My vet says sarcoids (which are skin lesions, and there are six different types) show similarity to bovine papilloma virus - cow warts - when tested. They believe flies transmit them from cattle to horses - but no definitive cause and no definitive treatment - some treatments work for some sarcoids, and you just hope you get lucky. No vaccinations for it - it isn't rare, but isn't common either - this is my first horse with a sarcoid in 40 years of horses in our family. Our vet has treated a dozen or so.
Here are pics from today, before I cleaned and retreated her - the whole bottom of the scab is separating, and Hunter - I almost laughed at your don't PICK IT comment because it is SOO TEMPTING TO PICK IT!! But I didn't, just cleaned it and reapplied the XTerra - I also started treating the one on her cheek with this 4 day on/4 day off cycle - we are on day 3 and I will start taking pics of that one tomorrow.
Really hoping that when this sloughs off it takes all the sarcoid with it.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela