Winter worries: Atypical myoglobinuria

Galeforce winds followed by sunny days, knee-deep snow and torrential rain — the joys of a British winter. Stormy weather can result in cases of atypical myoglobinuria, a serious muscle disorder associated with a sudden deterioration in weather conditions.

Atypical means it is a strange disease that does not behave as expected and myoglobinuria means that there is muscle damage, the end products of which are passed out in the urine in a similar way to a horse tying up with azoturia. However, unlike azoturia, this condition occurs in unworked or minimally worked horses.

Seen sporadically in a spring or autumn associated with foul weather, the key features to remember are:

• It usually affects several horses or ponies in a group kept at grass with little or no supplementary feeding.
• Outbreaks tend to appear after a period of heavy rain, when the animals may have become cold and wet.
• There is a sudden onset of stiffness unrelated to exercise and it can rapidly progress to recumbency and death. The sick horses appear very weak and tend to collapse easily, indicating muscle weakness, but affected muscle groups are not swollen or painful.
• Affected horses do not seem to be in severe pain and are usually able to eat and drink normally, even if they do not have the strength to stand up. Their pulse, temperature and respiratory rates tend to be normal.
• A clear clue that something is wrong is that their urine is an abnormally dark red to chocolate brown colour.
• Blood tests showing the muscle enzymes to be sky high, will prove the diagnosis, confirming it is a muscle disease.
• Unfortunately, there is no miracle cure and only supportive treatment, such as putting the horse on a drip or administering painkillers, is available. In any group of cases, it seems horses are either severely affected or not at all. With present knowledge, the prognosis for affected horses is poor.

Fortunately, this condition is still rare, but it causes concern, particularly if a group of horses collapses, which makes people panic about poisoning. In fact, atypical myoglobinuria is not due to poisoning, but may be linked to a toxic fungal overgrowth that flourishes in foul weather.

If one is aware of it, steps can be taken to reduce the risks. These include protecting susceptible horses by providing shelter or bringing them in when foul weather is forecast. Alternatively, ensure that they have supplementary feed, such as plenty of hay, while out at grass. Make certain that their general health is good and that problems that may influence their general condition, such as worm infestation, are under control.

• This veterinary feature was first published in Horse & Hound (1 February, ’07)