Pathological changes seen in horses in New Zealand grazing Mediterranean tall fescue (Lolium arundinaceum) infected with selected endophytes (Epichloë coenophiala) causing equine fescue oedema

AIM: To investigate whether Mediterranean tall fescue that is infected with novel fungal endophytes causes disease in horses when grown in New Zealand. METHODS: Seven horses were used in the experiment. Two were fed Mediterranean tall fescue that was infected with AR542 novel endophyte, two horses were grazed the same fescue that was endophyte-free and three horses grazed Mediterranean tall fescue that was infected with AR584 novel endophyte. Repeated blood samples were taken to detect changes in serum total protein and haemocrit. Any horse showing clinical evidence of disease was subject to euthanasia and necropsy. RESULTS: Within 6 days, both horses that were grazing fescue that was infected with AR542 became depressed and lethargic. After 5 days of grazing fescue that was infected with endophyte AR584, one horse was observed to be depressed and the other died shortly after being removed from the AR584 pasture. While the third horse in this group did not develop clinical signs within a 5-day feeding period, haemoconcentration and hypoproteinaemia was consistently detected in all horses that grazed Mediterranean tall fescue that was infected by AR542 or AR584 endophyte. No abnormalities were observed in horses grazing fescue that was endophyte-free. Necropsy of the 3 horses that had been subject to euthanasia revealed oedema within the gastrointestinal tract. Histologically, the oedema was accompanied by large numbers of eosinophils, but no necrosis. CONCLUSIONS: Horses grazing Mediterranean tall fescue that was infected by AR542 or AR584 consistently had reduced serum total protein and haemoconcentration. The toxic compound remains unknown but may result in leakage of plasma proteins into the gastrointestinal tract. Why horses appear uniquely susceptible to this toxin remains unknown. CLINICAL RELEVANCE: Mediterranean tall fescue that is infected by AR542 or AR584 should not be fed to horses. This intoxication should be considered in horses in which a rapid onset of hypoproteinaemia and haemoconcentration is detected. This intoxication should also be considered if marked gastrointestinal oedema is observed during necropsy.