When I first started working with my mare in January of 2011 she had a small (maybe the size of an almond with its shell) lump on the side of her neck, in the area you give IM shots. I had my teacher/boss look at it, but since she showed no discomfort with it, you could poke/prod/squeeze it without her so much as blinking, and she had no signs of being sick, we didn't call a vet and just kept an eye on it.
Since then, I've kept an eye on it, even had a vet tech friend look at it, but its never changed so I've never been concerned about it.
I didn't see my horse for a couple days, and it appears that on Thursday or Friday it suddenly grew, and late Friday/early Saturday it broke open and drained. I can't see anything in it, it still doesn't bother her to poke or prod, and she still has no fever or change in appetite.
My big question is, aside from keeping it clean and keeping an eye on her, do I need to worry about it being something she could pass on to other horses, and why would it take this long for it to abbcess?
It may have contained a bit of a foreign body or "sequestrum" that set up enough local inflammation to form a lump. Sometimes, eventually, the body just can't keep up with walling something like this off and the inflammation will rev up again, at which point the lump can become active or drain or start to swell and hurt. It could have been from her rubbing it on something, an inadvertent scrape, or just random.
I'd say just normal wound care is appropriate for starters, but you might want to have it excised for good just to make sure it doesn't become a chronic nuisance.
A foreign body trying to work its way out would be my guess.
Once of my horses had a big spinter in his shoulder that broke off as we tried to remove it. Quite deep, so we didn't want to completely open him up. It healed over. Not sensitive at all.
2 years later it opened up and a small piece of wood came out. It healed over again (leaving just a small lump). A year later it opened again and the rest oozed out, presumably with the last bit of foreign body (wood).
Just treated as any other wound. Never seemed to bother him.