Unexplained sudden death is thankfully rare in the horse, but it can be devastating when it does happen.

There is an important distinction to be made between a horse found dead and a genuine sudden death. A horse found dead may have had undetected yet serious or significant signs of illness or injury prior to the moment he actually died. A genuine sudden death is when a closely observed and previously healthy horse dies. In such circumstances, it can be remarkably hard to find the cause of death, unless there are obvious clues such as scorch marks associated with lightning strike.

When a horse dies in competition, news travels fast via social media and headlines can follow — indicating the high emotional and economic impact of an equine fatality. Aside from the trauma to all concerned, the safety of the rider and the public perception of animal welfare during equestrian events must be considered. More knowledge of and awareness about sudden death will help vets, officials and event organisers minimise potential risks and hopefully reduce future incidents.

Accurate information and statistics on this subject are limited. There are plenty of anecdotes yet few firm facts and figures — partly because one is dealing with a rare event. One analysis of sudden death and fatal musculoskeletal injury during FEI three-day eventing reported a prevalence of 5.8 deaths from 10,000 starts. Horses that sustained a catastrophic orthopaedic injury, trauma or fall were not included as true sudden deaths, since they died from known causes.

Who’s at risk?

More recently, studies involving the Swiss Equestrian Federation have been published, looking at sudden death in horses during and immediately after exercise.

Researchers from the USA and Switzerland investigated the details of cases in nine countries in North America and Europe.

They focused specifically on sudden death in sport horses, including only those that died during exercise or within an hour after finishing. Information was obtained through more than 100 owner/vet-completed surveys, 57 of which contained sufficient information to be included.

Findings showed that more than 70% of the horses died during exercise, the others collapsed afterwards. Most died during (or just after) training or pleasure riding rather than during competition.

From these reported cases of sudden death, 40% were event horses. While some died during or following the cross-country phase of an event, more than half died during or just after a training session outside of competition. There was a suggestion that the larger number of eventers may have been due to selection bias, with a greater response received from the eventing community.

It is easy to speculate that there is more risk of eventing sudden deaths as eventers tend to work at a greater intensity, but it cannot be proven from this survey that eventers are a higher risk group.

Statistical analysis also showed that deaths occurred at all levels, although they were more frequent in horses competing or training at more advanced levels. The authors emphasised that any horse can die suddenly during or after athletic activity: the remaining equine fatalities included nine leisure horses, eight showjumpers, five dressage horses, four hunters and four polo ponies, a few Western horses and one endurance athlete.

A third of the horses had previous medical problems, but few had issues, such as recognised heart conditions, that would stand out as being risk factors for sudden death. Most showed no signs of problems before the incident, except slowing down, disorientation or unsteadiness in a small number of cases.

Frustratingly, a definitive diagnosis for the cause of death was available in just 28% of cases. A cardiovascular cause involving the heart or circulation system was the most frequent explanation, followed by lung disease, although it is hard to be sure when relying on questionnaire data.

Subtle signs

The rupture of a major blood vessel, typically the aorta, has previously been blamed as a common cause of exercise-related death in horses — a theory backed up by these survey results.

The aorta is the largest blood vessel, carrying oxygenated blood from the heart to the rest of the body. The walls of the aorta, normally strong and tough, can develop a weak or thin area that ruptures, resulting in fatal internal bleeding. Although this sort of catastrophe is almost impossible to predict in advance, the fact that several of the horses that died were reported to be “less energetic than usual beforehand” suggests that any untoward signs should be checked out. Vets cannot always detect the possibility of sudden death in advance, but we can rule out some risks.

Proper postmortem (PM) examinations are not always performed after equine sudden death, perhaps because of a lack of nearby facilities or for financial or emotional reasons, but it makes sense to try to find an explanation. A PM is mandatory for any equine fatality at any competition under FEI jurisdiction, and is likely to be necessary if the horse is insured.

A meticulous PM involves a trained veterinary pathologist with the facilities to investigate in detail, especially to rule out foul play. This means sending the horse’s body to a vet school or equine hospital with the know-how to detect crucial tiny clues and run the right tests.

There is always the possibility that previous treatment or illness may be linked — as in the alarming case of at least 21 polo ponies in the USA, who received what was thought to be a boosting vitamin-type injection, but subsequently suddenly collapsed and died on the polo grounds.

Horses do not suffer heart disease as humans do, but heart issues cannot be ruled out if no other cause of death is found.

Heart damage may be obvious — for example, if a valve within the pump mechanism tears.

The heartbeat is basically an electrical impulse, however, so if this just stops, there will be nothing to see.

Occasionally, there may be subtle damage to the heart tissue. Blood tests during PM are not particularly informative, but this may change with modern technological advances.

Prevention of sudden death is the aim. Regular health reviews for any horse, including a thorough veterinary examination of the heart and lungs with a stethoscope, are potentially beneficial — especially if there is a change in fitness levels, more obvious fatigue or any other issues, such as lameness.

If you don’t look, you don’t find, so a proper check makes sense if you are in any doubt about your horse’s health.

Ref Horse & Hound; 26 July 2018