Behavior-altering parasites and parasitoids
Some forms of parasites and parasitoids cause changes in the behaviour of their hosts by directly affecting the hosts' nervous system. The acquired or modified behaviors assist the parasite in transmitting or spreading itself to additional hosts, and while the mechanism of change may be harmless to the host, the acquired or modified behaviors typically accelerate the host's demise.
Animal behavior is typically motivated by proximate level mechanisms that promote particular actions. In most cases, proximate level mechanisms are a result of the individual’s interaction with the environment but some parasites have been shown to interact and manipulate these mechanisms to produce behaviors beneficial to the parasite alone. Parasites are known to drastically affect how animals behave. Parasites most commonly target the central nervous system (CNS) in order to alter animal behavior. By affecting hormone secretions or by physical restructuring, parasites successfully change how an animal’s body functions and delivers, interprets and reacts to messages. Some parasites, like Toxoplasma, form vacuoles that travel through the nervous system interrupting key functions in intraneural communications. The emerald cockroach wasp alters behavior through the injection of venom directly into the host’s brain, causing hypokinesia, and while the circuitry in control of movement is functional, the nervous system acts from a depressed state. Movement in the host is controlled by dopamine and octopamine which affect transmission of interneurons involved in the natural response to escape, and the reduced motor activity results from a reduction of these amines.
Toxoplasma gondii is a noted case of unintended intermediary hosts. Typically, Toxoplasma infects animals from the Felidae family and oocysts are shed with the feces. When rodents consume the fecal matter, they become infected themselves and the parasite begins to alter their behavior. Rodents become more extroverted and less fearful of felines. There is some evidence that Toxoplasma, when infecting humans, alters their behavior in similar ways to rodents. Toxoplasma has also been linked to cases of schizophrenia. (For more details on the effects on humans, see here)
The jewel wasp or Emerald cockroach wasp (Ampulex compressa) parasitises its host, the American cockroach (Periplaneta americana) as a food source and home for its growing larvae. Unlike other similar parasites, the wasp induces the behavioral change prior to infecting the host; the wasp does that by injecting venom into the host's brain which paralyses the central nervous system and puts the cockroach into a state of hypokinesia, "a reversible long-term lethargy characterized by lack of spontaneous movement or response to external stimuli" (Banks and Adams). The cockroach, however, remains alive, and after dragging it to a burrow in the ground the wasp deposits an egg into its carcass and buries it for the growing larva to feed on. The adult wasp emerges after 6 weeks, leaving behind nothing but a hard outer cockroach shell.
- Klein, Sabra L. “Parasite manipulation of the proximate mechanisms that mediate social behavior in vertebrates.” Physiology & Behavior 79 (2003): 441-449. JSTOR. Web. 9 May 2012.
- Banks, Christopher N.; Michael E. Adams (2012). "Biogenic amines in the nervous system of the cockroach, Periplaneta americana following envenomation by the jewel wasp, Ampulex compressa". Toxicon 59 (2): 320–328. doi:10.1016/j.toxicon.2011.10.011. ISSN 0041-0101. Retrieved 2013-02-22..
- Lindova, Jitka, Lenka Priplatova, and Jaroslav Flegr. “Higher Extraversion and Lower Conscientiousness in Humans Infected with Toxoplasma.” European Journal of Personality 26 (July 2011): 285-291. Wiley Online Library. Web. 18 Apr. 2012.