sheperd and small dogs...arthritus

Question
we own a7.5 year old female shepherd,spayed and purchased as a pup from a breeder.after age 3 she has had 5 instances with a neighbors doxson and yorkie where she has caused both minor and major injury to.we installed an electric fence,but on those occasions it was down due to lightning,or other damage.she does this when she sees them running around closer to our yard.why this aggression towards small dogs? she is so gentle and loving,never has tried to attack any person,child,or other animals visiting home.(not ones that small a breed,though)thankfully our neighbors have been understanding...we pay the vet bill of course.It is very upsetting, and seems so out of character for her.       This same dog has been avoiding the use of her left rear leg.are they prone to arthritis?any otc's you could recommend?

Answer
I don't know how to train her out of attacking the smaller dogs, and I am not sure at that age a professional trainer or behaviorist could either.  

I don't think those fences draw much power.  Could you install a UPS for it?  Perhaps it would be better not to leave her out when you aren't around.  

There are only 2 OTC things I would suggest.  Aspirin, buffered aspirin is best, but none of the newer pain killers.  The dosage is 5 mg per pound of body weight every 8 hours.  Unless she is a large Shepherd, a regular 325 mg tablet might be close enough.  The other thing is glucosamine.  I am giving my old lab 750 mg per day, perhaps too much.  For more, see http://www.arthritis.org/arthritis-dogs-2.php