When used as directed all year round, heartworm preventatives are highly effective at protecting dogs against heartworm disease. Your veterinarian will need to regularly test your dog for heartworms as part of basic preventive health care. In some cases, your dog may test positive for heartworms even after starting a heartworm preventive routine. That doesn’t necessarily mean that the product you are using isn’t working. Let’s look at why. Heartworm preventatives work by killing the youngest juvenile-stage worms after the mosquito bite. As the worms mature, they become less likely to be killed by the medication. So if your dog was already infected before starting a monthly heartworm routine, those existing adult heartworms may eventually trigger positive test results. Also, if your veterinarian has previously treated your dog for heartworm disease You might be surprised to learn that it can take more than a year for your dog’s body to remove all the evidence of an infection from the bloodstream. And to make matters even more complicated, heartworm disease doesn’t show up in tests until five to nine months after your dog was bitten by an infected mosquito. So a “negative” test result is not a guarantee that your dog is free of heartworm infection. It simply means that the tests did not detect the presence of heartworms in your dog. Your dog could miss a dose of her monthly heartworm preventative, spit out or vomit the tablet without you realizing it, or be on the wrong dosage based on her body weight. These could all cause a positive test result at some point in the future. Bottom line? It’s important to test your dog for heartworms regularly – especially prior to starting a new heartworm preventative. Serious health issues can occur in dogs with a pre-existing heartworm infection if they begin to take a routine heartworm preventative. Your best bet is to start a heartworm preventative as soon as you can – and stick to it. If you still have questions about your dog’s test results or treatment program, talk to your veterinarian. Ask about Trifexis, the 3-in-1 combination product that kills fleas and prevents infestations, prevents heartworm disease, and treats and controls hookworm, roundworm and whipworm infections. See the full product label for complete safety information. Serious adverse reactions have been reported following concomitant extra-label use of ivermectin with spinosad alone, one of the components of Trifexis. Treatment with fewer than three monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. Prior to administration of Trifexis, dogs should be tested for existing heartworm infection. Use with caution in dogs with pre-existing epilepsy. The most common adverse reactions reported are vomiting, depression, itching and decreased appetite. To ensure heartworm prevention, observe your dog for one hour after administration. If vomiting occurs within an hour of administration, redose.