European outbreaks of atypical myopathy in grazing horses (2006–2009): Determination of indicators for risk and prognostic factors

Abstract

Appropriate management of atypical myopathy (AM) requires the establishment of an accurate diagnosis and prognosis. Furthermore, preventive measures to avoid AM need to be refined. The aims of the study were as follows: 1) to improve the diagnosis of AM; 2) to identify prognostic predictors; and 3) to refine recommended preventive measures based on indicators of risk factors. An exploratory analysis of cases in Europe between 2006 and 2009 reported to the Atypical Myopathy Alert Group was conducted. Based on clinical data, reported cases were allocated into 2 groups: confirmed or highly probable AM (AM group; further divided into survivors and nonsurvivors); and cases with a low probability of having AM or with another final diagnosis (non-AM group). Using Welch's test and odds ratios corrected for multiple comparisons, the AM vs. non-AM groups were compared to identify indicators for diagnosis and risk factors, and survivors vs. nonsurvivors in the AM group were compared to identify prognostic factors. Sensitivity, specificity and positive and negative predictive values were calculated for specific clinical signs related to final diagnosis and outcome. From 600 reported cases, 354 AM cases (survival rate of 26%) and 69 non-AM cases were identified, while there were insufficient data to categorise the remainder. Variables valuable for diagnosing AM compared with similar diseases were as follows: presence of dead leaves and wood and/or trees on pastures; sloping pastures; full-time pasture access; no food supplementation; normal body condition; pigmenturia; normothermia; and congested mucous membranes. Nonsurvival was associated with recumbency, sweating, anorexia, dyspnoea, tachypnoea and/or tachycardia. Survival was associated with remaining standing most of the time, normothermia, normal mucous membranes, defaecation and vitamin and antioxidant therapy. This study refines the list of risk factors for AM. Clinical signs valuable for diagnosis and prognosis have been identified, enabling clinicians to improve management of AM cases.