I had two colts sent off to a trainer earlier this year and about a week after they came back (end of May) they came up with a very snotty, thick nose discharge, cough, & lethargic. They got over it in about a week, and then I had an older (9yo) horse get it. He got really sick and ended up with a case of “strangles” which resulted in a deep abscess about the size of my fist in middle of his neck with that had to be lanced by the vet. He recovered and I had about a week with no snotty noses or cough, and then another horse got it, it lasted about a week and he recovered. It’s been maybe 2 weeks where everyone was healthy, no cough, snotty noses etc, then last Friday I notice another horse coughing and starting to get a snotty nose, over the weekend it’s developed into the cough, thick, snotty discharge, and lethargic. Last night one of the colts that first had this cold or whatever came up out of the pasture with a snotty nose starting as well.  

When I got my two colts back earlier this year I had them quarantined for 5 days and they didn’t show anything until I kicked them out in the pasture with the others. I’ve tried to keep whoever is sick well away from the healthy horses and always feed/care for them last to reduce contamination as much as possible, I’ve bleached & scrubbed everything as well.. I am so sick of this sickness that doesn’t seem to leave! Is there anything that I can give them to help boost their immune system?