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
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.