The clinical signs of atypical myopathy appear peracute. The rare premonitory signs described by owners consist of :
- decrease of appetite
- signs of colic
- stiffness or lameness.
Horses presenting atypical myopathy suffer from severe, generalised weakness and are frequently found in lateral recumbency. Sometimes, they are found dead on pasture without showing any premonitory signs. Signs of colic can be present, most probably due to the frequently observed difficulties to urinate. The distension of the bladder could indeed explain the signs of colic that usually disappear once the veterinarian empties the bladder. However, colic sign unresponsive to bladder emptying may indicate coexistence of abdominal colic and atypical myopathy.
Horses that are still able to walk present with stiffness, particularly visible at the hindquarter. It is important also to check other horses present on the same pasture for stiffness, because it can be an announcing sign of atypical myopathy.
In general, horses suffering from atypical myopathy are depressed. Often, they show difficulty or even inability to get on their feet and/or to stay standing. However some horses remain standing up. Muscular tremor and localized or generalized sudation can be observed. In spite of the severity of the clinical signs, often affected horses still want to eat (try to grasp any hay or grass that is near their mouth). No clinical signs have been reported related to troubles of the central nervous system (the horse doesn’t seem to be disconnected from his environment), only occasionally some paddling has been observed. This might be interpreted as a sign of suffering or/and anxiousness and/or desire to get up. It is important to mention that compared to the intense suffering of exertional myopathy, atypical myopathy seems less painful.
The emission of dark coloured urine is probably the most specific clinical sign of atypical myopathy. If possible, collect a urine sample to show your vet to help him make a correct diagnosis.
The mucosae are congested (abnormally red) or less often, cyanotic (purple). Taking the rectal temperature may reveal (∼ 30% of cases) a severe hypothermia (less than 36°C, whereas it is between 37 and 38.5°C in a healthy horse). This hypothermia is probably a consequence of the fact that the horse is laying on the pasture, not able to move or get up, often in the cold (most of the cases occur in autumn or spring, during cold nights). About 10% of cases present with hyperthermia (> 38.5°C)
Most horses show difficulties to breathe that worsen over time. Their heart rate is often increased (> 60 beats/min compared to 40 beats/min for a normal resting horse), and their respiratory rate sometimes also (> 15 respirations/min). Most of the time, the gut sounds are normal.