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Most owners have been in the position of wondering whether they should get “bloods” run on their horse.

This is part of a standard procedure in the sick animal, but when is it appropriate to take a blood sample from a performance horse? Which parameters will be evaluated — and what information can be gained?

Monitoring the general fitness of an equine athlete is important. Blood sampling may be one way to gain a greater insight into health or general fitness, but it is not possible to assess fitness truly from a sample obtained from a horse at rest. Blood profiles vary greatly between breeds, and also between horses being trained for different disciplines.

Because of this, it is a good idea to have a blood sample taken when your horse is healthy and competing normally, as a baseline. The timing of sampling, in relation to feeding and exercise, is also important — especially due to the ability of the spleen to release a huge reservoir of red blood cells in response to stress.

Once his blood profile is established, further sampling may be useful if you feel your horse’s performance is below par. Blood sampling may be useful in cases of athletic poor performance, lethargy, reduced appetite or change in demeanour. Sampling is of importance in a horse who travels often or over long distances.

In the red

Blood profiles seem to fall within a very narrow range in a group of optimally performing horses. Measures of assessing optimal performance become much less accurate when we move away from the racehorse — particularly when we look at the issue of blood cell counts.

Thoroughbreds have a higher red blood cell (RBC) count, as do horses used for sports demanding short bursts of energy. Endurance horses, on the other hand, which are usually Arabian, have lower RBC levels. These athletes tend to struggle most with dehydration issues, so a high amount of RBCs would make their blood more viscous (thicker) and could be detrimental to health.

In general, low RBC counts may reflect anaemia which, if it is detected, can be corrected with supplements.

White blood cell (WBC) counts can vary due to breeding and age, which makes drawing comparisons difficult. Those slightly outside the normal range of 6-10×10°/L are generally not significant — a healthy adult sport horse’s WBC count can be as low as 4-5×10°/L.

These huge variations between gender, breed and age group make it all the more important to have a baseline blood sample.

Infection or inflammation may be present without clinical signs and can lead to poor performance. While stand-alone WBC counts are not a good indicator of the presence of infection, this information can be used in conjunction with measurement of other blood components.

The combined findings of markers such as WBC count, serum amyloid A (SAA) in acute cases and fibrinogen in chronic cases, can be easily measured to detect problems. This is particularly useful in revealing sub-clinical respiratory infections, significant in young racehorses. Alterations in inflammatory profiles must be followed up to determine the exact source of the problem, so that it can be managed accordingly.

While bloods are a poor marker of electrolyte status, electrolyte abnormalities such as potassium, sodium and calcium imbalances are not usually an issue in sport horses. The exception may be those competing in endurance events, but what is lost in sweat is easily replaced orally. A paired sample of blood and urine may be necessary to interpret electrolyte status.

The muscle enzymes creatine kinase (CK) and aspartate aminotransferase (AST) are well understood and easy to measure. CK rises and declines quickly after muscle damage, whereas AST does so more slowly. It may therefore be possible to assess the age and activity level of a myopathy (muscle disease).

Blood sampling is recommended if a horse appears stiff or sore after exercise, or is showing signs of “tying-up”. These can range from general poor performance to heavy sweating, muscle tremor or muscles that are hard to the touch, red-brown urine or extreme pain.

For meaningful results, especially in mild cases, accurate timing of sampling is critical. Samples should be taken at a resting baseline and then at one-, four- and 24-hour intervals post-exercise.

Muscle enzyme profiles tend to be more variable in the unfit horse, and results in horses with a myopathy generally fall above the normal range by several thousands.

Some myopathies that cause tying-up may be caused by a genetic condition, such as polysaccharide storage myopathy (PSSM). Additional blood samples may then be submitted for genetic testing.

While liver failure is relatively rare in horses, liver disease is quite common due to the organ’s filtering and detoxifying role. Elevated liver enzymes and bile acids have also been implicated in poor performance.

Again, with such a broad range considered normal, baseline bloods may be useful — especially in horses that have had changes in management or feed, or those who travel. Elevated liver enzymes can be the result of contaminants in feed and roughage and are often caused by toxins.

Low-grade alterations are of little concern and tend to correct themselves quickly, but may be aided by alterations to diet and oral supplements. If liver parameters are greatly elevated, or persist at high levels, further tests such as liver biopsy should be performed to determine the cause.

The bigger picture

Blood sampling may be useful in horses competing in all disciplines and at all levels, but results must be interpreted with caution. The reference range is broad and a healthy horse’s results may well fall outside of these parameters. Abnormal results should be followed up with further diagnostics and evaluation, as they are rarely useful in isolation and are at high risk of over-analysis.

There may be some application for using blood samples in monitoring training progress, as well as part of poor performance or exercise intolerance investigations. There is little science or consistency, however, to reflect the value of this.

Baseline information is hugely helpful in understanding what is normal for your horse. Blood test results can then be compared with these findings during times of poor performance and sickness, so that causes can be managed accordingly.

Ref Horse & Hound; 6 July 2017