I thought you meant that the vet routinely wormed them once a month. Sounds like you need to treat them all at once or they will just keep getting them from each other.ChessaQ and Dawnben thank you guys.
I got worried because I saw little worms on her feces last week.
Same thing with my 2 months shih tzu female puppy. I literally have to pull a 7 inches long white worm from her rectum (i think that's the term. hahaha)
She never had any deworming since I got her. So I am planning this weekend to bring her to my vet.