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Summary

An investigation of 78 cases of typical Australian Stringhalt from 52 properties in Victoria was carried out from 1985 to 1987. Horses were either examined in the field (n = 52), referred to the Veterinary Clinical Centre (n = 13) or clinical details were obtained verbally (n = 13). In addition 10 cases of false or atypical stringhalt were examined. Detailed soil and pasture analysis was carried out on 14 properties where Australian Stringhalt had occurred. Information was also obtained on epidemiology of the condition from a survey of practitioners. Fifty of the 52 cases examined in the field occurred in horses that were dependent upon poor quality unimproved dry pasture. In all but a few cases, there was no pasture improvement or fertiliser application, leading to the development of weed-dominated pastures, particularly by flatweed, Hypochaeris radicata. The range of clinical signs exhibited by horses with Australian Stringhalt was described and a grading system proposed to classify horses according to severity of signs. Laryngeal abnormalities were present in 10 of 11 cases examined endoscopically and these horses exhibited increased electromyographic (EMG) activity in the long digital extensor muscle at rest and during hindlimb flexion. To a large extent, the EMG changes disappeared and digital extensor muscle atrophy improved in two horses that were monitored to recovery. Deep peroneal nerve conduction studies in four horses with Australian Stringhalt showed a substantial reduction in nerve conduction velocity and when stimulated at 50 Hz were unable to sustain activation of the long digital extensor muscle. EMG and evoked responses appeared to be sensitive indicators of the state of the disease. Spontaneous recovery over a period ranging from a few days to over 18 months occurred in 78 per cent (45 out of 58) of field cases available for follow up, although eight horses were destroyed on humane grounds or for pathological investigation.