Abstract
Infrared thermography (IRT) is a popular technology used for the detection of thermal changes given its non-invasive nature and lack of direct contact with the individual. Accordingly, the maximal eye temperature (MaxET) measured with IRT has been extensively applied in equine research. However, there is little information available about the potential limitations of the MaxET in field studies. Thus, the aims of this study were to 1) quantify the individual variation of MaxET in field conditions and the effects of individual, breed, body size (height at withers), eye side, sex and age, 2) determine the effects of environment and operator, and 3) explore the relationship between MaxET and rectal temperature (RT) at rest. To accomplish these aims, 791 MaxET measures from 32 horses were collected in Sweden in five different months and five farms over a period of 12 months. There was an effect of individual on IRT (P < .05) and individual MaxET varied from 29.4 to 37.6 °C. IRT was also affected (P < .05) by breed and sex (maximal difference 1.1 °C and 0.3 °C, respectively) but not by eye side, age and height at withers. There were significant effects of month and farm (maximal differences; 2.4 and 2.3 °C, respectively), between outdoor and indoor measurements (0.8 °C) and also between operators (0.2 °C). There were no correlations between MaxET and RT. These results demonstrate that in horses observed at rest in their home environment, MaxET is affected by endogenous (sex and breed) and environmental factors (farm, location and month of the year) and shows no relationship to RT. We strongly suggest that IRT technology should be used with great caution in field studies and only under conditions where these factors can be accurately accounted for.
Highlights
Body temperature is a critical homoeostatic parameter for survival in mammals and a well-established key indicator of vital signs along with blood pressure, heart rate and respiratory rate [1]
In the first experiment the effect of different endogenous and environmental factors on the eye temperature were studied in 32 horses (8 young individuals were less than 4 years old and 24 adult individuals with ≥ 4 years old), in 5 farms, at five occasions during months, and in the second experiment the relationship between eye temperature and HR at rest was studied in horses in two farms at one occasion
The linear mixed model analysis showed that there were in dividual differences on the maximal eye tempera ture (MaxET) measures (P < .05), and the pro portion of total variation attributed to the individual was 37.4%
Summary
Body temperature is a critical homoeostatic parameter for survival in mammals and a well-established key indicator of vital signs along with blood pressure, heart rate and respiratory rate [1]. The hy pothalamus receives input from warm and cold receptors in the skin, internal organs and the hypothalamus itself. Based on this information it can alter behaviour and local tissue blood flow thereby influencing local tissue temperature [2]. Heat that is dissipated through the skin can be monitored without contact and non-invasively using infrared thermog raphy (IRT) [5] As a result, this technology has the potential to provide new insights into a wide range of areas, from comparative thermo-physiological responses between captive and wild animals to injury and disease diagnosis [6]
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