Behavior-altering parasite or parasitoid

Some parasites and parasitoids cause changes in the behavior of their hosts by directly affecting the hosts' decision-making and behavior control mechanisms. The acquired or modified behaviors assist in parasite transmission, and often result in the host's demise.

Parasites may alter hosts' behaviors in ways that enhance the likelihood of parasite transmission from host to host (e.g. by host predation); result in parasite release at appropriate sites (e.g. by changes in the host's preferences for habitat selection); increase parasite survival or increase the host's likelihood of colonization by suitable mates for the parasite.

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 felines, 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.

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.

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