A Pharmacological Intervention Model to Assess Positive Affective States in Laying Hens

It is widely accepted that the absence of suffering no longer defines animal welfare and that positive affective experiences are important (Mellor 2015). However, there are few valid and reliable tools available to comprehensively assess positive affective states of hens, particularly practically on farm. Pharmacological interventions that disrupt specific neural pathways have been shown to be a useful tool when validating indicators of affective states and can assist to manipulate affective states in order to investigate novel indicators such as biomarkers associated with affective states (Nasr et al. 2013). With this approach, we aimed to validate a pharmacological intervention to block the subjective hedonistic phase associated with reward in laying hens via the administration of the opioid antagonist, nalmafene. We predicted that hens that did not experience a positive affective state would show minimal anticipatory behaviour when the same reward was later presented.

Eighty Isa brown hens at 80-85 weeks of age were randomly allocated to either a control (C)  or nalmafene (N) treatment group. On day 1 through to 4, hens were dosed intramuscularly into the pectoral muscle with 0.9% saline (C) or nalmafene dissolved in saline (N) at a dose rate of 0.4mg/kg derived from Savory et al (1989). Exactly 30 minutes after dosing, hens were presented with live mealworms in a transparent closed food container at front of their cage. Hens could see the mealworms and could reach the container but could not access the mealworms. After two minutes, the lid was opened and hens were provided with access to the meal worms for five minutes. Latency to peck the container and number of pecks when the lid was closed was calculated as an indicator of anticipation. Kaplan-Meier survival (latency and censored data), generalized linear models (pecks) or Mann-Whitney comparisons (behavior) with treatment, day and the interaction where appropriate.

More hens from the control group (40-100%) pecked the closed mealworm container and were quicker to do so than hens from the nalmefene treatment group (10 - 40%) on all days (day 1: C - 95.1 ± 8.8 s, N - 108.1 ± 8.3 s, ²(39,1) = 4.3, P = 0.038; day 3: C - 42.3 ± 11.4 s, N - 110.1 ± 7.1 s, ²(39,1) = 17.15, P < 0.001; day 4: C - 18.4 ± 7.9 s, N - 104.1 ± 8.7 s, ²(39,1) = 29.8, P < 0.001). The average number of pecks on the closed mealworm container increased over time by hens in the control group (interaction between day and treatment: ²(98,1) = 7.7, P = 0.005). Treatment hens pecked fewer times than the control hens at all-time points (²(98,1) = 408.7, P < 0.001). Behavioural analysis showed no indication that nalmefene treated hens were sedated, nauseous or fearful (immobility, resting, preening and alert all P > 0.05).

We provide evidence that nalmafene may be an effective pharmacological intervention to block the positive affective reward state in laying hens and may be utilised to develop science-based measures to identify novel biomarkers of positive affective state.

ACKNOWLEDGEMENTS: This work was funded by Australian Eggs.