Yes, we recently rescued a Yorkie who tested positive for heartworms. Missy had her first shot on May 28 and is doing great. She's had no problems and it's been easy keeping her calm and quiet, even with a 2 year old active "brother", Ace the puggle, who is constantly on the move! We take her back on July 1 for the second shot, and she will have to spend 2 nights in the animal hospital. Hopefully she will be heartworm free by the end of the summer!