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Normally, a yellowish discharge from the eye is an indication of an eye infection. You will need to have your vet check the eyes of these horses, so that the appropriate medication (ointment) for the eye can be prescribed. You want to be care of "self medicating" the eye with an ointment that you may already have as some contain steroids that are not appropriate for infections. Eye infections are contagious, so this would be the cause of why you are seeing it affecting several of your horses.
You can, however while waiting on your vet, clean the eye with "Eye Clear", an eye wash for the eye. If you do not have it on hand, you can clean the eye with a slightly warmed clean soft cloth to remove and of the discharge. Also, if you have "fly masks" I would suggest putting it on your horses and keep them stalled until your vet can check them. The masks will help keep "flies" and other debris out of the eye while you are medicating and treating it.
I hope this helps. If you need further information, please feel free to contact me.
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