Go to google and do a search under: heartworm map. It will tell you where there is heartworm in the country and one map tells you what months you may have heartworm in your state so the vet does not talk you into medication if your dog does not need it or only need it for a few months.
The vet may make you do a blood test which is probably not necessary to see if your dog has heartworm yet. The heartworm occurs when a mosquito with heartworm's larve is at 60 degrees for 30 days in a row, so if your temps go below that, your dog at this point could not have gotten heartworm and does not need a blood test. The heartworm map suggests giving meds one month before your dog can get heartworm in your state and for two months afterwards.
Heartworm meds can have many side effects and be fatal, so it's not something to be taken lightly. I live in a colder climate and my vet tried to talk me into a blood test and heartworm meds for my dog, then I saw on the internet that I don't even have that problem where I live!!!! Beware of vets that try to sell you their products, like Science Diet or Eukanuba food which are both bad.
Proheart6, I believe is taken off the market already for causing so many dog fatalities. Do a internet search for comparisons of different heartworm meds.
Bye, chilidog