My horses butt has swelling on both sides in each butt cheek, how to treat this and what is it?

An abscess is infection escaping the body, it’s where there’s is a foreign body, or damaged tissue (perhaps a stone bruise or something) that the body walls off and attacks until there is enough pressure in the puss for it to burst out. You’d have to be incredibly unlucky for a horse to pick up a systemic infection from an abscess bursting because it’s already walled off.

Likewise with injections, the site of the injection may become infected and abscess but if the injection site has no reaction it is very unlikely something got in there and moved to another part of the body.

It looks like the ham strings are in spasm, without a picture from further away it’s difficult to see the extent of the distribution. They could well have been strained from hopping around lame with the abscess, equally from hooning about once feeling better.
 
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That is some serious swelling! If he were mine, I would be calling the vet ASAP.
I did call the vet right after it happened. He came out and said this was the 5th hematoma he has seen in a month and until then had never seen a hematoma in his practice! Strange and he can't figure out what is causing them. I really think my mares edema travelled from her front legs directly to her butt and that there is a correlation here that all started with a high nail. Long story short , vet is doing nothing. Said it should resorb with time. Sometimes draining can do more harm than good. Said if it was a pendulous hanging hematoma he would consider draining but my horses he said are not that bad. So not really sure if this is a good idea to do nothing but I am watching it. On my own, I am still cold hosing it on warmer days and also putting epsom salt poutice on it a couple times a week, letting it sit a few days, then washing it off. Hoping it will go down. Worried though...
 
I did call the vet right after it happened. He came out and said this was the 5th hematoma he has seen in a month and until then had never seen a hematoma in his practice! Strange and he can't figure out what is causing them. I really think my mares edema travelled from her front legs directly to her butt and that there is a correlation here that all started with a high nail. Long story short , vet is doing nothing. Said it should resorb with time. Sometimes draining can do more harm than good. Said if it was a pendulous hanging hematoma he would consider draining but my horses he said are not that bad. So not really sure if this is a good idea to do nothing but I am watching it. On my own, I am still cold hosing it on warmer days and also putting epsom salt poutice on it a couple times a week, letting it sit a few days, then washing it off. Hoping it will go down. Worried though...
Actually, relooking at the original photos I posted I think it has come down a little bit. Could take a long time.
 
I did call the vet right after it happened. He came out and said this was the 5th hematoma he has seen in a month and until then had never seen a hematoma in his practice! Strange and he can't figure out what is causing them. I really think my mares edema travelled from her front legs directly to her butt and that there is a correlation here that all started with a high nail. Long story short , vet is doing nothing. Said it should resorb with time. Sometimes draining can do more harm than good. Said if it was a pendulous hanging hematoma he would consider draining but my horses he said are not that bad. So not really sure if this is a good idea to do nothing but I am watching it. On my own, I am still cold hosing it on warmer days and also putting epsom salt poutice on it a couple times a week, letting it sit a few days, then washing it off. Hoping it will go down. Worried though...
From everything I’ve read on an old post on another forum, ditto ^^^^ what your vet said.

While a few people never knew what caused such big eed as on the butt, the suspicions ranged from getting kicked, to bee stings or spider bites. To even getting cast in the stall.

These can take a long time to go away. One vet did tell one horse owner they could do the following in the hopes of speeding up the “disappearing“ process”:

Didn't drain, not always successful. Vet recommended hot packing with moist heat,(towel in hot water well wrung out) for 10 - 15 mins 2 x's a day. After a week started using DSMO and that worked like a charm. Really helped the body reabsorb the blood pool.
 
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