Unknown Infection - Spider bite, vasculitis, blood clot?

Now that we are in the recovery mode, I thought it interesting to share a recent really scary week with my prior/current horse. I sold my young horse who passed with a clean vetting, A+ bill of health. Horse from foal to now has no history of allergies, lameness, sensitivities. As clean as you can imagine.

Week after, horse is still in my care as new owners schedule hauling. Horse comes in a little dull one morning, jogs sound on straight line, but on circle dragging toe/lame. Negative to hoof testsers, no swelling in lower limbs. Place hand on inside of gasking/thigh, buring hot. Low-grade fever. No visual bites/scrapes, no rain-rot of skin scuzz.

Vet comes out, blood shows elevated white blood cells. Vet gives Excede and prescribes banamine 2x day. Says: Vasculitis. Leave out on turnout.

Two days later, mare is worse, still sound on straight line, but on circle cannot hold canter, hitches in stifle.

Take to vet who gives steroid shot and takes CBC. CBC comes back clear. She stays at clinic, now on stall rest. Next day, ultrasound. Horse has a small blood clot on inner thigh. She stays for 5-days, at clinic. She starts daily IV antibiotics, Baytril, aspirin. Horse fluctuates alertness to groggy/lame. 4th day give a different antibiotic before going to a local university clinic.

At university clinic re-ultrasound 24-hours in, Blood clot is gone. Horse begins to perk up. Antibiotics are lessoned. She’s going to her new owners tomorrow.

Vets have NO idea what caused this. She does live on 24/7 turnout with a creek/shade trees that she does enjoy spending afternoons by. Spider bite? Her mare-buddy did come in with a swollen nose last week (imagine bee sting swell), but no similar symptoms and is fine.

It’s been a week of breath-holding.

Are there ticks in your area? Lyme disease or another tick-borne disease (like anaplasmosis) would be high on my list of things to investigate.

From the Merck Manual:
anaplasmosis in horses

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Long time ago I had a horse do something similar. He pulled an adductor (inner thigh muscle) tearing around in the mud (don’t ask, it wasn’t planned). Anyway, healed up just fine in about a week, but it sure looked gruesome at first. I happened to be in vet school at the time so it was easy to have them check it out.