Infrared thermal imaging associated with pain in laboratory animals

Abstract

The science of animal welfare has evolved over the years, and recent scientific advances have enhanced our comprehension of the neurological, physiological, and ethological mechanisms of diverse animal species. Currently, the study of the affective states (emotions) of nonhuman animals is attracting great scientific interest focused primarily on negative experiences such as pain, fear, and suffering, which animals experience in different stages of their lives or during scientific research. Studies underway today seek to establish methods of evaluation that can accurately measure pain and then develop effective treatments for it, because the techniques available up to now are not sufficiently precise. One innovative technology that has recently been incorporated into veterinary medicine for the specific purpose of studying pain in animals is called infrared thermography (IRT), a technique that works by detecting and measuring levels of thermal radiation at different points on the body's surface with high sensitivity. Changes in IRT images are associated mainly with blood perfusion, which is modulated by the mechanisms of vasodilatation and vasoconstriction. IRT is an efficient, noninvasive method for evaluating and controlling pain, two critical aspects of animal welfare in biomedical research. The aim of the present review is to compile and analyze studies of infrared thermographic changes associated with pain in laboratory research involving animals.

Keywords: animal welfare; animals; pain; thermal images; vascular change.

Fig. 1.

A) Representative image of vasodilatation in the central artery (red) and marginal vein (lateral in blue) of a rabbit’s ear (dotted line in red), one of this species’ primary mechanisms for dissipating heat. B) Infrared thermographic image showing areas with temperatures above 38°C in the periocular and auricular regions. C) The left ear and, in white, the trajectory of the central auricular artery in longitudinal form from the base of the ear to the vertex. The auditory canal, marked in a green circle, indicates a temperature of 37.8°C.

Fig. 2.

External factors like significant changes in ambient temperature foster vasodilatation or vasoconstriction of blood vessels in the skin, mediated by afferent neurons of the noradrenergic type.

Fig. 3.

Nude mouse thermogram. A) Different temperature gradients between the skin surface and core (i.e., the Tcore visible in the white zones in the range of 34–36°C) of the mammal’s body. B) Anesthetics that cause indiscriminate vasodilatation result in a mixing of core and peripheral blood that reduces the Tcore.

Fig. 4.

The use of thermograms under anesthesia during a surgical process allowed adequate monitoring of the condition of laboratory animals and verification of the correct anesthesia. A) Auditory canal of the guinea pig as a thermal window for thermographic evaluation. B) The auditory canal of the rat offers an excellent thermal window for thermographic measurement. C) The tear caruncle and auditory canal of rabbits can be used as thermal windows for thermographic measurement.