Why are heartworm tests performed yearly?

Our veterinarians recommend testing for heartworm every year. Even if your dog is on heartworm preventative, he/she may not have swallowed one of the doses, a dose may have been missed, or, in very rare cases, the heartworm prevention may not have been 100% effective in your dog. The heartworm test we carry also tests for several diseases transmitted by ticks, including Lyme disease.