Sickness behavior
Encyclopedia
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
behavioral changes that develop in ill
Illness
Illness is a state of poor health. Illness is sometimes considered another word for disease. Others maintain that fine distinctions exist...
individuals during the course of an infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
.
They usually (but not necessarily) accompany fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...
and aid survival.
Such illness responses include lethargy, depression
Depression (mood)
Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behaviour, feelings and physical well-being. Depressed people may feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, or restless...
, anxiety
Anxiety
Anxiety is a psychological and physiological state characterized by somatic, emotional, cognitive, and behavioral components. The root meaning of the word anxiety is 'to vex or trouble'; in either presence or absence of psychological stress, anxiety can create feelings of fear, worry, uneasiness,...
, loss of appetite
Anorexia (symptom)
Anorexia is the decreased sensation of appetite...
,
sleepiness,
hyperalgesia
Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...
,
reduction in grooming and failure to concentrate
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....
.
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
Depression (mood)
Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behaviour, feelings and physical well-being. Depressed people may feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, or restless...
, and some aspects of the suffering that occurs in cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
.
History
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 adaptationsEvolutionary medicine
Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. It provides a complementary scientific approach to the present mechanistic explanations that dominate medical science, and particularly modern medical education...
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
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
in response to lipopolysaccharides (a cell wall component of Gram-negative bacteria). These cytokines acted by various humoral and nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
routes upon the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
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.
General advantage
Sickness behavior in its different aspects causes an animal to limit its movement; the metabolic energy not expended in activity is diverted to the fever responses, which involves raising body temperature. This also limits an animal’s exposure to predators while it is cognitively and physically impaired.Specific advantages
The individual components of sickness behavior have specific individual advantages. Anorexia limits food ingestion and therefore reduces the availability of ironIron deficiency (medicine)
Iron deficiency is one of the most common of the nutritional deficiencies. Iron is present in all cells in the human body, and has several vital functions...
in the gut (and from gut absorption). Iron may aid bacterial reproduction, so its reduction is useful during sickness. Plasma concentrations of iron are lowered for this anti-bacterial reason in fever. Lowered threshold for pain ensures that an animal is attentive that it does not place pressure on injured and inflamed tissues that might disrupt their healing. Reduced grooming is adaptive since it reduces water loss.
Immune control
Lipopolysaccharides trigger the immune systemImmune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
to produce proinflammatory cytokine
Proinflammatory cytokine
A proinflammatory cytokine is a cytokine which promotes systemic inflammation.Examples include IL-1 and TNF alpha....
s IL-1, IL-6
Interleukin 6
Interleukin-6 is a protein that in humans is encoded by the IL6 gene.IL-6 is an interleukin that acts as both a pro-inflammatory and anti-inflammatory cytokine. It is secreted by T cells and macrophages to stimulate immune response, e.g. during infection and after trauma, especially burns or other...
, and tumor necrosis factor (TNF). These peripherally released cytokines act on the brain via a fast transmission pathway involving primary input through the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
s, and a slow transmission pathway involving cytokines originating from the choroid plexus
Choroid plexus
The choroid plexus is a structure in the ventricles of the brain where cerebrospinal fluid is produced...
and circumventricular organs
Circumventricular organs
Circumventricular organs are so named because they are positioned at distinct sites around the margin of the ventricular system of the brain.They are among the few sites in the brain which have an incomplete blood–brain barrier...
and diffusing into the brain parenchyma by volume transmission. Peripheral cytokines may enter the brain directly. They may also induce the expression of other cytokines in the brain that cause sickness behavior. Acute psychosocial stress enhances the ability of an immune response to trigger both inflammation and behavioral sickness.
Behavioral conditioning
The components of sickness behavior can be learned by conditional associationClassical conditioning
Classical conditioning is a form of conditioning that was first demonstrated by Ivan Pavlov...
. For example, if a saccharin
Saccharin
Saccharin is an artificial sweetener. The basic substance, benzoic sulfilimine, has effectively no food energy and is much sweeter than sucrose, but has a bitter or metallic aftertaste, especially at high concentrations...
solution is given with a chemical that triggers a particular aspect of sickness behavior, on later occasions the saccharin solution will trigger it by itself.
Depression
It has been proposed that major depressive disorder is near-identical with sickness behavior, so raising the possibility that it is a maladaptive manifestation of sickness behavior due to abnormalities in circulating cytokines . Moreover, chronic, but not acute treatment with antidepressant drugs was found to attenuate sickness behavior symptoms in rodents . The mood effects caused by interleukin-6Interleukin 6
Interleukin-6 is a protein that in humans is encoded by the IL6 gene.IL-6 is an interleukin that acts as both a pro-inflammatory and anti-inflammatory cytokine. It is secreted by T cells and macrophages to stimulate immune response, e.g. during infection and after trauma, especially burns or other...
following an immune response have been linked to increased activity within the subgenual anterior cingulate cortex
Brodmann area 25
Brodmann area 25 is an area in the cerebral cortex of the brain and delineated based on its cytoarchitectonic characteristics.It is also called the subgenual area, area subgenualis or subgenual cingulate....
, an area involved in the etiology of depression. Inflammation-associated mood change can also produce a reduction in the functional connectivity of this part of the brain to the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...
, medial prefrontal cortex, nucleus accumbens
Nucleus accumbens
The nucleus accumbens , also known as the accumbens nucleus or as the nucleus accumbens septi , is a collection of neurons and forms the main part of the ventral striatum...
, and superior temporal sulcus
Superior temporal sulcus
The superior temporal sulcus is the sulcus separating the superior temporal gyrus from the middle temporal gyrus in the temporal lobe of the brain. The superior temporal sulcus is the first sulcus inferior to the lateral fissure....
.
Cancer side effect
In cancer, both the disease and the chemotherapyChemotherapy
Chemotherapy is the treatment of cancer with an antineoplastic drug or with a combination of such drugs into a standardized treatment regimen....
treatment can cause proinflammatory cytokine release which can cause sickness behavior as a side effect
Adverse effect (medicine)
In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as surgery.An adverse effect may be termed a "side effect", when judged to be secondary to a main or therapeutic effect. If it results from an unsuitable or incorrect dosage or...
.