Contribution to the understanding of unexplained death of pasturing horses in the North of Spain: a retrospective case series

The aim of this work was to study a retrospective case series of acute degenerative myopathy, which caused high mortality in adult horses grazing in a specific region of Spain, in relation to a possible etiopathogenesis. Outbreaks of myopathy occurred in December of 1999, 2003, 2009, and 2011 after an abrupt fall in temperatures and the first snowfalls. Ten horses were necropsied, and an exhaustive gross and histopathological examination was performed using specific histochemical stainings to evaluate muscle and cardiac damage. Intense myodegeneration, affecting the postural and respiratory muscles and the muscles involved in swallowing, was the main finding. A further consistent finding was necrosis of Purkinje fibers in the myocardium. Serum concentration values of creatine kinase, selenium, and vitamin E, as well as blood concentration values of glutathione peroxidase activity, were determined in 10 horses clinically affected and in another 12 horses with no clinical signs. In the affected horses, creatine kinase was high and reached values over 10,000 U/L. Selenium values were deficient in the horses affected, ranging from 4.2 to 10.5 mu g/g (reference range: >50 mu g/L) in selenium-untreated horses, and also, selenium values in 11 horses with no clinical signs were below the reference range. Glutathione peroxidase activity was below the reference range in all the analyzed sera, and vitamin E values were also below the reference range in four horses. The severe selenium deficiency together with a sudden arrival of unfavorable weather conditions seemed to provoke the arising of this disease. The presence of Eupatorium cannabinum in this valley is discussed as a possible source of the phytotoxin associated with rhabdomyolysis.Ministerio de Educación: AP2007-02007Peer Reviewe