Table 3: Risk and protective factors for the development of atypical myopathy.

Risk factorsProtective factors

Demographic data
 AgeYoung horses (<3 years)
 Sex*(i) Colts 
(ii) Stallions
 Body condition(i) Thin 
(ii) Normal weight
(i) Overweight

Management practices at the horse level
 DewormingFrequent deworming
 VaccinationRegular vaccination
 PasturingFull-time pasturing (all year round)(i) Not at pasture 
(ii) <6 H per day at pasture 
(iii) Weather-dependant pasturing in spring and in autumn

Food and water
 Supplementary feedsHay in autumn(i) Supplementary feeds in autumn (except hay), in particular silage and complete mix 
(ii) Supplementary feeds in winter, in particular straw, silage, complete mix, and corn 
(iii) Supplementary feeds in spring 
(iv) Supplementary feeds in summer 
(v) Salt block (all year round)
 Water supply(i) Distribution network 
(ii) In tank or bath tub

Pasture characteristics
 HistoryDead horses on the pasture in the past
 Grass landLush pasture in winter
 Incline(i) Sloping pasture 
(ii) Steep slope
Gentle slope
 Trees(i) Surrounded by or containing trees 
(ii) Presence of dead leaves 
(iii) Presence of dead wood
 Humidity(i) Humid pasture 
(ii) Pasture surrounded by or containing a stream/river

Management of the pastures
Spreading of manure

*Age was identified as a confounding variable regarding the risk factors associated with the animal’s sex. Sex is linked to age, as “gelding” is a status that follows castration, a procedure usually performed in males >18 months old (i.e., the most risky age for AM). Because females are more frequently kept at pasture than males, a higher percentage of AM cases is females.