For those interested, here are maps showing the reported incidence of heartworm positive dogs to area vet clinics: https://www.heartwormsociety.org/veterinary-resources/incidence-maps Note the changes (increases & spread across the country) between 2001 and 2013. 2013 is a lot higher than even 2010. Those in CA, I thought it was striking that there is an area of northwest CA that now has the incidence numbers equivalent to places like Southeast Texas, Mississippi, Georgia, & deep South. Of mainland US, only North Dakota seems relatively clear of heartworm. Nevada looks good, except for the Las Vegas area/southern tip. For those of us in AZ, the only low incidence area anymore seems to be a northeastern strip of the state.