Parasitism - Effects On Host Behavior

Effects On Host Behavior

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 jewel wasp alters behavior through the injection of venom directly into the host’s brain, causing hypokinesia. Parasitic life cycles can have the capacity to infect a singular host or a series of hosts. Direct life cycles are related to a single host while indirect life cycles, or complex life cycles, rely on a series of intermediate hosts in order to complete the life cycle. Indirect life cycles likely arose in order to increase the efficiency of spreading to new hosts. By using intermediary’s the parasite DNA can spread to several hosts instead of remain concentrated within a single host, therefore promising a higher likelihood of DNA continuation. In some cases, intermediary hosts are accidental.

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. Toxoplasma, however, has begun to infect humans. In doing so, human behavior has been altered in similar ways to rodents. Further, Toxoplasma has been linked to cases of schizophrenia.

The emerald jewel wasp (Ampulex compressa) parasitises its host, the American cockroach (Periplaneta americana) as a food source and home for its growing larvae. The wasp begins by injecting venom into the brain which paralyses the central nervous system system of the cockroach and puts it into a state of “hypokinesia”. “Hypokinesia...is a reversible long-term lethargy characterized by lack of spontaneous movement or response to external stimuli,” (Banks and Adams). After dragging the cockroach to a burrow, the wasp deposits an egg into its carcass, burying it for the growing larvae to feed off until it emerges in 6 weeks, leaving nothing but a hard outer cockroach shell. Although the circuitry in control of movement is functional, the nervous system acts from a depressed state. They are not killed by this hypokinesia, but would recover if not for the larvae eating them from the inside out. The movement is controlled by dopamine and octopamine which affect transmission of interneurons involved in the natural response to escape. Reduced motor activity results from a reduction of these amines.