Sycamore poisoning is a term which is commonly used to refer to atypical myopathy. The term is something of a misnomer, as there are other ways a horse might contract it than from the sycamore tree. That said, the disease’s scientific name now seems equally inaccurate, as the disease is no longer as atypical as it once was. Since the less scientific term is the easier of the two to remember, it’s achieved traction among equestrian circles – but new evidence suggests that another term might well be more appropriate.
Among such circles, there are more reports of the condition than ever before. This increase might be due to an increased awareness of the problem rather than an increase in its actual prevalence – nevertheless, it’s worth horse owners acquainting themselves with the symptoms of the condition and what can be done to guard against it.
What causes sycamore poisoning?
The precise cause of sycamore poisoning is a matter of some debate. One probable clue to the disease’s source comes from a fruit that triggers a disease with which the disease is associated, Jamaican vomiting sickness, which is often triggered after an animal ingests the ackee fruit. Thanks to modern botany, we have quite a detailed picture of the chemicals contained in such fruits, and the relationships they bear with the plants we find in a typical horse’s environment.
The investigation was rapidly drawn to hypoglycen A, a poisonous amino acid found in the Ackee fruit. Sure enough, the chemical was then found in box elder trees around the sites of several outbreaks of sycamore poisoning in the US. The theory has been further supported by subsequent studies on both sides of the Atlantic, and we can conclude with some certainty that hypoglycen A is the cause of the disease.
When it finds its way into a horse’s body, it begins to interfere with the production of energy in the animal’s muscles, preventing the mitochondria there from properly oxidising fatty acids, and preventing fatty acids from being transported. The muscles affected include those found in the cardiac and respiratory systems, and thus the disease is extremely serious.
What risk factors are there?
Since the sycamore’s seed is designed to travel a long way as it falls to earth, the chance of it finding its way into a paddock is much higher than that of other seeds. Consequently, the vast majority of cases of sycamore poisoning in Europe have been the work of the tree after which the disease is named.
That said, there are several other trees which contain hypoclycen a – including maple trees. It is estimated that a horse can develop toxic symptoms after eating anywhere between 32 and 9,000 seeds – but there is tremendous variation among horses in terms of toxicity level.
The disease is at its most prevalent during wet and windy times of year – though, for reasons we’ve already mentions, the reason for this is more likely the latter than the former. Fallen leaves and dead wood on the ground will also heighten the risk, as will a sloping pasture. It will come as no surprise, then, that the overwhelming majority of cases occur in the autumn, when horses are grazing in the open in paddocks with deciduous trees nearby.
Diagnosing the problem
Horses which are at risk will display several signs before they develop severe mypopathy. These signs are all associated with muscle weakness. They include sweating, tremors and lethargy. If the horse is also passing dark red/brown urine, then this all but confirms the presence of the condition – as there are few other conditions which cause this symptom. A vet will be able to perform laboratory tests on a horse’s muscles. The main indicator is the enzyme creatine kinase, which will rapidly increase as soon as the myopathy commences.
As you might expect, the prognosis for horses diagnosed with sycamore poisoning is invariably bleak. Depending on the study you believe, anywhere from half to a quarter to just 3% of horses survive. We should treat these findings with scepticism, however, only the most effected horses will be submitted for treatment.
Since it’s very difficult to tell whether a horse has a good chance of surviving or not, we should endeavour to be as proactive as possible when treating the condition. If the horse is to survive, then they will generally begin to recover three days after contracting the condition.
If you know what you’re looking for, however, then determining the horse’s chances of recovery is markedly easier. If a horse is able to stand and pass faeces, and the mucous membranes appear normal, then the chances of a recovery are generally good. It’s a good idea to monitor the creatine kinase levels of the horse, too – as a reduction will tend to precede a full recovery.
Treating the condition is an enormously taxing endeavour – both physically and emotionally. Affected horses will need constant care, and ideally transport to a dedicated facility. Wherever the treatment takes place, it will typically be multi-pronged and extensive.
Horses will need to be administered with heavy painkillers. Fluid therapy may also be necessary, as the horse will be unwilling to drink. The fluid will need to be carefully judged on a case-by-case basis, as some horses will need more than others. Horses will need to be fed a maintenance diet as they recover, and given respiratory support. While the horse is lying prone, they will need to be regularly turned in order to prevent muscle sores from developing. Some vets may suggest a course of relaxants – but this is controversial, as it may exacerbate the condition.
As we’ve seen, treating this condition is hugely difficult, as not a great deal is known about it. With more study will come better methods and a better prognosis for effected horses, but until that time comes, the best approach is to prevent your horse from contracting the condition in the first place. Do this by minimising your horse’s exposure to trees, frequently checking your paddock for fallen leaves, and reducing stocking density during autumn – so that your horses are all able to access the grass they need.