I had a 1 month old colt last summer who had that, since a few years ago I had a run in with strangles I panicked. But, the colt was not sick, was acting normal, no discharge, no fever, eating normally too. I called the vet and they said to just watch it as it was probably just a reaction to his teeth coming in. So I watched and watched and kept an eye on my other foals too. Well he cleared up just fine and no one else got sick, so the vets were right!!