I know more about tendon sheath infections than I’d like to, because I went through this with my then yearling last year. I did a lot of research and posted here, if you’d like to read the thread: https://www.chronofhorse.com/forum/forum/discussion-forums/horse-care/9899464-tendon-sheath-infection
My horse is now 2 and seems to be totally sound and the swelling is all but gone, however it has taken a full year for the swelling to go, and it hasn’t been ultrasounded since. I will have it scanned before breaking him in, 12 months from now.
Two of my friends have recently lost horses to tendon sheath infections. One after a 3 month battle, after which the infection was controlled however the damage left behind meant a life time of pain. In her poor horse’s instance, there were extensive adhesions, damage to the tendon, and damage to the joint cause by the infection. The other was euthanised a week post surgery as the skin on the horse’s leg started sloughing away under bandages, the tendon became exposed and was about to snap.
Your first challenge is eliminating the infection. Have vets recently drawn samples of fluid to check this? The next challenge is assessing the damage the infection has caused. As yours, like mine, is a hind leg this is a good thing as they cope better with hind leg issues and there are less risks of supporting leg laminitis.
My research and personal experience did tell me that tendon sheath infection healing is a roller coaster, but your horse’s extremes are, well, extreme, ie comfortable some days and 3 legged on others. Is the wound still draining? My friend who’s horse was euthanised 3 months post surgery found her horse was more comfortable when the wound was draining, but once it sealed up it was extremely painful. Another factor that can cause the ups and downs in comfort levels is adhesions, these are very painful as they inhibit the gliding of the tendon in the sheath, however, sometimes they can break loose (a good thing).
I would suggest discussing with vets ultrasounding it at this point to get an idea of what’s going on in there, and sending a fluid sample away to ensure the infection is under control.
I mentioned in my thread that PEMF therapy seemed to work wonders with my guy, reducing swelling, and improving movement and pain levels.
I am sorry you are going through this and hope it is a good outcome. Tendon sheath infections are more serious than joint infections - I count myself extremely lucky to have a horse that’s survived one, and that will hopefully be sound enough for a performance career. Keep us posted.