Development of a model for investigation of perennial ryegrass toxicosis in sheep

Abstract

AIMS

To develop a clinical model of perennial ryegrass toxicosis (PRGT) based on feeding a known dose of lolitrem B and ergotamine, and to produce a consistent clinical presentation for assessment of disease pathophysiology, neurological changes and neurohistopathology.

METHODS

Male lambs, aged between 10–12 months, were randomly assigned to either Treatment (n=9) or Control (n=9) groups. Lambs in the Treatment group received feed containing a novel endophyte-infested perennial ryegrass seed, commencing on Day 0 of the Feeding phase with a low induction dose, then increasing after 3 days to provide 0.16 mg/kg live bodywight (LBW)/day of lolitrem B and 0.054 mg/kg LBW/day ergotamine. Lambs were examined daily and when defined signs of PRGT were observed they were transferred to the Testing phase. Neurological examinations, assessment of gait, surface electromyography (EMG) and mechanosensory nociceptive threshold testing were carried out and blood samples collected during both phases of the trial, with a full necropsy, histopathological examination and measurement of faecal cortisol metabolites (FCM) performed on Day 2 of the Testing phase.

RESULTS

Typical clinical signs of PRGT, including ataxia of vestibulocerebellar origin leading to stumbling, were observed in all Treatment lambs. The median interval from the start of the Feeding phase to entry into the Testing phase was 21 (min 18, max 34) days. Histopathological characterisation of neurological lesions included the presence of Purkinje cell vacuolation, pyknotic granular layer neurons and proximal axonal Purkinje cell spheroids. Lesions were most apparent within the vestibulocerebellum. Mean root-mean-square voltages from triceps EMG increased in Treatment lambs between Feeding phase Day 0 and Testing phase Day 2 (p<0.001). Daily water intake during the Testing phase for the Treatment group was less than in Control group lambs (p=0.002), and concentrations of FCM at necropsy were higher in Treatment compared to Control lambs (p=0.02).

CONCLUSIONS AND CLINICAL RELEVANCE

Lolitrem B and ergotamine dosing in feed on a live weight basis combined with neurological/gait assessment provides an effective model for investigation of PRGT and potential therapeutics. Assessment of gait changes using defined criteria and RMS voltages from EMG appear to be useful tools for the assessment of the severity of neurological changes.

KEY WORDS:

• Perennial ryegrass toxicosis
• lolitrem B
• tremor
• ataxia
• sheep
• faecal cortisol metabolites

Acknowledgements

The authors wish to thank Cate Chandler, Alice Birckhead, Emily Birckhead, the staff of the Veterinary Teaching Hospital and the Veterinary Diagnostic Laboratory, Charles Sturt University, for their excellent technical assistance during this study. Thanks to G Musk for assistance with nociceptive threshold testing and equipment, and Adam Hamlin for assistance with perfusion fixations.

JC Quinn was supported by funding from Meat and Livestock Australia and a Research Fellowship from the Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries). MD Combs was supported by a technical assistance grant from Meat and Livestock Australia.

ORCID

EJ Narayan http://orcid.org/0000-0003-2719-0900

Notes

1 WJ Mace, AgResearch, Palmerston North, NZ