Sickness behavior

Sickness behavior is a coordinated set of adaptive behavioral changes that develop in ill individuals during the course of an infection. They usually (but not necessarily) accompany fever and aid survival. Such illness responses include lethargy, depression, anxiety, malaise, loss of appetite,sleepiness,hyperalgesia, reduction in grooming and failure to concentrate. Sickness behavior is a motivational state that reorganizes the organism's priorities to cope with infectious pathogens. It has been suggested as relevant to understanding depression, and some aspects of the suffering that occurs in cancer.

Sick animals have long been recognized by farmers as having different behavior. Initially it was thought that this was due to physical weakness that resulted from diverting energy to the body processes needed to fight infection. However, in the 1960s, it was shown that animals produced a blood-carried ‘‘factor X’’ that acted upon the brain to cause sickness behavior. In 1987, Benjamin L. Hart brought together a variety of research findings that argued for them being survival adaptations that if prevented would disadvantage an animal’s ability to fight infection. In the 1980s, the blood-borne factor was shown to be proinflammatory cytokines produced by activated leukocytes in the immune system in response to lipopolysaccharides (a cell wall component of Gram-negative bacteria). These cytokines acted by various humoral and nerve routes upon the hypothalamus and other areas of the brain. Further research showed that the brain can also learn to control the various components of sickness behavior independently of immune activation..

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