Behavioural indicators of infectious disease in managed animals

Identifying reliable and valid behavioural indicators of infection in managed animals is increasingly important as the risks of emergent diseases increase alongside concomitant concerns over pathogen resistance and the environmental and safety impacts of traditional therapeutic treatments. Early behavioural detection of disease could help to curb transmission, assist in selecting resilient animals and guide facility design to help animals avoid infection and to support their recovery. This review explores the adaptability and flexibility of animal responses to pathogens, including behaviours that (i) favour disease avoidance (ii) are associated with immune activation (iii) directly resist pathogens (iv) are dysregulated leading to hypersensitivity and (v) are associated with pathogen tolerance. A key theme is that all of these behavioural responses are strongly modulated by contextual factors such as pain, hunger and social priorities. Efforts to develop and validate, and increasingly automate, behavioural indicators of infection have so far primarily focused on infection-induced changes in core behaviours such as feeding and general activity or on hypothesis-free machine-learning comparison. However, such approaches have limited specificity, sensitivity and may be hard to generalise across contexts. The current review suggests ways in which specificity could be improved by monitoring changes in behaviours that are more closely linked to immune activation e.g. sleep, attention and motor function, by a more granular focus, and by integration with clinical symptoms. It also proposes that sensitivity could be improved by monitoring pliant (“luxury”) behaviours and by intentional challenges or tests. Improved knowledge of how animals behave when infected could be used to design environments where the costs of resistance or tolerance are reduced and where recovery is promoted. • Behaviour can be used as a tool to assess infectious disease, an important component of animal welfare. • Animals can respond to pathogenic threat by avoidance, immune resistance, direct behavioural resistance or tolerance. • Each of these strategies is associated with behaviours that could be detected using manual or automated methods. • Behavioural responses to infection are modulated by competing motivational priorities. • Behavioural indicators of infectious disease will be more accurate and generalisable if they are hypothesis-based.