Hi, I'm Dr. Jill, an equine/large animal veterinarian. I'd like to ask a few questions to help better answer your question.

Have you taken his temperature and if so, has he had a fever at any point? Are the knots firm, or soft and squishy (or with a point on them that's squishy)? Any change in feces? And may I ask what area of the country you live in?

The swelling along his abdomen...does it extend all the way across like a several inch thick swelling, or is it individual swellings? Any heat that you've felt in that area, or is he sensitive to being touched over the swellings? Any change in the hair over them?

Last thing for now...has he been examined by a veterinarian yet?

Well, my first thought is Pigeon Fever. I'm out in California where we see it a lot (also occurs commonly in Texas) and it often starts with swellings around the sheath and inguinal region and can cause swelling over their entire belly up to their front legs. Also often obstructs the lymph drainage causing a limb to be swollen too. It can take several weeks, but we try to avoid antibiotics to let the abscesses mature (can also apply ichthamol or another poulticing agent) and then cut them open when they form a soft spot. Usually happens in the late summer/fall, though it can happen in winter.

However, the abscesses usually get bigger and are hot and painful. By 3 weeks, I would expect he would have ruptured one by now.

Did your vet do a rectal exam and feel anything?

I certainly hear your frustration too! Never fun to not know what's going on!

How big are the knots?

Oops...re-read...apple sized :)

Oh, just one swelling?

I'd also be inclined to put an ultrasound probe over it to see what's in there...

Yes, that'd be great!

I think there's a paperclip icon that lets you load images directly to the chat window.

Ha :). There should be a little button that looks like a paperclip...if you click on it, should be able to select the picture file to load. We're not supposed to give out personal information (rules of the site), but let me know if this doesn't work or your screen looks different.

I also think computers break just about every time I touch them :).

Sadly, no...lived in Texas for a summer (loved it), but back out in California. I think diagnostics I would start with are a good physical exam and bloodwork, so that's good info to have that the bloodwork's normal. Helps rule out infections and organ dysfunction. Possibly also a rectal exam to see if I could feel that area internally.

Since it sounds like that hasn't helped with a diagnosis, I'd next go to ultrasound to see what is in there, then possibly stick a needle in it depending on the ultrasound.

Could still be an abscess, hematoma (i.e. broken blood vessels), or could be some other kind of tissue mass (like a very enlarged lymph node), or possibly a break in the body wall allowing intestine to slip out (exept those usually feel soft and squishy).

Could still be pigeon fever as well (Corynebacterium infection) causing an abscess which would mature over time, develop a soft spot, then burst open.

If he's still eating fairly well and seems relatively comfortable, I'd personally be inclined to give him more time before putting him down.

If he's made it 21 years as a ranch/calf horse, he must be a great one!!!

You could always repeat bloodwork to see in anything's changed, but I think you're money is probably best spent on the ultrasound and rectal palpation to feel around that area on the inside (if it hasn't been done already). Your vet might also want to do a needle biopsy depending on what is seen on the ultrasound.

If you get a chance, I'd love to hear how things go! I think you should be able to log back into the same question even if an answer's been accepted. Otherwise, I think there's a way to message me directly through the site as well. (my username is JJ-DVM)

Hope this helps! If you have any other questions, please don't hesitate to ask either here or through the private messenger.