Abstract
Eye temperature measured using infrared thermography (IRT) can be used as a non-invasive measure of autonomic nervous system (ANS) activity in cattle. The objective of this study was to evaluate if changes in eye temperature (measured using IRT) can be used to non-invasively measure ANS activity in sheep. Twenty, 2 to 4-year-old, Romney ewes were randomly assigned to receive either epinephrine (EPI) or physiological saline (SAL) for 5 min administered via jugular catheter (n = 10 ewes/treatment). Eye temperature (°C) was recorded continuously using IRT for approximately 25 min before and 20 min after the start of infusion. Heart rate and heart rate variability, measured using the root mean square of successive differences (RMSSD) and the standard deviation of all inter-beat intervals (SDNN), were recorded for 5 min before and up to 10 min after the start of infusion. Blood samples were taken before and after the infusion period to measure plasma epinephrine, norepinephrine, cortisol and packed cell volume (PCV) concentrations. During the infusion period, maximum eye temperature was on average higher (P<0.05) in sheep that received epinephrine than those that received saline. On average, heart rate was higher (SAL: 87.5 beats/min, EPI: 123.2 beats/min, SED = 7.07 beats/min; P<0.05), and RMSSD (SAL: 55.3 ms, EPI: 17.3 ms, SED = 14.18 ms) and SDNN (SAL: 54.3 ms, EPI: 21.5 ms, SED = 10.00 ms) lower (P<0.05) in ewes during the 5 min post-infusion period compared with ewes that received saline. An infusion of epinephrine resulted in higher geometric mean epinephrine (P<0.05) and cortisol (P<0.05) but not norepinephrine (P>0.05) concentrations in ewes compared to an infusion of saline. PCV concentrations were higher (P<0.001) by 7 ± 1.0% (mean±SED) in ewes after an epinephrine infusion. These results suggest that heart rate variability is a sensitive, non-invasive method that can be used to measure ANS activity in sheep, whereas change in eye temperature measured using IRT is a less sensitive method.
Highlights
Animals respond to stressors both behaviourally and physiologically, which enable them to adapt to these challenges; these changes can be measured and used to assess an animal’s welfare
These results suggest that heart rate variability is a sensitive, non-invasive method that can be used to measure autonomic nervous system (ANS) activity in sheep, whereas change in eye temperature measured using infrared thermography (IRT) is a less sensitive method
There was a tendency for a treatment x time effect on root mean square of successive differences (RMSSD) (F2,33.3 = 2.65, P = 0.085; Fig 3B) and a statistically significant treatment x time effect on standard deviation of all inter-beat intervals (SDNN) (F2,33.8 = 3.41, P = 0.045; Fig 3C)
Summary
Animals respond to stressors both behaviourally and physiologically, which enable them to adapt to these challenges; these changes can be measured and used to assess an animal’s welfare. Catecholamines (e.g., epinephrine and norepinephrine concentrations) can be challenging to measure due to their short half-life in plasma and the need to handle animals in order to collect blood samples, which in itself can stimulate the release of catecholamines. Heart rate reflects both the sympathetic and parasympathetic activity of the nervous systems, whereas HRV provides information on the balance between the activity of the sympathetic and parasympathetic branches of the ANS [2]. IRT can potentially be used as a non-invasive method to measure stress in animals [5]
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