Clinical expression of perennial ryegrass (lolitrem-B) intoxication in New Zealand horses : a thesis presented in partial fulfillment of the requirements for the degree of Master of Veterinary Science at Massey University, Manawatu, New Zealand

Abstract

Perennial ryegrass staggers (PRGS) is a neurological mycotoxicosis caused by the ingestion of lolitrem-B. In this study, seven horses split into two separate groups were exposed to lolitrem-B by feeding them perennial ryegrass seed and hay containing 2 ppm lolitrem-B. Paired data was collected prior to and after two weeks exposure to lolitrem-B including video-documented neurological examination, clinical examination, brainstem auditory evoked (BAEP) and magnetic motor evoked (mMEP) potentials, blood and cerebrospinal fluid, and a frusemide challenge. All horses developed tremor when exposed to lolitrem-B. The degree of tremor varied between individual horses and also depended on the level of activity, increasing during feeding and exercise. Using an ophthalmoscope a subtle, rapid (~5 Hz) tremor of the eyeball was detected in six of the seven horses. Subtle signs of ataxia were observed during handling, and motor dysfunction was exaggerated when blindfolded. Ataxia primarily involved a truncal sway and irregular, but predictable, limb placement that compensated for the lateralisation of the center of gravity. Results indicate that lolitrem-B may lengthen the peak V latency of BAEP traces. mMEPs also showed a lengthening in take-off latency and peak latency. The frusemide challenge revealed that renal K+ secretion was impaired significantly (p = 0.003) during the first 15 minutes after frusemide administration. During the treatment period resting heart rate increased significantly (p = 0.018) but stayed within normal values. No relevant changes were observed in respiration rate, rectal temperature, gastrointestinal auscultation or complete blood count, while changes in serum biochemistry require validation. No change was detected in urine lolitrem-B levels and although plasma lolitrem-B increased during the treatment period, levels did not correlate with the severity of clinical signs displayed. This study provides a clearer appreciation of the clinical signs and variability of perennial ryegrass intoxication in horses. The clinical effects of lolitrem-B intoxication in horses primarily involve action-related tremors and symmetrical vestibular ataxia. Results from the frusemide challenge indicate that lolitrem-B disrupts renal large-conductance Ca2+-activated K+ channels, indicating a potential diagnostic avenue. Further research is required to establish the significance of increased mMEP and BAEP latencies.