' hormonal balance, to encourage nurturing behavior similar to that seen in females.
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.
[47] 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.
[49] The mechanistic details underlying the increase in dopamine levels and the way it affects the rat's behavioral change remain elusive.
[46] The emerald cockroach wasp alters behavior through the injection of venom directly into the host's brain, causing hypokinesia.
The trematode Schistosoma mansoni secretes opioid peptides into the host's bloodstream, influencing both its immune response and neural function.
[51] Mermithid nematodes infect arthropods, residing in their haemocoel (circulatory cavity) and manipulating their hemolymph osmolality to trigger water-seeking behavior.
[52] 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.
[53] The adaptive manipulation hypothesis posits that specific behavioral alterations induced in a host can be used by parasites to increase their fitness.
The induced behavioral change in the host thus leads to the parasite's increased success in completing its life cycle.
