Abstract
The reliable assessment and management of avian pain is important in the context of animal welfare. Overtly expressed signs of pain vary substantially between and within species, strains and individuals, limiting the use of behaviour in pain studies. Similarly, physiological indices of pain can also vary and may be confounded by influence from non-painful stimuli. In mammals, changes in the frequency spectrum of the electroencephalogram (EEG) recorded under light anaesthesia (the minimal anaesthesia model; MAM) have been shown to reliably indicate cerebral responses to noxious stimuli in a range of species. The aim of the current study was to determine whether the MAM can be applied to the study of nociception in birds. Ten chickens were lightly anaesthetised with halothane and their EEG recorded using surface electrodes during the application of supramaximal mechanical, thermal and electrical noxious stimuli. Spectral analysis revealed no EEG responses to any of these stimuli. Given that birds possess the neural apparatus to detect and process pain, and that the applied noxious stimuli elicit behavioural signs of pain in conscious chickens, this lack of response probably relates to methodological limitations. Anatomical differences between the avian and mammalian brains, along with a paucity of knowledge regarding specific sites of pain processing in the avian brain, could mean that EEG recorded from the head surface is insensitive to changes in neural activity in the pain processing regions of the avian brain. Future investigations should examine alternative electrode placement sites, based on avian homologues of the mammalian brain regions involved in pain processing.
Highlights
In the context of animal welfare, pain assessment is important both in terms of identifying when an animal is in pain, as well as in the development of effective analgesic strategies
Analysis of variance showed there was no effect of stimulus or time, or their interaction, on the F95 or PTOT of the chicken EEG (Table 2)
The application of supramaximal cutaneous thermal or mechanical noxious stimuli to the chicken failed to elicit changes in any spectral EEG parameter, whilst the application of a noxious electrical stimulus induced a significant increase in F50, but no change in PTOT or F95
Summary
In the context of animal welfare, pain assessment is important both in terms of identifying when an animal is in pain, as well as in the development of effective analgesic strategies. It is widely accepted that birds, like mammals, are capable of experiencing pain. This belief is supported by multiple lines of evidence. Cutaneous mechanical, thermal, chemical and polymodal nociceptors have been identified in birds, and these respond to noxious. Effects of noxious stimuli on the chicken EEG stimulation in a similar manner to the equivalent mammalian nociceptors [1, 2]. Behavioural and physiological responses to nociceptive stimuli consistent with those observed in mammals have been described in birds subjected to mechanical, thermal and chemical noxious stimulation Like mammals, birds have been shown to possess endogenous pain modulation mechanisms [9, 10]
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