Eye and Ear Temperature Using Infrared Thermography Are Related to Rectal Temperature in Dogs at Rest or With Exercise

Rectal body temperature (BT) has been documented in exercising dogs to monitor thermoregulation, heat stress risk, and performance during physical activity. Eye (BT(eye)) and ear (BT(ear)) temperature measured with infrared thermography (IRT) were compared to rectal (BT(rec)) temperature as the reference method and assess alternative sites to track hyperthermia, possibly to establish BT(eye) IRT as a passive and non-contact method. BT measures were recorded at 09:00, 11:30, 12:30, and 02:30 from Labrador Retrievers (N = 16) and Beagles (N = 16) while sedentary and with 30-min play-exercise (pre- and 0, 15, 30-min post-exercise). Total exercise locomotor activity counts were recorded to compare relative intensity of play-exercise between breeds. BT(rec), BT(eye), and BT(ear) were measured within 5 min of the target time. Each BT method was analyzed by analysis of variance for main effects of breed and time. Method differences were compared using Bland–Altman plots and linear regression. Sedentary BT differed by breed for BT(rec) (p < 0.0001), BT(ear) (p < 0.0001), and BT(eye) (p = 0.06) with Labs having on average 0.3–0.8°C higher BT compared to Beagles. Readings also declined over time for BT(eye) (p < 0.0001) and BT(ear) (p < 0.0001), but not for BT(rec) (p = 0.63) for both breeds. Total exercise (30-min) activity counts did not differ (p = 0.53) between breeds. Time and breed interaction was significant in response to exercise for both BT(rec) and BT(ear) (p = 0.035 and p = 0.005, respectively), with a marginal interaction (p = 0.09) for BT(eye). All the three methods detected hyperthermia with Labs having a higher increase compared to Beagles. Both BT(ear) and BT(eye) were significantly (p < 0.0001) related to BT(rec) in all dogs with sedentary or exercise activity. The relationship between BT(eye) and BT(rec) improved when monitoring exercise hyperthermia (r = 0.674) versus measures at rest (r = 0.381), whereas BT(ear) was significantly related to BT(rec) regardless of activity (r = 0.615–0.735). Although BT readings were significantly related, method bias (p < 0.02) was observed for BT(eye) to slightly underestimate BT(rec), whereas no bias was observed between BT(ear) and BT(rec). This study demonstrates that IRT technology effectively measures both ear and eye temperature and enables effective monitoring of BT changes at rest, with exercise, and between breeds. However, ear, and not eye, temperature is a better reflection of rectal temperature.