Join Date: Nov 2011
Location: Central Oregon, USA
Another vote for "bleeder" or EIPH (exercise induced pulmonary hemmorage) if it happens again. I had a bleeder who started bleeding when he was 19, so instead of treatment we semi-retired him (no more eventing) and managed his environment. Dust makes it worse, so we soaked hay and slightly dampened his stall bedding.
Like others said, it is treatable, too, and likely worth it for a performance horse still in their prime.
Or, it could have been a one-off incident, either a busted vessel, like humans get time to time, or a scrape in the nostril from a stem in the hay.