When evaluating the risk of heartworm disease, it's important to use valid information. The following is a map compiled by the
Heartworm Society.
The map is from 2019 (them most current available).
I've practiced in an endemic area where a dog would get heartworm if not on preventive medication. The only variable was when based on how long the animal was outside or exposed to mosquitoes.
The current heartworm preventative medications also treat/prevent most intestinal parasites as well. Because of the broad spectrum of benefits and the very low potential of side effects, my choice would be to administer preventative medications everywhere in the US.
Doug, DVM