Pleasure horses with neurological signs: EHV1-infection or mycotoxin intoxication?

In the course of several days most of the 40 riding-school horses turned ou t in paddocks developed ataxia of variable severity. Five of these horses s howed severe ataxia and tremors, became paralyzed and were euthanized Eleve n privately-owned horses which were stabled on the same premises showed no clinical signs. The most likely diagnosis seemed to be the 'neurological fo rm of EHV1', although the signs were not entirely typical. A few weeks late r a second outbreak occured among the riding-school houses and one of the p rivately owned horses also showed signs of ataxia. In the meantime it had been shown that EHV1 titers in paired serum samples had not increased and that the cerebrospinal fluid of one of the severely a ffected horses was normal. Toxicological examination of hay, delivered just before the first outbreak and stored for the winter, showed a significantl y increased concentration of lolitrem B mycotoxin (5-6 mg/kg). The hay appe ared to have been made of ryegrass used for lawns and playing fields. Retro spectively it became probable that this hay occasionally been fed to the ho rses just before the onset of clinical problems. It is concluded that the h orses showed the 'ryegrass-stagger syndrome.