Sickness behaviours in ducks include anorexia but not lethargy

The characteristics of mammalian sickness behaviour are often conferred on the avian phyla despite there being little scientific evidence that birds change their behaviour when the innate immune system is activated during disease or infection. We used bio-logging techniques to measure activity and feed intake in birds exposed to different pathogen associated molecular patterns (PAMPs). Even though the PAMPs of Gram-negative and Gram-positive bacteria, as well as double-stranded RNA viruses, evoked innate immune responses, none of them induced lethargy in free-roaming Pekin ducks (n = 12). But each of the PAMPs induced anorexia in the ducks. The degree of anorexia was strongly correlated with the fever response that each PAMP evoked. It is well known that when birds have an increase in their core body temperature, they become anorectic. Therefore, it is possible that the fever response, which is mediated by the pro-inflammatory cytokines that are released from immune cells post immune activation, may be responsible for the anorexia that our birds developed, rather than the pro-inflammatory cytokines directly modulating the motivation for feeding. From our results it is clear that sickness-induced lethargy cannot be considered a clinical sign or a diagnostic criterion for the onset of pathogenic infection in certain lines of poultry.