Just like some vaccines make kids and adults ouchy, some make dogs ouchy too. That’s probably what the growling was about. And as previously said, your small dog got the same dose as a great dane. He could be sore all over from it.
If you want to not give heartworm prevention over the winter, DO NOT go by dates on a calendar. Actually track the temperatures. When you live in the south, you can’t count on it being cold enough or staying cold enough according to the dates. A couple years ago, we had an entire week of freezing weather in November, but didn’t have anything colder that sweatshirt weather the entire rest of the year.
