Table 1.
Characteristics of depression and sickness behavior.
| Clinical depression | Sickness behavior | |
|---|---|---|
| Basic symptoms |
Depressed mood most of the day Decreased interest or pleasure in almost all activities Anorexia, and/or significant weight loss or weight gain Insomnia or hypersomnia Psychomotor agitation or psychomotor retardation Fatigue or loss of energy Decreased ability to think or concentrate |
-- Disinterest in social interactions Anorexia and weight loss; no weight gain Sleepiness Reduced locomotor activity; no agitation Lethargy Failure to concentrate |
| Existential symptoms | Feelings of worthlessness or guilt Suicidal ideation or behavior |
-- -- |
| Melancholic dimension | A distinct quality of depressed mood (anhedonia) Non-reactivity Diurnal variation Early morning awakening Psychomotor retardation Excessive weight loss |
Reduced intake of sweetened milk (anhedonia) Behavioral inhibition -- -- Reduction of locomotor activity and exploration Important weight loss |
| Anxiety dimension | Tension; physiological behavior; respiratory symptoms; genito-urinary symptoms; autonomic symptoms; anxious behavior at interview (general) | Anxiety |
| Physio-somatic dimension | Flu-like malaise; aches and pain; muscle tension (in some of the patients) | Malaise and hyperalgesia (key symptoms of sickness) |
| Pyrexia | Slightly increased body temperature | Pyrexia |
| Onset Course |
Insiduous Waxing and waning or relapsing-remitting Chronic Sensitization of episodes Seasonal variation (Hypo)manic episodes |
Acute onset Acute adaptive response, maximal 19 to 43 days May be prolonged, but then is maladaptive -- -- -- |
| Pathways | (Sub)chronic inflammation with increased PICs CMI activation Sensitization of inflammatory and CMI pathways Activation TRYCAT pathway O&NS Damage by O&NS Autoimmunity Neuroprogression |
Acute inflammation with increased PICs Activated -- Maybe activated TRYCAT pathway Unknown but probably yes -- -- -- |
| Triggers | Multiple, not well defined Psychosocial stressors, medical inflammatory illness, neuroinflammatory disorders, inflammatory conditions |
Acute, highly defined Acute pathogens and tissue injury |
| Episodes tend to become autonomous from trigger | Is always a response to a defined trigger | |
| General | Inflammation-related chronic progressive disorder | Inflammation-induced adaptive behavioral response that is conserved through evolution |
| PICs' Janus-face | Bad 'chronic' side: a chronic disorder with positive feedback loops between (neuro)inflammation and (neuro)degenerative processes | Good 'acute' side: supports inflammation, redirects energy to immune cells, conserves energy and prevents negative energy balance, helps eradicating the trigger, and has anti-inflammatory effects |
CMI, cell-mediated immune; O&NS, oxidative and nitrosative stress; PICs, pro-inflammatory cytokines; TRYCAT, tryptophan catabolites.