Pathological observations on an outbreak of paralysis in broodmares

A description is given of the pathological changes present in 8 mares which died or were euthanased in the course of an outbreak of paralysis on a Thoroughbred studfarm. In all cases the principal changes were in the central nervous system (CNS) and consisted of a severe and widespread vaculitis in the brain, cord, sheaths of nerves, capsules of ganglia and occasionally elsewhere in the body. Associated with the damage to vessel walls there was haemorrhage and exudation of plasma into the perivascular tissues. Involvement of neurones was minimal and no neuronophagia was present. Six cases had malacic foci in the brain, related to thrombosis of local arteries. Acinar necrosis in the mammary gland was present in one case. Examinations of the CNS and cerebrospinal fluid (CSF) for the presence of equid herpesvirus 1 (EHV1) or other microbial agents proved negative but EHV1 was isolated from the spleen of one mare and from the pharyngeal mucos membrane in another. The pathogenesis of this type of paralysis is discussed.