79 Veterinaria Italiana 2021, 57 (1), 79-82. doi: 10.12834/VetIt.1077.5873.2 Accepted: 03.02.2020 | Available on line: 27.07.2021 1Dipartimento di Medicina Veterinaria, Università degli Studi di Milano, Milano, Italy. 2Dipartimento di Scienze Mediche Veterinarie, Università di Bologna, Alma Mater Studiorum, Bologna, Italy. * Corresponding author at: Dipartimento di Medicina Veterinaria, Università degli Studi di Milano, Milano, Italy. E-mail: sara.barbieri@unimi.it. Sara Barbieri1*, Zita Talamonti1, Eleonora Nannoni2, Eugenio Ugo Luigi Heinzl1, Michela Minero1 and Elisabetta Canali1 Keywords Animal welfare, Body temperature, Health status, Infrared thermography, Swine. Summary This study aims to assess the correlation between surface temperature estimated by infrared thermography and core temperature measured with rectal thermometer in weaning and fattening pigs. A total of 108 pigs were used in this study. Thermal images of the eye of each animal were recorded with a thermal imaging camera, rectal temperatures were measured using a calibrated digital thermometer. The average rectal temperature was 38.9  ±  0.4  °C (MIN = 37.9 °C; MAX = 40.1 °C) and the average eye temperature was 36.7 ± 0.1 °C (MIN = 34.8 °C; MAX = 38.8 °C). Our results showed that the mean eye temperature estimated by infrared thermography was significantly correlated (r = .581, P < .01) with rectal temperature. The correlation was significant and strong for weaners (r = .739, P < .01), significant although weak for fatteners (r = .236 P < .05). Thermography could be a valid method to estimate the core temperature of pigs under farm condition. Use of thermography in pigs: relationship between surface and core temperature SHORT COMMUNICATION temperature; based on thermal images it is possible to perform accurate temperature measurements (Speakman and Ward 1998). As IRT is a non-contact procedure, data can be collected on animals that are difficult to reach or to approach; furthermore, the short measuring time allows the recording of data from moving animals (Kastberger and Stachl 2003). IRT has been used in several species. The medial posterior palpebral border of the lower eyelid and the curuncula lacrimalis has been demonstrated to be the location of the eye region showing the maximum temperature (pony: Johnson et  al. 2011; cattle: Stewart et  al. 2008; sheep: Stubsjøen et al. 2009). Only a few studies have investigated the use of IRT in pigs and even fewer have investigated the use of IRT as a tool to identify increases in temperature (Bates et al. 2014, Schmidt et al. 2013, Traulsen et al. 2010). The aim of the study was to assess the relationship between surface temperature estimated by IRT and core temperature measured with a rectal thermometer in weaning and fattening pigs. The experimental protocol included only procedures of a common clinical examination and animals were kept in compliance with the EU Council Directive Consumer and European Union (EU) policy demand for consistent enforcement of welfare legislation in food producing animals has been increasing over the last decades. In response to this demand, the assessment of animal welfare at farm level needs to develop a science-based multidimensional approach (Mason and Mendl 1993). The welfare assessment aims at determining the actual status of animals, including both physical and mental state, using animal based indicators able to address areas of concern in this field (EFSA 2012a). Several studies report that body temperature in pigs is a valid indicator for welfare assessment (Tosi et al. 2003, EFSA 2012b) and fever is the earliest and one of the main clinical signs of many diseases. However, body temperature is difficult to measure under farm conditions, as the accepted methods for measuring core temperature need handling and restraining of animals (Stewart et al. 2005). Infrared thermography (IRT) is a non-invasive technique to estimate the body temperature by detecting infrared radiation emitted by each body (Mitchell 2013, Speakman and Ward 1998, Stewart et  al. 2005). IRT uses thermal radiation emitted by objects to visualize and measure their surface 80 Use of infrared thermography in pigs Barbieri et al. Veterinaria Italiana 2021, 57 (1), 79-82. doi: 10.12834/VetIt.1077.5873.2 of the day. Frequency distributions and Pearson correlation between core and surface temperatures of the pigs were calculated. Cases of animals with rectal temperature higher than a reference limit (39  °C) were selected. Mean high temperatures of selected animals were compared to those of the other animals using a T test. Data was normally distributed (Kolmogorov-Smirnov test) (IBM 2014): the average rectal temperature was 38.9  ±  0.4  °C (MIN  =  37.9 °C; MAX  =  40.1  °C) and the average eye temperature was 36.7 ± 0.1 °C (MIN  =  34.8 °C; MAX  =  38.8 °C). Our results showed that the mean eye temperature estimated by IRT was significantly correlated (r  =  .581, P < .01) with rectal temperature. The correlation was significant and strong for weaners (r = .739, P < .01), significant although weak for fatteners (r  =  .236 P  <  .05), showing that IRT can be reliably used on pigs of different ages (Figure 2). We considered the eye region in agreement with IRT studies on different species, which have identified this location as the one that corresponds most to rectal temperature and that is less affected by other factors (Johnson et al. 2011, Stewart et al. 2008). The absence of hair around the eye allows heat dispersion that amounts to a greater emission of infrared radiation (Mitchell 2013). Chung and colleagues (Chung et al. 2010), comparing rectal and infrared thermometry in piglets, reported a significant linear relationship for surface temperature measured on three different locations of the body (central abdomen, cranial dorsum and perianal regions), while no significant relationship was found for lower eyelid. However, under farm conditions, the measurement at body regions such as flank and back may be negatively influenced by external factors, e.g., dirtiness, contact with other pigs and with the ground. On 2008/120/EC that stipulates minimum standards for the protection of pigs. A total of 108 pigs (28 weaners of 47 day old and 80 fatteners of 232 day old) were used in this study. The experiment was carried out in the facilities of the Department of Veterinary Medical Sciences of the University of Bologna (Italy): pigs were kept in groups of 5 animals on a slatted floor and under controlled temperatures ranging from 20 to 27 °C, according to the age of animals. A clinical examination was performed before the measuring in order to exclude animals with clinical signs of diseases. Pigs received a commercial diet, according to the Consortium for Parma Ham production rules (Consortium for Parma Ham 2015), and water was available ad libitum. Thermal images of the eye of each animal were recorded with a thermal imaging camera (Nec Avio TVS500). To optimize the accuracy of the thermographic image and to reduce sources of noise, before every work session the same image of a Lambert surface was taken to define the radiance emission and to nullify the effect of surface reflections on tested animals (Mallick et al. 2005). Only perfectly focused images were used. To determine the temperature of the eye, Grayess IRT Analyzer 4.8 (Informer Technologies, Inc., USA) was used and the maximum temperature (°C) within a circular area traced around the curuncula lacrimalis was measured (Figure 1). This maximum value was used for subsequent analysis. Rectal temperatures were measured using a calibrated digital thermometer, checked before the examination and compared to a certified mercury thermometer. In accordance with the manufacturer’s instructions, the thermometer was inserted into the anus and positioned in contact with rectal mucosa for 10 seconds, until hearing the acoustic signal. During the measurements, animals were not manually restrained. For each animal the capture of thermal image was immediately followed by the measurement of rectal temperature; temperatures were recorded at the same time Figure 1. Thermal image of a pig’s head showing the position of the measurement point on the eye. 38.0 36.4 34.8 33.1 31.5 29.9 28.3 26.6 25.0 °C Rectal temperature (°C) > 39 °C P < .01 34.00 35.00 36.00 37.00 38.00 39.00 < 39 °C Ey e te m p er at u re (° C ) Figure 2. Mean surface temperature estimated by infrared thermography in pigs of different ages with core temperature higher and lower than the reference limit (39 °C). 81 Barbieri et al. Use of infrared thermography in pigs Veterinaria Italiana 2021, 57 (1), 79-82. doi: 10.12834/VetIt.1077.5873.2 that can be used for early disease detection. IRT applied at eye level is a valid method to estimate the core temperature of pigs under farm condition; however, the results should be interpreted with caution because of the limited sample size and further research is needed. Moreover, external environmental and physical conditions can negatively influence IRT measurements collected in the field and these factors need to be controlled in the design of experiments in order to have a clear interpretation of temperature outcomes (Church et al. 2014). IRT might be a useful non-contact method to measure the core temperature of pigs under farm conditions, being valuable for a non-invasive assessment of physiological state and for monitoring pig welfare. Thermal imaging cameras are still relatively expensive, but appear to be reliable under field conditions and IRT provides instantaneous results since software for data analysis in real time is incorporated. Therefore, such a non-contact method would save time and reduce stress on the animals. the contrary, Schmidt and colleagues (Schmidt et al. 2013) measured body surface temperature in sows at different body regions and concluded that, under farm conditions, the back of the ear and the eye are the most promising locations to measure body temperature in pigs. Our results suggested that IRT surface temperature measured at eye level is higher in animals with rectal temperature higher than the reference limit of 39 °C. 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