A Boston-based animal protection group says it’s preparing for a possible influx of canine influenza after a terrier in Massachusetts came down with symptoms resembling those of a dog flu strain that has sickened more than 1,000 dogs in the Midwest. Officials at the Angell Animal Medical Center announced Friday that they suspect a 5-year-old female terrier from Watertown that was brought to the center’s Waltham location May 3 had the virulent flu strain, which doesn’t affect humans.