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Are the Ticks out in your area yet?

Tick paralysis?
Lymes?

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Sounds like they suspect a tick disease since they are treating your dog with Doxycycline. A young dog with sudden onset lameness and swollen joints, the things I would suspect are:
*tick diseases (Lyme, Rocky Mountain Spotted Fever, Erlichosis)
*Neurologic Disease
*Autoimmune Illness (Rheumatoid, Lupus)

If the initial bloodwork was normal, I probably would submit a tick titer, and place the dog on steroids (Prednisone) and doxycycline and see how the dog did after a few days, which sounds like what you vet is doing. If the dog isn't doing worse, have patience and give the meds time to work.

Keep us updated.

Buckmaster, I've posted it for you. There's sometimes heavy traffic there so over the weekend you may need to go back a few pages to find the post "Help, ill Pup". WVVet seems to have given you some good advice. We need more dog knowledgable folks on this site! Thanks WVVET, more than just the folks posting the specific question are learning from your responses. Thanks again, Juniorpc.

Is there any chance your chessie was bit by a coon? Or came in contact with coon saliva or feces or something? If so, do a google search (or other search engine) on Coon Hound Paralysis. My dog had this last year and ended up dying as a result.

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