Prevalence of Obesity and Equine Metabolic Syndrome Among Grazing and Non-Grazing Horses

Introduction

Obesity is a big challenge in horses' welfare. Obese horses are predisposed to some metabolic disorders such as equine metabolic syndrome (EMS), which consequently results in laminitis, subfertility, and hyperlipidemia. Obesity may also cause osteoarthritis and hypertension (McCue et al., 2015). Obesity is an issue that is neglected by owners sometimes. High prevalence of obesity has been reported in the UK (Robin et al., 2015). In the United States, 51% of light breed (Thatcher et al., 2012) and 24% of Australian pleasure horses are fat (Potter et al., 2016). Obesity is determined by body condition scoring; for example in Henneke scoring system, the body condition score (BCS) above 7 is assumed fat (Henneke et al., 1983). Some horses have regional fat accumulation (regional adiposity), most commonly in their nuchal ligament region (cresty neck), tailhead, and preputial or mammary gland region. BCS ≥7/9 (Henneke scoring system) and cresty neck score ≥3 (Carter et al., 2009) are two main features of obese horses.

EMS is a set of clinical features and a risk factor for endocrinopathic laminitis (Durham et al., 2019). EMS occurs as a result of over-nutrition, exercise limitation, and, consequently, obesity (McGowan et al. 2013). The main feature of EMS is insulin dysregulation (ID) (Durham et al., 2019), which is characterized by hyperinsulinemia. Other manifestations of this syndrome with variable occurrences are: hypertriglyceridemia, hyperleptinemia, arterial hypertension, altered reproductive cycling in mare, and increased systemic inflammation response (Durham et al., 2019). ID is the core feature of pathophysiological events in EMS. Hyperinsulinemia appears after insulin resistance and is a trigger for other manifestations.

There are two hypotheses linking obesity to insulin resistance (IR): first, adipokines and cytokines of adipose tissue that downregulate the insulin signaling pathway; second, the accumulation of intracellular lipids in tissue (lipotoxicity). Because of intracellular accumulation of lipid in the liver of obese horses impairment of insulin clearance is another possible factor that results in hyperinsulinemia in EMS horses (Potter et al., 2016).

It is not well documented how hyperinsulinemia causes laminitis. Nonetheless, endothelial dysfunction and digital vasoconstriction (Fitzgerald et al., 2019) play major roles. Insulin is a double-edged sword; it normally causes vasodilation by the synthesis of nitric oxide (NO) (Muniyappa et al., 2008). On the other hand, by the activation of sympathetic pathways through endothelin-1 (ET-1), vasoconstriction is induced (Muniyappa et al., 2008). In IR (Insulin Resistance) human, it has been found that NO synthesis is impaired, and in contrast, ET-1 synthesis is increased as a compensatory mechanism for hyperinsulinemia.

Current knowledge for the management of EMS horses includes physical activity, dietary restriction, and medical management of hyperinsulinemia. Owners should be aware that control of EMS is much easier than treatment, so the balance ratio and exercise are the main approaches (Marycz et al., 2018).

It has been shown that removal of horses from pasture, limitation of forage, and soaking of forage in water can help obese horses lose weight. EMS horses should be fed with 1-1.5% of BW of hay with a non-structural carbohydrate (NSC) content of ≤10%, or alternatively, long time soaking (8-16 hours) can reduce NSC (McGowan et al., 2013).

This study aimed to evaluate the prevalence of obesity and EMS among Iranian horses and also evaluate the role of feeding management and exercise in occurrence of obesity and EMS. Till now all of the studies have been done in countries with similar management policies based on pasture feeding. We assumed that in Iran with different management styles, the prevalence of EMS could be different. Moreover, the authors did the study in three provinces with different pasture access to better evaluate the roles of feeding management in EMS occurrence.

Materials and Methods

Ethical approval was obtained from the University of Tehran Research Council. The study was performed on 570 horses aged 3-18 years, which were divided into two categories, including 3-10 years and 11-18 years. One hundred and ninety horses were grazed in pasture, while 380 horses were fed indoors and did not have access to pasture. Three hundred fifty-two mares, including 146 pregnant (first 186 days of pregnancy) and 206 non-pregnant, as well as 218 male horses, including 209 geldings and 9 stallions, were considered for this study. The study was done during the spring and summer of 2020 in three provinces, including Tehran, Alborz, and Golestan. The overweight horses with BCS ≥7 (Henneke scale) or horses with local adiposity such as cresty neck ≥3 (Carter's score) were identified by observation. Each horse was asked to walk and trot to evaluate its lameness. Lame horses were clinically examined to confirm their laminitis. Given the effect of stress on endocortical axis, we tried to separate the horses from stressful situations before sampling. In this regard, horses with recent transportation or any environmental stress were excluded from the study.

Horses with obesity criteria and horses with laminitis at the initial visit or with a history of endocrinopathic laminitis were included in the study even if their BCSs were under 7. Based on their premises and ration, horses were classified as grazing and non-grazing.

EMS horses were defined as horses with plasma insulin concentrations higher than 20 μIU/mL.

In obese horses, blood samples were taken in the morning before feeding (6-8 hours off-feed). The blood sample was collected in a plain and EDTA evacuated tube to measure insulin and blood glucose levels. The samples were sent as whole blood to the laboratory. The blood samples were placed in ice, and processing was done in the laboratory for more evaluation. Centrifugation was done for 10 minutes at 3000 g. Glucose and insulin were measured. Glucose was measured using the AutoAnalyzer device.

All of the harvested plasma samples of all individual horses were tested using a horse-specific ELISA kit (Hangzhou EastBiopharm Co. LTD., China) for the measurement of plasma concentrations of insulin. The light absorption of microwells was read by an ELISA plate reader device (Biotech ELX, USA) at 450 nm wavelength. Based on the light absorption of standard microwells, standard curves were drawn for each peptide. Then, using these standard curves, the concentration of insulin was determined.

Statistical Analysis

Chi-square test and Fisher's exact test were conducted to compare the frequencies of surveyed factors between the obese and non-obese and also between the obese horses affected by EMS and those not affected by this condition. SPSS software version 20.0 (SPSS Inc., Chicago, IL., USA) was used to perform statistical analysis, and P-value<0.05 was considered statistically significant.

Results

As abovementioned, horses were divided into two categories based on age, including 3-10 years and 11-18 years. Hence, 53 out of 347 horses (3-10 years) and 29 out of 223 (11-18 years) were fat. Moreover, 7 and 6 horses had EMS at 3-10 years old and 11-18 years, respectively (Table 1). There was no significant correlation between age and obesity or EMS occurrence.

Among the 352 mares, 67 were fat, and 11 had EMS. Furthermore, out of 218 males, 15 were fat, and 2 had EMS (Tables 2 and 3). Gender significantly affected obesity, but there was no correlation with EMS occurrence.

There were 3 breeds in this study, including Turkman (117), Thoroughbred (73), and warm-blood (380). Since the feeding strategies were the same for Turkman and Thoroughbred horses, and also for better analysis of data, the authors considered both breeds as one category named light-saddlebred. They were kept in studs which were located in the same area with access to pasture. Forty-eight light saddlebred and 34 warm-blood horses were fat; 8 and 5 of them had EMS, respectively. Breed had a significant correlation with obesity, but authors believed this correlation was more affected by feeding strategies since, as mentioned, light-saddlebred horses had access to pasture grazing while warm-blood ones were fed indoors.

In this study, 485 horses had routine physical activity; either professional training or day long walking (at least 20 min), and trotting on the lunge. Eighty five horses rarely worked and only spent a long time in the stall. Moreover, 54 and 28 horses were fat in both groups, respectively (Table 2). Six horses with physical activity and 7 with sedentary behavior had EMS (Table 3).

Table 1. The frequency and number of horses in each variable

Among 570 horses, 82 were defined as fat (BCS≥7 and/or CN≥3; 14%); among the horses with pasture access, 48 were fat (25%), while in the group which did not graze, 34 horses were fat (9%). The prevalence of obesity in the total population was 14% (84/570). Obesity had a significant correlation with feeding management (Table 2). Nine out of 48 obese horses grazed on pasture had EMS (18%); while in indoor feeding, 4 out of 34 obese horses had EMS (12%). In general, 13 out of 570 horses had EMS (2.5%) in this study. It means the prevalence of EMS among obese horses was 16% (13/82). EMS occurrence did not have a significant correlation with feeding strategy (Table 3).

Table 2. Frequencies of studied variables in all horses differing in obesity

^*Significant difference between obese and non-obese horses (P<0.05)

Table 3. Frequencies of studied variables in obese horses differing in metabolic status

^*Significant difference between EMS and non-EMS horses (P<0.05)

Discussion

ID is the core and main feature of EMS (Carslake et al., 2021). An increase in plasma insulin level is the main laboratory indicator of a suspected case of EMS. In some studies, very variable results have been reported for the prevalence of EMS (Pleasant et al., 2013; Morgan et al., 2014; Carslake et al., 2021). This difference in results is probably due to the effect of different criteria such as the cutoff value of insulin, breeds of the studied population, and methods of insulin evaluation. Walsh and colleagues (Walsh et al., 2009) used 70 IU/mL as a cutoff value, while Carslake (Carslake et al., 2021) and Morgan (Morgan et al., 2014) used 20 IU/mL. Pleasant (Pleasant et al., 2013) studied light-bred horses in the United States, while Carslake (Carslake et al., 2021) and Morgan (Morgan et al., 2014) worked on ponies. In the present study, we considered the 20 IU/mL plasma insulin concentration as the criterion, and light-saddlebred and warm-blood horses were sampled.

We reported a much lower prevalence of EMS (2.5%). We believe this low prevalence is the effect of feeding management on warm-blood horses. The prevalence of EMS in light-saddlebred horses was 4.2% (8/190); in warm-blood ones, it was 1.3% (5/380).

In horses grazed on pasture, EMS prevalence was 4.7% (9/190) compared to 1% (4/380) in indoor horses. Carslake and colleagues reported a relationship between the increasing age and the occurrence of EMS (Carslake et al., 2021). This relationship was previously reported by Morgan (Morgan et al., 2014). In this study, no correlation was found between the EMS occurrence and obesity in different age groups and gender, while obesity was affected by both. The study of Carslake et al. was the first one to show a gender effect on EMS occurrence, with mare at a higher percentage than gelding and stallion. These authors did not find any significant effect of pregnancy on EMS occurrence.

This study strongly supported the correlation between obesity and EMS occurrence. The prevalence of EMS in obese horses was 16% versus 0% in normal or underweight horses, as well as supporting the results of other studies (Heliczer et al., 2017., Durham et al., 2019., Fitzgerald et al., 2019., Carslake et al., 2021). Vick et al. showed an inverse relationship between the BCS and insulin sensitivity. They also found a positive relationship between adiposity and an increase in inflammatory markers (Vick et al., 2006). This relationship could be explained by the anti-inflammatory property and insulin sensitivity of adiponectin. In the study of Zak et al., no relationship was found between the obesity, ID, and the occurrence of low-grade, chronic inflammation in the horses with EMS (Zak et al., 2020).

Another study had anecdotal results; Carter et al. concluded that weight gain in horses simultaneously occurred with diminishing insulin sensitivity. Thirteen geldings were overfed for 16 weeks, causing an increase in their BCS from 6/9 to 8/9. In their study, obesity resulted in hyperinsulinemia and hyperleptinemia (Carter et al., 2009).

Insulin sensitivity was unchanged in thoroughbred geldings fed high fat and fiber diet, while in a group of horses fed a diet high in starch and sugar, insulin sensitivity was decreased. The authors concluded that dietary energy sources are more determinative than weight gain (Quinn et al., 2008).

Decreased pasture time in summer was a risk factor in one study (Carslake et al., 2021), the result of which is in contrast with our results. We found a strong relationship between grazing on pasture, obesity, and EMS occurrence. As Quinn et al. (2008) stated, a high-sugar diet causes insulin resistance. Pasture is mostly high in sugar, starch, and fructan, especially in the growing season. High NSC in a fresh pasture is associated with obesity and laminitis (Reynolds et al., 2019).

Conclusion

This study concluded that the most crucial factor in obesity and EMS incidence is feeding management, and diets with high NSC are the most critical risk factors.

Our results, in summary, show that gender, breed, sedentary lifestyle, and feeding management could affect the obesity in horses. In contrast, feeding management is the key factor for EMS occurrence among fat horses. We concluded that in the same group of horses or ponies, factors such as breed, age, sex, and are not as important as diet. Therefore, the best way to control EMS is to appeal to an appropriate ration management system.

Acknowledgments

The author gratefully acknowledged the owners who cooperated in this study and gave the data to the authors accurately. The authors supported the study financially.

Conflict of Interest

The author of this study have no financial or personal relationship with other people or organizations that could inappropriately influence or bias the content of the study