AOral ulcers can occur due to a host of inciting causes including viral infections, caustic chemicals, mechanical irritants, poisonous plants, feed additives, grass seed contaminants in hay, sharp enamel projections on teeth, and foreign bodies. In this horse’s case, I shall assume that a primary dental problem has been ruled out. Most viral agents would have cleared within a few weeks, so while it’s possible that he could be suffering lingering signs of a viral infection, it is not likely.

As a 7-year-old, his canine teeth should be fully erupted and mature. However, some horses mature late in this regard, and it is possible that erupting canines could be causing irritation and resultant ulcers in that area. That should resolve once the canines are fully erupted.

Chemical irritants can be tricky to identify, but chemical culprits like wood preservatives, herbicides, or insecticides should be investigated. Small splinters can act as foreign bodies and break open as ulcers, as can some species of grass seeds. Also, just because there have been no new hay deliveries doesn’t mean that one, two, or a dozen bales out of the same field are not contaminated with weeds or chemical irritants. The same is true of commercial feeds and supplements. Accidental contamination can happen. New containers of the horse’s feeds and supplements might be necessary. I’d also recommend riding without a bit for a few weeks, even though the bit seems to fit just fine, and see if the problem resolves.

If the horse might be immunocompromised in any way, odd things like bacterial or fungal infections might be explored. But I’d rule out the more common causes first