black lab developing sort of a twitch in leg
I have been noticing an apparent "muscle twitch" in his hind legs. He is an outdoor dog on a farm and is about 10 years old. He was diagnosed with heartworm around 7 years ago but was treated and has been "negative" on all tests since. A few days ago, he had trouble getting up the three steps on our front porch; it was as if he slipped, fell backward and then tried again after coaxing him. Sometimes he comes out of his doghouse, a brand new larger one, with his left foot in the air as if it had "fallen asleep", limps for a bit, then seems fine.
Can you give me some idea of what this might be and if I need to get help for him immediately? We love our dog and don't want to see him suffer. Thanks for any suggestions.