The blood test for heartworms is to determine that they do NOT have heartworms before medicating them. The heartworm medication can be fatal if they already have heartworms. Monthly medication prevents them from getting heartworms from mosquitoes. I give my boy and girl heartworm medication monthly year round, though the vet still tests them annually, just to be sure. It's expensive to keep them medicated, but the cost of trying to rid them of heartworms is super expensive and dangerous. |