Evaluation of Heat Distribution for the Diagnosis of the Hoof with Abscess by Infrared Thermography in Horses

1. INTRODUCTION

The hoof complex comprises the hoof capsule, sole, frog, digital cushion, ungual cartilages, and the deep digital flexor tendon [1, 2]. These biological structures are susceptible to trauma and various disease processes, including infections (hoof abscesses, puncture wounds, and keratomas), white line disease, and canker. These diseases generally originate in the foot’s ground surface, penetrate the hoof capsule, and extend toward/into the foot’s dermal structure [1]. Many factors may play a role in the development of hoof diseases or abscesses, including the environment, hoof quality, trauma, and shoeing conditions [1].

Hoof abscesses are probably the most common cause of acute severe lameness in horses encountered by veterinarians and farriers [1]. In particular, the soles of horses that exercise a lot on stony ground are often wounded by sharp pebbles. Such wounds may suddenly result in severe (acute) lameness. It is difficult to diagnose because the degree of lameness varies from being subtle in the early stages to non-weight bearing [1]. Hoof abscess, defined as the focal accumulation of purulent exudate between the keratinized and germinal layers of the epithelium, is often associated with a defect in the white line [1]. Bacteria or foreign material that penetrate the stratum corneum induce an inflammatory response that causes the abscess [1]. Surgical treatment and medication are combined in the treatment of hoof abscess, which often includes paring out the abscess to establish drainage and the use of poultice to promote draining [1, 3].

Shin et al. recently reported the prevalence of hoof disorder in racing and riding horses in South Korea, including thrush 4.2% (122/2,891), superficial hoof wall cracks 1.2% (35/2,891), white line lesions 1.0% (29/2,891), hoof wall separation 0.6% (17/2,891), hoof wall defect 0.5% (14/2,891), laminitis 0.3% (8/2,891), and wounds 0.2% (6/2,891) [4].

Infrared Thermography (IRT) has the advantage of enabling a quicker and more accurate diagnosis of physiological changes in the region of interest (ROI) compared with conventional imaging techniques [5]. IRT has been used as a diagnostic tool in the veterinary field since the mid-1960s. This technique has been used mainly for the diagnosis of fractures and ruptures in the orthopedic field [6-8]. IRT is a non-contact, non-invasive diagnostic technique, which measures the physiological function by recording the exothermal status of the emitted infrared rays [9-11]. The skin temperature depends on the underlying tissue metabolism and local blood circulation. Therefore, the changes in physiological activity result in abnormal thermal patterns. Alterations in cutaneous blood flow and sweat patterns occur in the case of overload and inflammation [12]. Despite the various benefits of IRT, few studies have been reported that were used to diagnose hoof-related diseases with IRT.

The study of normal temperature distribution for the diagnosis of hoof disease of horses using IRT is rare therefore, we aimed to investigate whether IRT could aid in the diagnosis of hoof disease in horses; we also aimed to propose a new temperature measurement analysis system in terms of detecting differences in the heat distribution of the hoof.

2. MATERIALS AND METHODS

2.3. Data Analysis

The heat pattern of the hoof area was evaluated using images taken of those hooves that were healthy and those diagnosed with hoof abscess and divided into seven regions of interest (ROI 1 – 7) (Fig. 1). The first ROI represented the location of the central cleft. ROI 2 and ROI 3 reflected the lateral clefts on the medial and the lateral, respectively. ROI 4 reflected the frog, and ROI 5, ROI 6, and ROI 7 reflected areas of the sole (medial, lateral, and middle between the medial and lateral regions, respectively). Each horse had different hoof sizes, but the ROIs covered as much of the area as possible. The average temperature for each area was calculated using specialized software (FLIR Tools Professional, FLIR Systems Brazil, Brazil). The thermal images were analyzed by a rainbow color scheme, and the temperature assigned to each color was uniform for all images.

The estimation of the main trends of basic ROI temperatures for all 66 plantar was performed using an unpaired t-test with Welch’s correction or the Mann–Whitney test. ROI temperatures were also compared for grouped data using The Kruskal–Wallis test with Dunn’s multiple comparisons test to be able to discuss the data obtained in the context of relevant scientific articles. Sensitivity and specificity were measured using receiver operating characteristics (ROC) curves. The sensitivity (%) reflected the ability to detect a hoof abscess given that an abscess was present, while specificity (%) estimated the ability to avoid classifying a healthy hoof as a hoof with abscess. All statistical analyses were performed using SPSS software (version 25, IBM SPSS Statistics, USA), where probability (p) values < 0.05 were considered to indicate statistical significance.

3. RESULTS

3.2. Feasibility of IRT in the Detection of a Hoof Abscess

The ability of IRT to predict a hoof abscess according to ROI classification is summarized in Table 2. In general, the cut-off value was higher in the affected area (lateral cleft and sole regions). IRT classified hooves with hoof abscess with high sensitivity (mean: 74.3%, range: 60% – 100%) and specificity (mean: 87.5%, range: 62.5% – 100%) across all ROIs, and analysis of the area under the curve (AUC) showed excellent discrimination (mean: 0.934, range: 0.881 – 0.964, Fig. (3) and Table 2).

Table 1.

ROI	Hoof area	T1 ± SD	T2 ± SD	p	ΔT
1	Central cleft	24.80 ± 3.56	30.32 ± 2.79	< 0.0001	5.52 ± 3.45
2	Lateral cleft (left side)	27.67 ± 3.61	33.66 ± 1.65	< 0.0001	5.99 ± 3.39
3	Lateral cleft (right side)	27.97 ± 3.56	33.63 ± 1.55	< 0.0001	5.66 ± 3.33
4	Frog	25.52 ± 3.55	31.05 ± 2.55	< 0.0001	5.53 ± 3.42
5	Sole (left side)	24.78 ± 3.83	31.30 ± 2.56	< 0.0001	6.52 ± 3.67
6	Sole (right side)	24.66 ± 3.91	31.85 ± 2.25	< 0.0001	7.19 ± 3.71
7	Sole (middle side)	24.98 ± 3.44	31.79 ± 2.43	< 0.0001	6.81 ± 3.31
1 – 7	Entire hoof body	25.77 ± 3.87	31.94 ± 2.60	< 0.0001	6.17 ± 3.71

ROI: region of interest, T₁ ± SD: mean and standard deviation of temperature obtained in ROI of healthy hooves (°C), T₂ ± SD: mean and standard deviation of temperature obtained in ROI of hooves with abscesses (°C), ΔT: mean difference and standard deviation between T₁ and T₂ for each particular ROI (°C); Unpaired t-test with Welch’s correction or Mann–Whitney test (significance level, p < 0.05).

Table 2.

ROI	Cut-off value (°C)	Sensitivity (%)	Specificity (%)	AUC	SE	Significance
1	27.75	80	75.0	0.881	0.050	< .0001
2	32.35	90	91.1	0.963	0.030	< .0001
3	32.05	80	85.7	0.951	0.030	< .0001
4	28.75	70	76.8	0.884	0.055	< .0001
5	29.05	90	87.5	0.945	0.041	< .0001
6	29.40	90	87.5	0.964	0.021	< .0001
7	28.80	80	83.9	0.950	0.027	< .0001

ROI: region of interest, AUC: Area under the curve: SE, Standard Error.

4. DISCUSSION

Puncture wounds to the sole of the foot can introduce bacteria and debris to the solar surface of the distal phalanx, which frequently results in fracture or septic pedal osteitis. Septic pedal osteitis involves bone lysis of the distal phalanx and is accompanied by the presence of purulent exudate (which differentiates this condition from non-septic pedal osteitis) [15]. Radiography is sometimes useful to identify a gas or fluid pocket; however, it is difficult to locate a suitable drainage location by radiography. Moreover, drainage is possible only by experienced veterinarians or farriers. This study did not investigate whether the location of the inflammation matches the radiographic pockets of the abscessed hoof and the thermal pattern of thermography. Because, in the study, the thermal pattern showed the affected range as temperature distribution, but the exact location of the drainage could not be specified [16].

In this study, we demonstrate the usefulness of IRT as a diagnostic tool for detecting hoof abscess by correlating the temperature of healthy hooves and hooves with abscess. IRT is used for medical diagnostic purposes because it can measure the distribution of surface temperature and display the relationship between the surface temperature and lesions in the underlying anatomical structures [17]. As noted by Turner and Palmer, normal thermal patterns can be tabulated to accommodate the vascular system of horny tissue, which are highly symmetrical between the left and right sides of the body [8, 18]. Any asymmetry observed in thermal patterns would reflect an abnormality. Several studies have concluded that a temperature difference within 1 °C between the compared parts of the body is considered as normal [8, 19-21]. Most horses try to avoid stepping on the floor with minimal force when a sore leg occurs. That way, even without pain or other illness, the load on one side increases, whereas all other sides are resting, resulting in a high temperature. Therefore, temperature comparison with the other three hooves except for the hoof with an abscess in the same horse was excluded. Nevertheless, in this study, the range in temperature difference in the hoof was wide, even in the absence of inflammation or clinical signs. This cannot exclude the possibility of asymptomatic inflammation.

As shown in this study, the mean temperature of ROIs in hooves with abscess was higher than that of healthy hooves, a finding that was especially prominent for sole regions. The temperature range of healthy hooves (15.9 °C – 33.6 °C, Fig. (2a) was wider than that of hooves with hoof abscess (26.5 °C – 37.0 °C, Fig. 2b). This difference might reflect various influencing factors, such as season, exercise, and environment. Based on the average temperature of each ROI, the optimal cut-off value for discriminating hoof abscess was calculated through ROC analysis. The ROC curve is used to determine the usefulness of direct or indirect measurements of screening tests. The sensitivity and specificity of the sole region, where hoof abscesses mainly develop, were statistically significant at 96.7 and 88.3, respectively (p < 0.0001).

IRT aids in disease diagnosis by detecting the heat released by inflammation or increased blood circulation. The temperature changes are observed as an image using infrared heat sensing technology. Although it is an important early diagnosis technique that can provide a preventive diagnosis (screening) for each individual or group prior to a detailed diagnosis, it has not been as widely used in animals as in humans. Therefore, this study aims to investigate the utility of IRT as a new temperature measurement analysis system for determining the heat distribution of the hoof, which may be useful in the case of hoof abscesses. In conclusion, we believe that IRT would prove to be useful not only for pre-diagnostic purposes but also for determining the healing process in case of hoof diseases. In future research studies, the results of this study may be extrapolated not only to other hoof diseases (such as acute laminitis) but also to hoof diseases in other animals.

CONCLUSION

This study was conducted to establish IRT as a new temperature measurement analysis system and to use its indicators for detecting hoof abscess in horses. Through this study, we promote the early screening of horse hoof diseases, which will greatly contribute to the prevention of losing of high-valued horses .

REFERENCES