RESUMEN
Morphological information is presented for syntype material of the etiologic agent of equine protozoal myeloencephalitis, Sarcocystis neurona. A clinical description of the horse from which the organism was isolated and the methodology used to immunosuppress the horse in an attempt to increase parasite numbers are also given. The description includes microscopic details observed both with light and transmission electron microscopy. Mainly stages from tissue are illustrated, but information is also presented on the development of the organism after inoculation onto monolayers of bovine monocytes. It is believed that the large numbers of organisms observed in this horse were due to its having not received prior treatment with trimethoprimsulphonamide and the large amounts of corticosteroids that were administered in order to facilitate isolation of the pathogen.