Let's start with the scooting. You were very much correct to suspect allergies, either or both, environmental allergies (atopy) or food allergies.
Of course, anal glands, are notorious for scooting problems. But, keep in mind, even after an anal gland expression, scooting can persist for a few days because of the inflammation in the rectal area.
My previous dog was a Cocker. Suffered from allergy issues, and was a chronic scooter (limited by allergy management).
Here's the problem: Benadryl, or any other antihistamine, will only help about 1 in 3 or 1 in 4 dogs to really control their allergies.
So, I am still very very suspicious that there is a allergy problem.
Now, the periodic episodes of back leg shaking is not at all consistent with allergy symptoms. But, at his age, I absolutely cannot rule-out age-related weakness, muscle atrophy, and absolutely, include some hip/lower back/knee arthritis.
So, I would ask your vet about just doing a low dose trial with a steroid. The steroid may actually be able to help the arthritis as well as the allergies!
Normally! A vet would prescribe a non-steroidal anti-inflammatory for arthritis. But, in this case, if your vet agrees, and they want to maybe help the allergies a bit, giving a regular steroid is a better option.
Also, definitely time to start some joint support therapy, as well know at this age, some degree of joint problems is hiding in that body!
A few common medications prescribed by vets (not all options,but many of the most common choices):
Currently, one of my favorite joint support supplements on the market is Dasuquin or Cosequin. Here is some more information about it. Many veterinarians and clients have observed that after a few months on a quality joint support product, they can often reduce the dose of the non-steroidal anti-inflammatory medications there pets may already be taking. A supplement can take several weeks to observe benefit, so it is important additional anti-inflammatory or pain medications are used in the meantime. Product Information about Dasuquin / Cosequin
Also, an incredible clinically tested food to help with joint problems is J/D. J/D Diet
Another supplement to review would be Adequan injections for joint health. These injections are also quite popular and have helped many pets with arthritic disease. Adequan injections
Notice, I included the two most common pain medications!
I would also ask for some Tramadol. I am concerned that the intermittent shake you see is also a symptom of discomfort. So, do a little trial. See how he responds to some pain medication.
Hope that information has been helpful.
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