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 (Lolium arundinaceum (Schreb.) Darbysh. (syn Festuca arundinacea)) infected with selected fungal endophytes (Epichloë coenophiala (formerly Neotyphodium coenophialum)) caused equine fescue oedema when grown in New Zealand, and to examine the pathological changes associated with this intoxication.

METHODS: Horses were grazed on Mediterranean tall fescue that was infected with the endophytes AR542 (n=2), or AR584 (n=3), or Mediterranean tall fescue that was endophyte-free (n=2). Blood samples were taken up to 7 days after the start of feeding to detect changes in concentrations of total protein in serum and packed cell volume. Any horse showing clinical evidence of disease was subject to euthanasia and necropsy.

RESULTS: Within 6 days, both horses grazing fescue infected with AR542 became depressed and lethargic. One horse grazing fescue infected with endophyte AR584 became depressed within a 5-day feeding period while another horse in this group died shortly after being removed from the AR584 pasture. The third horse in this group did not develop clinical signs within the 5-day feeding period. However, haemoconcentration and hypoproteinaemia was detected in all horses grazing Mediterranean tall fescue that was infected by AR542 or AR584 endophyte. No abnormalities were observed in horses grazing fescue that was endophyte-free. Necropsy examination was performed on two horses grazing fescue infected with AR542 and one horse grazing fescue infected with AR584. All three horses had marked oedema of 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 developed hypoproteinaemia and haemoconcentration, most likely due to leakage of plasma proteins into the gastrointestinal tract. This suggests that these selected endophytes produce a compound that is toxic to horses, although the toxic principle is currently unknown.

CLINICAL RELEVANCE: Mediterranean tall fescue that is infected by AR542 or AR584 should not be fed to horses in New Zealand. 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.