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.
EPIDEMIOLOGY, RISK FACTORS, AND SUSPECTED CAUSES
Historical Aspects
Disease outbreaks and case descriptions highly suggestive of AM have been reported for at least 70 years in northern Wales, United Kingdom (UK), and possibly even longer, because old veterinary textbooks describe a condition of “enzootic hemoglobinuria with dysphagia.” The occurrence of outbreaks, however, has markedly increased, at first in the 1980s in the UK, and in the last 10 years in western, middle, and northern European countries, where the disease was previously unknown. Case reports in the literature indicate that the same or a similar syndrome is also observed in the United States, Canada, and Australia.
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.
Specific Environmental and Individual Risk Factors
All horses in which AM was confirmed were or had been at pasture for most or all of the time. Affected horses usually had little or no supplemental feeding, although feeding hay in autumn has been identified as a risk factor. Atypical myopathy has developed in horses on pastures of various types and quality. Typically, however, pastures were stripped of grass and often bordered by trees, which may result in branches and dead wood falling onto the fields during storms. Accumulation of dead leaves in a pasture has been identified as a risk factor, as have slopes, humidity, proximity to a waterway, and mechanical spreading of feces.
Reports in the literature suggest that atypical myopathy may affect horses of various breeds. So far, no clinical cases in other equidae have been documented, but subclinical AM has been observed in donkeys. There is no sex predilection. Although horses of a large age range may be affected, young animals (less than 3 years of age) are at higher risk. Typically, affected horses are in fair body condition. Although thin individuals may also be affected, AM is not a disease of neglected or malnourished animals.
Further Risk Factors and Suspected Causes
Plant intoxications have been repeatedly investigated as a possible cause of AM, but no rhabdomyolysis-inducing plants have been found in pastures with confirmed cases in Europe. Toxins such as ionophores and tremetol have not been detected, and suspected cofactors, such as nitrites and other pollutants, have not been confirmed. There is no evidence that decreased antioxidant status plays a role because nonaffected pasturemates have had selenium concentrations similar to those in horses with AM. An unidentified viral agent and a genetic disorder have been proposed as possible causes or cofactors, but there is presently no direct or indirect evidence to substantiate these possibilities.
On the basis of epidemiologic findings and identified risk factors, an ingested or enterically produced toxin seems the most probable cause of AM. Accumulation of dead leaves in a humid environment provides a favorable environment for mold development and could lead to the production of mycotoxins. In particular, a potential causative role for trichothecene mycotoxins has been proposed.
Ingesta and fecal samples of affected horses have been investigated for the presence of Clostridium spp. Preliminary results indicate that Clostridium sordellii and Clostridium bifermentans producing lethal toxin may play a role in AM as a contributory or causative factor.
DIAGNOSIS
Atypical myopathy is suspected and diagnosed clinically on the basis of a typical history of a horse or horses affected while at pasture, especially during autumn and spring, with clinical signs of stiffness, muscle weakness, tremors, myoglobinuria, and laboratory findings that include markedly to highly elevated serum creatinine kinase activity. Diagnosis is confirmed postmortem by demonstration of acute severe rhabdomyolysis, predominantly in postural and respiratory muscles.
History
Being turned out to pasture, typically for 12 hours per day or more, is one of the most important and consistent findings linked with AM. Humid and cool conditions and windy weather are reported as possible triggering factors. Premises with affected horses are often sloping and contain a swampy area or waterway. There is no breed or sex predilection. Young horses are more frequently affected than mature horses, but AM has also been confirmed in old horses.
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