Thermoregulation mechanisms and perspectives for validating thermal windows in pigs with hypothermia and hyperthermia: An overview

Abstract

Specific anatomical characteristics make the porcine species especially sensitive to extreme temperature changes, predisposing them to pathologies and even death due to thermal stress. Interest in improving animal welfare and porcine productivity has led to the development of various lines of research that seek to understand the effect of certain environmental conditions on productivity and the impact of implementing strategies designed to mitigate adverse effects. The non-invasive infrared thermography technique is one of the tools most widely used to carry out these studies, based on detecting changes in microcirculation. However, evaluations using this tool require reliable thermal windows; this can be challenging because several factors can affect the sensitivity and specificity of the regions selected. This review discusses the thermal windows used with domestic pigs and the association of thermal changes in these regions with the thermoregulatory capacity of piglets and hogs.

Keywords: heat stress; hog; hypothermia; infrared thermography; piglet.

Figure 1

Flow chart showing the criteria used during the search of scientific articles for the overview.

Figure 2

Anatomical-physiological challenges of piglets related to hypothermia and of hogs related to hyperthermia.

Figure 3

Thermoregulatory mechanism in domesticated swine. When peripheral thermoreceptors are activated to recognize cold or hot stimuli, the information is transmitted to the hypothalamus (MnPO) and other brain structures involved in the sympathetic control of peripheral blood vessels. When exposed to cold environments, these vessels vasoconstrict to conserve heat, but under exposure to hot temperatures, they vasodilate to dissipate heat. Both responses influence the amount of infrared radiation observable using techniques like IRT, though this also depends on the thermal window evaluated. IML, intermediolateral column; MnPO, median preoptic nucleus; POA, preoptic area; rMR, rostral medullary raphe; TRP, transient receptor potential vanilloid (V), melastatin (M), ankyrin (A).

Figure 4

Sequence of the events that culminate in the death of hogs due to the effects of high ambient temperatures.

Figure 5

Ocular window in pigs. (A) The periocular window is outlined by an ellipse (El1) that surrounds the entire ocular surface, including a few millimeters of the upper and lower eyelids. (B) Irrigation to the ocular region, mainly by the supraorbital and infraorbital arteries and veins, which are branches of the maxillary artery and vein. IRT can detect changes in the muscle tone of these vascular structures and the heat radiated through them.

Figure 6

Auricular window in pigs. (A) The window is delimited by an ellipse (E1) that includes the central cartilage. (B) Circulation from the facial artery and external jugular vein, with their respective branches, spreads into the ramus auricularis medialis, intermedius, and lateralis, which are responsible for transporting blood to the ear pavilion and that respond to activation of the ANS.

Figure 7

Thermal nasal window. (A) To mark the perinasal window, a large circle is drawn around the entire snout with the upper lip as its ventral limit. (B) The nasal region and vasomotor changes there depend on circulation through the lateral and caudal nasal arteries and the dorsalis nasi vein, a branch of the facialis vein. These structures respond with vasodilation or vasoconstriction, according to the stimulus perceived.