Originally Posted by Left Hand Percherons
More than likely you are talking about a summer sore. What that is is simply fly larvae that have been deposited on a break in the skin. The easiest way to deal with it is to rub ivermectin paste dewormer on the sore until it goes away. Maybe 3-4 days.
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Our horse is very prone to this. Our vet advises the same as you noted as well as use Ivermectin orally every two weeks until the would is healed over. I do gently clean the wound with saline first and that seems to also support healing. I wish the sores our horse gets would heal in 3-4 days, but they generally take a few weeks. They will start to look better in the shorter period.
FYI: This pix is a pretty blown up sore. They pop up pretty quickly, so the sooner you catch it the better.