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Heartworm Disease

Every year your veterinarian performs a heartworm test on your dog and recommends twelve months of heartworm prevention to administer monthly until your dog’s next annual health check. Trust me, your dog thanks you for taking care of his or her heart and lungs by administering this prevention as prescribed each month! Heartworm disease is a serious and potentially fatal illness that is much easier and less expensive to prevent than treat.

Heartworm disease is caused by long worms that live in the heart, lungs, and surrounding blood vessels of a dog. These worms can live up to 7 years in the heart of an infected dog. Heartworm disease cannot be transmitted directly from dog-to-dog because a mosquito vector is needed for the worms to develop.

An infected dog produces baby heartworms, called microfilaria, that are released into his or her bloodstream. When a mosquito bites an infected dog, it picks up the microfilaria which then develop into juvenile heartworms, called larvae. To complete the lifecycle, that mosquito bites another dog in the neighborhood and infects that dog with the larvae that develop into adults. Without prevention, this lifecycle continues with mosquitos and dogs in the community.

Above we discussed what heartworm disease is, how it is transmitted, and how to prevent infection in your dog. Now, what happens if you are concerned your dog may have been exposed to heartworms?

It is important to have your veterinarian test your dog for heartworm disease every 12 months. Heartworm prevention is highly effective, but no medication works 100% of the time. Heartworm testing is performed by collecting a very small blood sample from your dog to look for evidence of mature female adult worms. It takes up to 6 months for the juvenile heartworms, called larvae, to develop into mature adults in a dog. Therefore, it is best to test a dog that missed a month of prevention six months after the month of prevention was missed. Otherwise, your test may not be able to detect the presence of infection (as the test can only detect mature adult worms).

The prevalence of heartworm disease is expanding throughout the United States. As you will see on the American Heartworm Society’s 2016 incidence map, all areas of Michigan had new confirmed cases of heartworm in 2016. In West Michigan, each veterinary clinic diagnosed 6-25 cases of heartworm disease in 2016!


Let’s discuss heartworm disease treatment of infected dogs.

Infected dogs with heartworm disease show a wide range of clinical signs pending the severity of infection. Early, light infections often have no clinical signs associated with them. Heavy, advanced infections are associated with coughing, exercise intolerance, decreased appetite, and can progress to heart failure.

If your dog is diagnosed with heartworm disease, your veterinarian will perform a series of blood tests, urine tests, and x-rays to determine the severity of his or her infection prior to treatment. Light infections are associated with less side effects of treatment than severe infections, so these tests allow your veterinarian to develop the safest treatment plan for your dog.

Treatment of heartworm disease is a lengthy process that includes numerous oral and injectable medications over many months. It is critical that your dog stays quiet and inactive throughout treatment to allow for the best outcome. Dying heartworms cause inflammation to the heart and lungs; therefore, your dog must stay calm for many months to allow his or her body to heal.

As treatment is costly and hard on a dog’s body, please remember to keep your dog on monthly heartworm prevention year-round. And, better yet, provide your dog with double the protection by using a monthly flea product with mosquito repellent to prevent mosquito bites in the first place!