Atypical Myopathy

CHAPTER 137 Atypical Myopathy

Atypical myopathy (AM), also termed atypical myoglobinuria or, occasionally, pasture myodystrophy, manifests as peracute to acute rhabdomyolysis. AM affects grazing horses in autumn and spring under humid, cool, and sometimes windy conditions. In recent years, it has been observed in several European countries with increasing frequency of outbreaks. A retrospective study of horses with nonexertional pasture myopathies in Minnesota suggests that AM might also affect horses in the United States. Predominant clinical signs are weakness, recumbency, muscle stiffness, tremors, sweating, and myoglobinuria. High serum muscle enzyme activities confirm severe muscle damage, which is mainly localized to the postural and respiratory muscles and occasionally also involves the myocardium. Histologic evaluation of these muscles reveals multifocal degeneration, necrosis, and increased intracellular lipid content. The mortality rate is high, and the cause remains unknown. Epidemiologic characteristics suggest that the condition is induced or precipitated by factors associated with pasturing and likely caused by an ingested or enterically produced toxin or toxins.


Seasonal and Regional Patterns

So far, all confirmed cases of AM in Europe have arisen in the fall or early winter and again in early spring. Outbreaks are frequently associated with distinctive climatic and meteorologic conditions: wet, slightly frosty, and sometimes windy weather. A retrospective epidemiologic study of cases in Belgium that were confirmed as being AM revealed that average October sunshine durations were lower during outbreak years. Furthermore, most (78%) confirmed cases were observed on days with minimum daily air temperature between 0° C and 8° C, and on 80% of these days, average relative humidity was greater than 90%. On most of the 10 days preceding an outbreak, less than 1.5 hours of sunshine per day was recorded, with an excess of precipitation or relative humidity, but no frost. Only two of 57 confirmed cases in Belgium were observed on days with minimum daily air temperatures below 0° C.

Locations of outbreaks range from individual fields to large geographic regions. Although these seasonal and regional patterns are distinct, the individual occurrence of outbreaks is unpredictable. Horse grazing pastures on which cases have occurred have an increased risk, but large outbreaks have occurred in regions where the disease was previously unknown. Furthermore, the incidence in such regions may vary considerably from year to year, with large outbreaks being followed by years in which there are few or no cases, and then by years in which outbreaks are seen again.

Some clinicians have the impression that there are fewer cases in a given region and that the clinical picture is overall less severe in years following the first outbreak. Epidemiologic data do not confirm this notion, however, but indicate that subsequent outbreaks can be just as devastating as initial outbreaks.

May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Atypical Myopathy
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