The good news is that are a lot of options for prevention of heartworm disease. The preventatives work by killing the larval stages that have already been injected into the dog or cat. So a dose of preventative works retroactively, killing the larval stages of heartworms for 45 days. For that reason you can get away with only giving most preventatives every 45 days. Veterinarians recommend dosing every 30 days because it is easier to remember to give preventatives at the same time every month. The other benefit of most heartworm preventatives is that they also contain an intestinal dewormer.
ON THE TOPIC OF TESTING For heartworm disease, different veterinarians have different philosophies about whether to test yearly or every other year. There is no hard or fast rule on this. You can put a dog on preventative without testing, it is just not ideal. You want to know if your animal has heartworms. Also in some area's, such as the most northern states, it is not necessary to keep pets on preventative in the middle of the winter (unless you travel with your pets), since ther are not any mosquitoes buzzing around during a Wisconsin January :)
It is important to know that your pet does not have heartworms before starting a preventative medication as it can cause your animal to become very ill.
BEING DILIGENT WITH A DEWORMING PROGRAM BENEFITS NOT ONLY THE HEALTH OF PETS, BUT THE HEALTH OF PEOPLE AS WELL.
Written by Susan Reabe