Behavior-altering parasite

Behavior-altering parasites are parasites capable of causing changes in the behavior of their hosts species to enhance their transmission, sometimes directly affecting the hosts' decision-making and behavior control mechanisms. By way of example, a parasite that reproduces in an intermediate host may require, as part of their life cycle, that the intermediate host be eaten by a predator at a higher trophic level, and some parasites are capable of altering the behavior of the intermediate host to make such predation more likely; a mechanism that has been called parasite increased trophic facilitation or parasite increased trophic transmission. Examples can be found in bacteria, protozoa, viruses, and animals. Parasites may perform bodyguard manipulation to increase protection of the parasites or their offspring.

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 may cause them to become attracted to the smell of cat urine, both of which increase their risk of predation and the parasite's chance of infecting a cat, its definitive host.

Parasites may alter the host's behavior by infecting the host's central nervous system, or by altering its neurochemical communication (studied in neuroparasitology).

Mechanisms

Direct and indirect manipulation

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.