Maybe it would help if you told us Sasha's breed and why you were giving the invermectin? I must say, I'm surprised to see that someone in Edmonton gives it year round. How come is that?
Not as cold where I am and I only use it in summer, just started two weeks ago.
i mention breed because my sister's Vet advised to NOT give anything with invermectin in it to her dogs, both English Shepherds, both collie types, neither tested for MDr1.