Behavioral Effects of Parasitism in Animals

Parasites are found in and on all living organisms. The ways in which a parasite manipulates the behavior of the host to its own advantage are wide ranging and are summarized in this article. Some of these methods aim to increase the rate of transmission from intermediate to definitive host by increasing or reducing activity that results in increased vulnerability of the intermediate host to predation, suppressing a host's fear response, or its ability to recognize a predator. Others aim to downregulate the reproductive fitness of a host (parasitic castration), allowing the parasite to maximize nutrient-taking from the host without reducing the survival ability of the host. In an evolutionary “arms race,” hosts have counteracted by evolving resistance to parasite infections and/or developing a tolerance against their harmful effects. These defense mechanisms include reducing the chances of getting infected via a change in habitat or food preference; generating behavioral fever; and recognizing and avoiding infected individuals, both as potential prey species and sexual partners. If an infection could not be prevented, the parasites can be attacked or removed by the host's behavior, such as scratching, grooming, and immersion in water. Behavioral changes (e.g., lethargy, inappetance, scratching, general malaise) are generally some of the most common reasons why an owner brings an animal to a veterinary practitioner, and these clinical signs can be indicative of a parasite infection. This article reviews some of the signs in exotic animals for which a veterinarian should be observant, especially if the patient is suspected of having a parasitic infection.