Introduction
Numerous factors have been associated with elimination of endurance horses from competitions, with changes in electrolyte concentrations, acid-base imbalances, and dehydration most frequently cited.1–5 Hypokalemia, hypochloremia, metabolic alkalosis, and hyperproteinemia are common and may be associated with failure to finish (elimination from competition).6,7 Although a loss of electrolytes and water during competition has been proposed as one of the primary causes for these changes, it is also possible that some horses begin the competition with abnormalities in electrolyte concentrations or hydration status. Preride electrolyte concentrations have been reported but not specifically compared with concentrations prior to transport.2
For horses that begin a competition with low electrolyte concentrations or a water deficit, these imbalances must have developed during 1 of 3 time periods: before transportation to the event while horses were at home, during the transport process itself, or while horses were housed onsite between the time of arrival and the start of competition, especially if the horses were not eating or drinking well. Understanding when electrolyte losses and dehydration occur could allow for interventions that would improve the welfare of horses during endurance rides and, potentially, other competitive events.
Illness prior to competition has been suggested in previous studies as a reason for poor performance or inability to successfully compete.8–10 Most of these studies have evaluated for infectious diseases, but other causes have also been investigated. However, electrolyte concentrations and hydration status prior to transport to a competition have not been major areas of study, and it is possible that electrolyte losses or dehydration can occur before horses even begin a competition.
The act of transporting horses, particularly over long distances, has been shown to cause a variety of problems, including electrolyte loss, dehydration, gastric ulcers, and respiratory problems.11,12 On the basis of these findings, it would seem likely that transportation to equine competitive events may be a possible contributing factor for electrolyte and hydration abnormalities during competition. However, the relative importance of transportation is unknown. Many studies2,13,14 of endurance horses used data collected during prerace check-in the day before an event as baseline data. One study14 investigated variables while horses were still at home, but these values were not specifically compared with values obtained immediately prior to competition. Thus, more research is needed comparing preride values with values obtained before horses are transported to an event.
Compared with research on the transportation of horses, there is much less research evaluating the effects of overnight stabling in a new location on electrolyte concentrations and hydration status of equine athletes. Decreased feed and water intake are especially likely if horses are less experienced with housing at campgrounds or showgrounds overnight. Horses may not eat in a new or stressful environment, and this anorexia can have effects on hydration status.15
Given that electrolyte and hydration abnormalities have been documented during intense and prolonged endurance competitions, a prospective study over multiple time points is needed to determine when deficits are most likely to occur. Identification of the timing of abnormalities could lead to changes in transport or stabling that would improve the welfare of horses at competitive events. Such findings might also provide insights into other disciplines for which transport and stabling prior to the event could be a factor in morbidity or suboptimal performance.
The purpose of the study reported here was to evaluate changes in electrolyte concentrations and hydration status that take place in endurance horses prior to the start of a competition and determine whether these changes would be associated with elimination. We hypothesized that changes in electrolyte concentrations and hydration status would occur prior to the start of a competition and that these changes would be associated with success or failure in the competition.
Materials and Methods
Nineteen horses enrolled in the 2016 Tevis Cup 100-Miles (160 km) One-Day Western States Trail Ride held on July 23 that year were recruited to participate in the study. The study was approved by the Animal Care and Use Committee of the University of California-Davis, and client consent was obtained from all owners. A 3-mL heparinized blood sample was collected from horses at each of 5 time points during the competition: 3 to 5 days prior to transport to the ride site (T0), during check-in the day before the ride (T1), 1 to 2 hours before the start of the ride (T2), at the approximately 15-km mark of the ride (T3), and at the approximately 55-km mark of the ride (T4). Additional samples were not collected after a horse was eliminated from competition.
Sample analysis
Plasma biochemical (sodium, potassium, chloride, urea nitrogen, glucose, and bicarbonate) concentrations were determined with a commercial point-of-care analyzer (i-STAT handheld analyzer; Abbott Point of Care Inc). Packed cell volume was determined with the microhematocrit method. Plasma total protein concentration was measured by means of refractometry.
Additional data collection
Additional data collected for each horse consisted of age, breed, sex, and finishing status (finisher or nonfinisher). The transport distance (ie, the distance from where the T0 sample was collected to where the T1 sample was collected, or the distance from home to the start of the ride) was recorded for each horse; however, the transport time was not recorded.
Statistical analysis
The Shapiro-Wilk test was used to test whether continuous data were normally distributed. Data are reported as mean ± SD for normally distributed data and as median (range) for nonnormally distributed data. For variables that were normally distributed, values were compared among time points by means of 1-way ANOVA followed by the Tukey test for multiple comparisons. For variables that were not normally distributed, values were compared among time points by means of the Friedman test followed by the Dunn test for multiple comparisons. If the value for a particular variable was missing for any given time point, values for that horse for all time points were removed for analysis of that variable. If the value for an individual horse was > 2 SDs less than or greater than the mean for a given time point, values for that horse were removed for analysis of that variable.
To evaluate associations between changes in measured variables and finishing status, 2-way ANOVA followed by the Sidak test for multiple comparisons was used. For a given variable, time points were included in the analysis only if values for ≥ 6 horses were available for each group (finishers vs nonfinishers). For this reason, values for the 55-km time point were not included in the 2-way ANOVA, because values were available for only 4 horses in the nonfinisher group.
All statistical analyses were performed with standard software (GraphPad Prism version 8.2.1; GraphPad Software). Values of P < 0.05 were considered significant.
Results
Mean ± SD age of the 19 horses included in the study was 12 ± 3 years. There were 17 geldings and 2 mares. There were 16 Arabians, 2 Arabian crosses, and 1 mule. The median distance horses were transported from home to the start of the ride was 140 km (range, 106 to 411 km). All horses were housed at the starting area the night before the ride and kept in corrals or tied to trailers where access to feed and water was available. Quantity of feed and water consumed prior to the competition was not recorded.
One of the 19 horses enrolled in the study was eliminated after the preride check-in and did not start the ride. A second horse would not allow a blood sample to be collected at the 15-km mark of the ride. Two additional horses were eliminated on the basis of results of veterinary examinations before the 55-km mark of the ride. Hence, the maximum number of horses included in statistical analyses for a given time point was 15. Twelve (63%) horses finished the ride, and 7 did not. By comparison, 87 (53%) of the 165 horses that entered the ride finished.
Significant differences were detected among plasma sodium, potassium, and urea nitrogen concentrations measured prior to the start of the ride (ie, at T0, T1, and T2; Table 1). For all variables except chloride and bicarbonate concentrations, significant differences were detected between values obtained prior to the start of the ride and values obtained during the ride (ie, at T3 and T4). Maximum increases and decreases in values between consecutive time points were calculated (Table 2).
Plasma biochemical values obtained at various times before and during the competition for 19 horses that competed in the 2016 Tevis Cup 100-Miles (160 km) One-Day Western States Trail Ride.
| Variable | Timepoint | No. of horses | P value | Reference range | ||||
|---|---|---|---|---|---|---|---|---|
| T0 | T1 | T2 | T3 | T4 | ||||
| Sodium (mmol/L) | 137 (l34-l39)a,b,c | 139 (l38-l42)a | l39 (l38-l42)d | 141 (l38-l45)c,d | l40 (l35-l44)b | 15 | < 0.001 | 128-142 |
| Potassium (mmol/L) | 3.6 ± 0.5a,b | 3.4 ± 0.3c,d,e | 4.4 ± 0.6a,c,f | 4.7 ± 0.6b,d,g | 3.0 ± 0.8e,f,g | 13 | < 0.001 | 1.9-4.1 |
| Chloride (mmol/L) | l02 ± 3 | 104 ± 1 | l04 ± 2 | l04 ± 3 | l04 ± 3 | 13 | 0.12 | 100-111 |
| Urea nitrogen (mmol/L) | 18 ± 3a,b,c | 20 ± 2a,d,e | 20 ± 3f,g | 22 ± 2b,d,f,h | 25 ± 4c,e,g,h | 13 | < 0.001 | 11-27 |
| Glucose (mmol/L) | 95 (87-l03)a | l06.5 (96-l83) | 95.5 (70-l06) | 117 (6l-l8l)b | 82.5 (68-l04)a,b | 14 | < 0.001 | 62-l34 |
| Bicarbonate (mmol/L) | 30.6 ± 1.3 | 28.8 ± 1.3 | 28.8 ± 2.7 | 28.5 ± 2.1 | 27.6 ± 3.7 | 15 | 0.05 | 24-30 |
| PCV (%) | 36 (3l-44)a,b | 37 (33-48)c,d | 36 (33-42)e,f | 51 (47-54)a,c,e | 46 (40-50)b,d,f | 15 | < 0.001 | 32-53 |
| Total protein (g/dL) | 6.8 ± 0.4a,b | 6.5 ± 0.4c,d | 6.7 ± 0.4e,f | 7.8 ± 0.4a,c,e | 7.5 ± 0.4b,f,f | 15 | < 0.001 | 5.2-7.9 |
Data are reported as mean ± SD for normally distributed variables and as median (range) for non-normally distributed variables. Blood samples were collected from horses 3 to 5 days prior to transport to the ride site (T0), during check-in the day before the ride (Tl), 1 to 2 h before the start of the ride (T2), at the approximately 15-km mark of the ride (T3), and at the approximately 55-km mark of the ride (T4). Additional samples were not collected after a horse was eliminated from competition, and horses were removed from the analysis for a particular variable if the value was missing for any time point.
a-fValues with the same superscript letter were significantly different (for sodium,P < 0.05a, < 0.05b, < 0.001c, and < 0.01d; for potassium,P < 0.05a, < 0.01b, < 0.01c, < 0.01d, < 0.01e, < 0.001f, and < 0.001g; for urea nitrogen,P < 0.05a, < 0.01b, < 0.001c, < 0.05d, < 0.001e, < 0.05f, < 0.00lg, and < 0.01h; for glucose,P < 0.001a and < 0.001b; for PCV,P < 0.001a, < 0.01b, < 0.001c, < 0.05d, < 0.001e, and < 0.05f; and for total protein:P < 0.001a, < 0.01b, < 0.001c, < 0.001d, < 0.001e, and <0.01f).
Maximum increase and decrease in plasma biochemical values between consecutive time points for the horses in Table 1.
| Variable | T0 vs T| | T| vs T2 | T2 vs T3 | T3 vs T4 |
|---|---|---|---|---|
| Sodium (mmol/L) | ||||
| Increase | 5 | 1 | 7 | 3 |
| Decrease | 2 | 4 | NA | 7 |
| Potassium (mmol/L) | ||||
| Increase | 1.1 | 2 | 1.6 | 0.8 |
| Decrease | 1.4 | 0.3 | 0.5 | 3 |
| Chloride (mmol/L) | ||||
| Increase | 6 | 3 | 3 | 5 |
| Decrease | 7 | 4 | 6 | 5 |
| BUN (mmol/L) | ||||
| Increase | 8 | 7 | 4 | 8 |
| Decrease | 1 | 5 | 4 | NA |
| Glucose (mmol/L) | ||||
| Increase | 80 | 60 | 86 | 12 |
| Decrease | 70 | 80 | 31 | 113 |
| Bicarbonate (mmol/L) | ||||
| Increase | 3.7 | 7.1 | 5.5 | 9.2 |
| Decrease | 4.7 | 4.5 | 7.1 | 9.2 |
| PCV (%) | ||||
| Increase | 17 | 9 | 18 | 1 |
| Decrease | 10 | 13 | NA | 10 |
| TP (g/dL) | ||||
| Increase | 0.8 | 1.2 | 1.6 | 0.6 |
| Decrease | 1.0 | 0.4 | NA | 1 |
NA = Not applicable (ie, there was no increase or decrease in the variable between the 2 time points).
See Table 1 for remainder of key.
In the 2-way ANOVA for associations between measured variables and finishing status (finisher vs nonfinisher), only bicarbonate concentration at the 15-km mark of the ride was significantly (P = 0.04) associated with finishing status. At this time, mean bicarbonate concentration for the finisher group (n = 11 horses) was 29.1 ± 2.0 mmol/L and mean bicarbonate concentration for the nonfinisher group (6 horses) was 26.2 ± 1.4 mmol/L. No other variable was significantly different in the 2-way ANOVA between the finisher and nonfinisher groups.
Discussion
Results of the present study suggested that significant changes in plasma sodium, potassium, and urea nitrogen concentrations can occur prior to the beginning of a competition in endurance horses. Specifically, plasma sodium concentration significantly increased during the transportation period (ie, between T0 and T1) in horses in the present study. In a previous study,12 sodium concentration increased in horses being transported without water, but to the authors’ knowledge, there are no other studies comparing sodium concentration in horses before and after transport to competitive events. One study14 sampled endurance horses while at home in training and then the day before competition, but these values were not directly compared.
In a study2 evaluating endurance horses prior to the start of competition, but not prior to transport, sodium concentration was increased in the group of horses that did not finish, compared with the concentration in horses that successfully completed the competition. However, we did not find the same relationship in the present study. Interestingly, in that previous study, preride sodium concentrations were higher than reference ranges reported elsewhere.16 These findings are consistent with the concern that electrolyte concentrations and hydration status determined at ride check-in the day before a competition may be different from the true baseline values obtained at home before transport.
Increase in sodium concentrations in the present study would be consistent with free water deficits that could be clinically relevant. However, without accounting for the volumes of sweat and fecal losses, it would be difficult to estimate the exact change. A rough estimate for a 500-kg horse with an extracellular fluid volume of 23% would be that an increase of 2 mmol/L in plasma sodium concentration would correspond to an approximately 2-L loss in free water, assuming no substantial losses or gains in total body sodium.16 The maximum increase in plasma sodium concentration during the transportation period for horses in the present study was 5 mmol/L, which would correspond to a > 4-L loss in free water. Given that intracellular fluid losses likely accompanied, and could have been larger than, any extracellular losses, the total fluid loss before horses arrived at the competition could have been clinically important.
Similar to serum sodium concentration, plasma urea nitrogen concentrations increased significantly during the transportation period for horses in the present study. Potential causes for an increase in BUN concentration in horses include renal disease, gastrointestinal bleeding, and dehydration,17–19 but it is likely that dehydration was the most important factor in these horses. Similar to the change in sodium concentration, the increased BUN concentration would support the conclusion that horses experienced some degree of dehydration after leaving home and before arriving for the preride check-in. However, the change in BUN concentration was small enough that its clinical importance may be limited.
Total protein concentration did not significantly change during the transportation period for horses in the present study. It is possible that some protein was lost during this period but that the total protein concentration did not change because of the loss of fluid volume.20 In addition, PCV can be affected by stress and splenic contraction, and it is possible that horses were more or less stressed at the various times when blood samples were collected.20
In the present study, plasma potassium concentration significantly increased between the time of the preride check-in (T1) and the start of the competition (T2). This finding was somewhat unexpected, but it may have been due to electrolyte supplementation prior to the ride, which is a common practice at endurance events.21 Plasma potassium concentration at the 55-km mark was significantly decreased, compared with the concentration immediately before the ride started, consistent with the loss of potassium in sweat that takes place during long-distance competition in horses.22
At the 15-km mark, horses that would later be eliminated from the ride (nonfinishers) had a lower bicarbonate concentration than did horses that successfully finished the ride (26.2 ± 1.4 mmol/L vs 29.1 ± 2.0 mmol/L, respectively). This finding was particularly interesting because an increased bicarbonate concentration has been a feature of endurance horses in some studies.3,23 However, another study found a decrease in bicarbonate concentrations in horses competing > 159 km.24 To the authors’ knowledge, a decreased bicarbonate concentration during an endurance ride has not been previously linked to finishing status. The magnitude of the decrease in bicarbonate concentration was unlikely to be clinically important, but a decreased concentration may represent a marker for horses that are having difficulty during a ride.
The blood sample that was collected at the 15-km mark was unusual in that it was not collected at a ride checkpoint that required a veterinary exam or wait time. Horses had just climbed 610 m to the location where blood samples were collected, and many had not had any substantial rest time. Comparison data for this type of checkpoint during an endurance ride do not exist, to our knowledge; therefore, the decrease in bicarbonate concentration should be interpreted with caution. A potential cause for the decrease in bicarbonate concentration might be that nonfinishers had a higher lactate concentration than finishers after this large climb in elevation, and a future study could test whether horses with a higher lactate concentration following a steep climb during a ride would be more likely to be eliminated later during the ride. Unfortunately, lactate concentration was not measured as part of the present study, and chloride concentration could not be measured in 3 samples at this time point. Therefore, the anion gap or strong ion difference could not be reliably calculated from the data available. Interestingly, among the small number of horses for which anion gap was available, the horse with the highest anion gap (41 mEq/L) was eliminated later in the ride because of metabolic failure and the horse with the lowest anion gap (34 mEq/L) was eliminated later in the ride because of lameness. Measuring lactate concentration immediately after a large climb could be a novel way of identifying endurance horses that may be in distress or at risk of elimination.
The electrolyte abnormalities observed before the start of the ride among horses in the present study could potentially be addressed with changes in transport or housing. For example, horses could be transported to the ride site 3 or 4 days in advance of the ride to allow electrolyte concentrations to return to baseline. Additionally, horses could be administered fluids IV after arrival at the ride site to replace any deficits that may have developed. These management changes would need evaluation in a future study before specific recommendations could be made.
Perhaps one of the greatest weaknesses of the present study was the unexpectedly small number of horses that were eliminated. If the study group had more closely paralleled the overall ride population, an additional 2 horses would likely have been included in the nonfinisher group. Additionally, some of the eliminated horses were removed from competition before collection of the last blood sample, making the nonfinisher group even smaller for analyses. Repeating the study with a larger number of horses might have identified more clinically relevant abnormalities.
On the basis of results of the present study, the number of horses that might be needed to detect statistically significant differences between finisher and nonfinisher horses at the preride check-in can be estimated through a power analysis. For potassium concentration, which has been shown to be lower in horses at risk for failure or with metabolic problems,4 power analysis with an α of 0.05, power of 80%, and difference in potassium concentration between groups of 0.5 mmol/L indicated that approximately 32 horses (16 in each group with an elimination rate of 50%) would be needed to detect a significant difference. A future study of this size could easily be designed to determine whether a decrease in potassium concentration at the preride check-in was indeed associated with elimination.
In the present study, 3 of the same analyzers were used to measure biochemical concentrations so that samples could be processed as quickly as possible. Thus, it is possible that variations among instruments could account for measured differences. An alternative approach would have been to collect all samples and test them with the same analyzer; however, this would have prolonged storage times for some samples, compared with others. An additional study weakness was that the distance traveled for the horses to reach the competition was relatively short (median, 140 km; range, 106 to 411 km). Horses may travel up to 5,000 km to participate in this ride, and electrolyte and hydration changes might be more substantial in horses that are transported farther. However, some of these horses that travel a very long distance are brought to a local stable many weeks before the ride. To maintain consistency of the results, horses that were selected for the present study lived at distances that would allow the same analyzers to be used for all samples. In a future study with a larger number of horses, distance traveled could be compared with changes in electrolyte concentrations to determine whether an association exists between these variables.
A final weakness of the present study that should be acknowledged was the lack of data concerning administration of electrolytes and water during transport and prior to the start of the ride. For this study, the horses originated from many locations and were entered in an actual competition, during which it would not be possible to force riders to standardize electrolyte supplementation. The study could be repeated under experimental conditions to determine the importance of electrolyte supplementation on the observed changes.
In conclusion, the present study confirmed our hypothesis that significant changes in electrolyte concentrations can occur in endurance horses prior to the start of a competition. Changes were identified both during the transport period and when horses were stabled overnight before the event. Additionally, a lower bicarbonate concentration following a steep climb during the competition was associated with later elimination. This represents an area for future research and could be a novel way to identify endurance horses that are starting to show signs of distress.
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