Do dogs need to have a heartworm test every year?

The heartworm preventative will protect your dog from the heartworm as long as it is properly given every month. The reason vets insist on yearly tests is to 1) protect themselves from lawsuits and 2)protect your dog from fatal use of meds.
The heartworm preventative is fatal if given to a dog with heartworms and ignorant owners will forget, lie, and try to save money in cold weather months, when mosquitos don't bite.
It only takes one mosquito to infect a dog and they can live in warm conditions, like a compost shed, in winter months, so if a vet prescribes H.W. meds without yearly testing and it kills your dog, he is sued by a distraught (and guilty) pet owner!
I had a hard time accepting this extra cost of testing also, until my vet patiently explained all of this and I researched it online, since I never miss a dose for my dogs.
One of my rescues had heartworms and went through the treatment (which can be fatal, itself) and that cured me from ever missing a dose or forgetting! It was heartbreaking to see this poor dog recover from the 2 poisonings-to-the -brink-of-death treatment that is administered for it!