Cool and safe walks: heat stress threshold for domestic dogs living in hot and sunny regions.

This study proposes heat stress thresholds for dogs during outdoor walks in a hot and sunny tropical region. A total of five mixed-breed dogs had their body temperatures (from their coat, skin, paw pad and rectum) and respiratory rate measured at rest, during 12 min of walking, and during 20 min of recuperation, when water intake was recorded. A total of 135 walks were undertaken throughout the night and daytime phases, thereby exposing dogs to levels of black-globe temperature (BGT) ranging between 19 and 50 °C. Dogs kept their rectal temperature within a narrow range (38-39 °C), without signs of hyperthermia. However, walking with the dogs when the BGT was above 35 ºC resulted in substantial increments in hair-coat temperature (+ 20.5 ºC), skin temperature (+ 7.2 ºC), respiratory rate (increasing markedly, from approximately 47 to 374 breaths min⁻1) and water intake (+ 215 mL) compared to walks at a BGT below 30 ºC. The set of dogs' responses exposed to different levels of radiant heat load was summarized into a single variable, the thermal stress index (TSI), with values ranging from 90 to 220. The best prediction of TSI accounted combined effects of air temperature and thermal radiation (R2 = 0.92). Maximum respiratory rate was recorded at TSI of 170 (Plateau = 287 breath min-1 or 4.78 breath s-1), and maximum skin temperature at TSI of 190 (Plateau = 38 ºC). In conclusion, walking dogs when TSI exceeds 170 should be approached with caution, particularly during prolonged or repeated exposure, as panting appears to become less effective beyond this threshold.