I liv in Fresno California and my vet told me the same thing. He said there is not really need becuase he has not any reported cases in all the years he has been in practice. However, last year when we went camping he gave me heartworm medication for Cosmo to take even though he said it wasn't necessary. I now have both my pups on the monthly medication in addition to Advantage flea medication "just in case." In the summer we boat and camp so I just feel safer knowing they are protected.
By the way my parents have two dogs a Jack Russell and adopted Chihwawa (spelling?) and they go to another well known vet in our area who does not have their dogs on heartworm medication either...
Just get more facts and then decide what is best for your peace of mind
