Discussion

Aims: To compare data of a large outbreak of equine atypical myopathy (AM) in the Netherlands, with literature described so far with the aim of increasing knowledge of aetiology, risk factors and possible preventive measures. Methods: Detailed information of 44 cases with a very high probability of being affected with AM and 10 AM confirmed cases during the period 9 October 2009 to 7 June 2010 was collected from veterinarians and horse owners of (suspected) AM cases through interviews with veterinarians and horse owners, medical records from veterinarians and standard surveys. Results: The mortality rate (74.5%) of this outbreak was lower than described in earlier studies (up to 90%) and new demographic (age, gender, breed) and geographical features have been reported. All affected horses had been on pasture before they developed the first clinical signs of AM. The Dutch Warmblood, the Icelandic Horse and the Friesian were affected with AM more often. Of 35 AM cases it was known that the horses where pastured near maple trees, of which in 15 cases the maples were known to be infected with the fungi Rhytisma acerinum. A sudden change in weather occurred in the days before horses started suffering from AM. Conclusions: Lower mortality can be explained by the fact that the syndrome is probably better known and recognised by veterinarians than before in the Netherlands. Young and old horses and specific breeds are often kept on pasture and therefore may have increased exposure to the aetiological agent. The weather conditions that were found could have caused more maple tree leaves to fall off trees and get infected with Rhytisma acerinum. Practical significance: Studying the aetiological, climatological and geographical features of an outbreak may give tools to further anticipate and prevent atypical myopathy in the future.