Role of infrared thermography in animal reproduction

Introduction

The modernization of animal husbandry has driven a demand for technologies that enable real-time, accurate and automated monitoring of animal physiology (Zheng et al., 2022) [28] . Infrared thermography (IRT) is a non-ionizing, passive technology that detects infrared radiation emitted by an object and converts it into a radiometric image, or thermogram (Borah et al., 2022) [1] . Because all objects with a temperature above absolute zero emit infrared radiation, allows IRT for the quantification of surface temperatures based on the Stefan-Boltzmann law (Usamentiaga et al., 2014) [25] . In the context of reproduction, IRT is valued for its ability to detect microcirculatory changes driven by hormonal fluctuations, metabolic activity and inflammatory processes (Mota-Rojas et al., 2021) [15] . Unlike invasive techniques which can induce stress-related hyperthermia and skew diagnostic results, IRT permits remote monitoring and thereby improving animal welfare (Stewart et al., 2005) [23] . IRT is helpful in detecting estrus, diagnosing pregnancy, assessing male fertility and identifying reproductive pathologies across various livestock species including bovines, buffaloes, small ruminants, equines and swine.

Biophysical Principles and Thermal Windows

The clinical application of IRT relies on the evaluation of "thermal windows". These are specific anatomical regions characterized by a high density of blood vessels, presence of arteriovenous anastomoses and often lack of thick hairs or fur (Mota-Rojas et al., 2021) [15] . These features facilitate heat exchange between body core and environment.

Vascular Dynamics and Heat Transfer:

The surface temperature of an animal is largely determined by the core body temperature and the blood flow to the periphery (Borah et al., 2022) [1] . Vasodilation controlled by the autonomic nervous system increases blood flow to the skin facilitating heat dissipation and appearing as "hotspots" on a thermogram (Ghezzi et al., 2024) [6] . Conversely, vasoconstriction reduces blood flow resulting in cooler surface temperatures (Mota-Rojas et al., 2021) [15] . In reproductive physiology, these vascular changes are often modulated by hormones such as estrogen and progesterone (Skliarov et al., 2023) [22] .

Key Reproductive Thermal Windows

• Vulva or Perineum: A critical window in females where surface temperature correlates with blood flow changes driven by the estrous cycle (Talukder et al., 2014) [24] . • Scrotum: In males, the scrotal surface is the primary window for assessing testicular thermoregulation which is essential for spermatogenesis (Kastelic et al., 1996) [8] . • Mammary Gland: The udder surface reflects vascular changes related to lactation and inflammatory responses such as mastitis (Colak et al., 2008) [3] . • Ocular/Muzzle: While not reproductive organs, these zones are often used as proxies for core temperature and stress assessment during reproductive procedures (Mota-Rojas et al., 2021) [15] .

Applications in Female Reproduction Estrus detection

Accurate detection of estrus is pivotal for optimizing timing of insemination. Traditional visual detection is inefficient particularly in species like buffaloes that exhibit "silent heat" et al., 2023) [18] .

• Cattle and Buffalo: Research indicates that the temperature of vulvar skin (regio urogenitalis) increases significantly during the follicular phase due to increased vascularity (Skliarov et al., 2023) [22] . Studies have shown that vulvar surface temperature rises by approximately 0.5 o C to 1.2 o C during the 24 to 48 hours preceding ovulation (Talukder et al., 2014) [24] . In Murrah buffaloes, a strong inverse correlation (r 2 = 0.70) has been observed between vulvar temperature and plasma progesterone levels; as progesterone drops during estrus, vulvar temperature rises (Ruediger et al., 2018) [19] . Thermograms of cows in estrus display "hot" color palettes (red/white) in the external genitalia, contrasting with the "warm" (orange) colors seen during diestrus (Skliarov et al., 2023) [22] . Automated IRT systems utilizing Support Vector Machine (SVM) and logistic models have achieved estrus detection rates exceeding 81% by monitoring ocular and vulvar temperatures (Wang et al., 2023) [26] . • Swine: In sows, the vulva is a reliable thermal window. Thermographic monitoring has demonstrated significantly higher vulvar temperatures during estrus compared to diestrus with a temperature difference of 2-3 o C often observed (Scolari et al., 2011; Skliarov et al., 2023) [20,22] . This hyperthermic response is attributed to the estrogen-dependent increase in blood supply to the external genitalia (Simõeset al., 2014) [21] .

• Small Ruminants: In goats and sheep, IRT has been effectively used to differentiate phases of the estrous cycle with vulvar temperatures rising by over 1 o C during estrus (Skliarov et al., 2023) [22] . • Mares: In Mares, the external genitalia can be 3-4 o C hotter than the surrounding croup tissue during estrus due to hyperestrogenemia (Skliarov et al., 2023) [22] .

Pregnancy Diagnosis

The application of IRT for pregnancy diagnosis yields mixed results depending on species-specific anatomy.

• Equines: IRT has demonstrated success in diagnosing pregnancy in mares, particularly during late gestation (Bowers et al., 2009) [2] . Pregnant mares exhibit higher surface temperatures in the flank and abdominal regions compared to non-pregnant mares attributed to the metabolic demands of the fetus and increased uterine blood flow (Bowers et al., 2009) [2] . • Ruminants: In dairy cattle, abdominal IRT is generally ineffective for pregnancy diagnosis because the metabolic heat generated by the large, physiologically active rumen masks the thermal signals of the fetus (Knížkováet al., 2007) [10] . However, some studies suggest that vulvar temperature may offer indirect diagnostic support by reflecting the maintenance of high progesterone levels (Radigondaet al., 2017) [17] . • Exotic Species: IRT has been successfully used to detect pregnancy in giraffes, zebras and giant pandas, likely due to anatomical differences that make the fetal heat signature more detectable than in domestic ruminants (Benesch and Hilsberg, 2003; Durrant, 2006) [7, 5] . [3,11] . Udder surface temperature correlates positively with somatic cell counts (SCC) and California Mastitis Test (CMT) scores (Polat et al., 2010) [16] .

Mammary Glands

Applications in Male Reproduction Testicular Thermoregulation and Semen Quality

Mammalian testes typically require a temperature 2-6 o C lower than the body core for effective spermatogenesis (Kastelic et al., 1996) [8] . The scrotum functions as athermoregulator and IRT is an ideal tool for assessing this function.

• Thermal gradients: A healthy bull scrotum exhibits adistinct thermal gradient, decreasing in temperature from the scrotal neck (top) to the fundus (bottom) (Kastelic et al., 1995) [9] . The vascular cone cools arterial blood before it enters the testes (Skliarov et al., 2023) [22] . • Semen Quality Correlation: Disruptions in this gradient, such as warmer fundus or "hot spots", indicate compromised thermoregulation and are associated with poor semen quality (Yadav et al., 2019) [27] . In buffalo bulls, a scrotal temperature gradient of ≥6.5 o C was linked to higher sperm concentration and motility while gradients ≤4.0 o C correlated with poor quality (Yadav et al., 2019) [27] . Abnormal thermograms are also associated with higher percentages of sperm morphological defects (Menegassi et al., 2015) [14] .

Diagnosis of Pathologies

IRT is useful for detecting inflammatory and degenerative conditions in the male reproductive tract.

• Inflammation: Conditions such as orchitis, epididymitis and balanoposthitis (inflammation of glans penis and prepuce) present as hyperthermic zones on thermograms (Kozumplik et al., 1989; Skliarov et al., 2023) [12,22] .

• Varicocele: Scrotal thermography is noted for diagnosing varicocele (enlargement of veins) which disrupts normal cooling mechanisms and appears as localized heating (Comhaireet al., 1976) [4] . • Degeneration: Conversely, testicular dystrophy and hypogonadism often present with reduced temperature gradients and "cold" spots due to decreased blood flow and metabolic activity (Skliarov et al., 2023) [22] .

Factors Influencing Thermographic Data

To ensure accuracy, several influencing factors must be managed during IRT data collection.

Technical factors

• Emissivity(ϵ): Accurate temperature calculation requires the correct emissivity setting. Biological tissue generally has high emissivity (0.98) but this can vary (McManus et al., 2022) [13] . Inaccurate emissivity values can lead to significant measurement errors (Borah et al., 2022) [1] . • Distance and Angle: The distance between the camera and the animal, as well as the shooting angle can affect temperature readings; standardizing these parameters is crucial for reproducibility (McManus et al., 2022) [13] .

Environmental Factors

• Solar Loading: Direct sunlight can artificially increase surface temperature readings making it a critical confounder (Mota-Rojas et al., 2021) [15] . [15] .

Animal Specific Factors

• Coat Characteristics: Thick hair or wool acts as an insulator blocking infrared radiation from the skin. Therefore, glabrous regions like eye, muzzle and vulva are preferred (Mota-Rojas et al., 2021) [15] . • Pigmentation: Darker hair coats absorb more solar radiation than lighter coats, potentially skewing data (Mota-Rojas et al., 2021) [15] . • Behavior and Stress: Physical activity or stress prior to imaging can cause stress-induced hyperthermia altering surface temperatures (Stewart et al., 2005) [23] .

Conclusion

Infrared thermography represents a valuable, non-invasive tool for monitoring animal reproduction. It offers significant advantages in detecting estrus, assessing male fertility, and diagnosing inflammatory conditions like mastitis without the stress associated with invasive methods. While highly effective in specific applications such as scrotal thermoregulation assessment and mastitis detection, its utility in pregnancy diagnosis is species-dependent showing limitations in ruminants due to rumen heat. The accuracy of IRT is contingent upon strict control of environmental and technical variables. Future integration with artificial intelligence and automated systems promises to enhance the reliability and practical application of IRT in commercial livestock production and breeding.

References

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