The Heartworm Society doesn't recommend climate dosing. Are you on the right page? American Heartworm Society
The website you linked is an article from 2007.
Here is what AHS says -
AHS recommends year-
round administration of chemoprophylactic
drugs to prevent heartworm disease, control
other pathogenic and/or zoonotic parasites, and
enhance compliance, the latter being particularly
important in light of the documented presence of
DogAware.com Articles: Heartworm Prevention in Dogs she has good information and is a proponent of less meds and natural alternatives, but still gives hr dogs year round heartworm pills at the normal dose.
Based on new information regarding possible reduced heartworm preventative efficacy, I now feel it’s best to give heartworm preventives year-round, not only for better protection against infection, but also to ensure that the manufacturer will pay for treatment should your dog become infected with heartworms.