Management, Training and Other Less Common Causes of Poor and Loss of Performance in Sports Horses

In the absence of obvious clinical signs of injury or disease, the investigation of poor performance or loss of performance in the performance horse can be challenging. Any investigation would of course involve taking an accurate history and a detailed clinical examination of the animal concerned followed by appropriate diagnostic investigation. However, when this fails what other steps can a veterinary surgeon take in order to try and identify the underlying problem or problems?

The first step in a more detailed investigation is to try and ascertain if the situation involves a true loss of performance or poor performance. In the case of poor performance, a horse is reported by the owner, trainer or rider to be performing below their expectations, based primarily on breeding and confirmation. Previously less common but now beginning to be encountered more frequently, the classification of poor performance may also be made based on objective measurements of gait, cardiac or locomotory muscle characteristics or measurements of heart rate and blood lactate during standardised exercise tests. Unfortunately, in many cases poor performance due to lack of ability is the correct but an unpopular diagnosis. Studies of the heritability of performance in horses in a variety of disciplines most commonly show a value of around 0.1 to 0.2. If the performance of the offspring could be perfectly predicted from that of its parents then the value would be 1, and 0 if there was no relationship. In one recent study of sport horses in Germany a heritability value of 0.61 was reported. This appears to be the highest value reported to date.

For a horse to be classified with “loss of performance”, it should previously have been performing more successfully than at the time of referral or investigation. For a sport horse with several years competition records this may be relatively straightforward to confirm. However, in the case of horses that have only been competing for les than a year, it is possible that the previous high performance is not truly representative. This may especially be the case if the horse in question had been competing against poor class horses.

Another important step to take is to try and ascertain if the problem affects only the horse presented or whether other horses may also be affected, but simply not be presented as they appear healthy. Furthermore, horses not being competed are unlikely to be presented for loss of performance.

Routine haematology and biochemistry can be extremely helpful in these investigations, but even more so when good baseline values are available when the horse was considered healthy and performing well. Good practice for obtaining reliable results from blood samples is to collect pre-feed and pre-exercise, usually early in the morning. In addition, care should also be taken to try and standardise for the previous days exercise as this can result in differences in a number of measurements, particularly CK/AST. [...]