You & Your Pet by Dr. Wise

Rhode Island Veterinary Medical Association 11 South Angell Street #347 · Providence, RI 02906 (401) 751-0944(401) 780-0940 · Fax · rivma@rivma.org

If I give my dog heartworm preventive medicine year-round do I still have to test him?

This is a great question. If heart worm disease was easily treated and caused only mild disease the answer would be no. However, within 6 weeks of the arrival of adult heartworms in the heart and nearby blood vessels, irreversible damage has already occurred. The signs of advanced disease include coughing, difficulty breathing, and reluctance to exercise. There are no obvious signs of the disease during the early stages.

Although newer treatments have much lower rates of complications, treatment is not risk free. For treatment to be most successful the disease must be detected well before the onset of clinical signs. Over the long run, giving heartworm preventive medication and testing annually, is much less expensive, both emotionally and financially, and is far better for your dog. Skipping the annual blood test would assume the medicine was 100% effective, it was given on time and properly, and that your dog actually absorbed the medicine. Although these assumptions are generally valid, a mistake leads to permanent heart and lung damage and could cause the death of your dog.

When it comes to heartworm disease "an ounce of prevention is worth a pound of cure".

My neighbor's dog has heartworms.  I don't want my dog to catch it.  What should I do?

It's great that you are concerned about keeping your dog from contracting this deadly disease.  The mosquito is the culprit that spreads heartworm disease from pet to pet (even cats can get it, although rarely.)  The mosquito takes a blood meal from an infected dog and picks up immature heartworm (microfilaria).   When the mosquito feeds on another pet, the young heartworm larvae are deposited into the pet.  The heartworm travel to the heart where they develop into adults and soon produce more microfilaria.  The cycle continues with the next mosquito bite.  

Once a dog becomes infected with the adult heartworm, several months may pass before any clinical signs appear.  Meanwhile, the animal's heart is working harder and harder to pump blood throughout the body because the worms take up so much space in the chambers of the heart.  Adult heartworms are about 12 - 14 inches long and cause permanent changes or damage to the heart and lungs.  (Cats suffer from damage to the heart and lungs more quickly and more severely initially, because their hearts are smaller.  Only one or two adult heartworm can impede the function of a cat's heart.)

There is treatment available for heartworm; however, it is only approved for use in dogs once it's contracted.  The treatment can be detrimental to the pet if there are any other medical problems.  It is very important to discuss treatment and post-treatment care in depth with your veterinarian.  The good news is that heartworm disease can be prevented.  There are several products available by prescription for dogs and cats.  You and your veterinarian can decide which one best suits your needs and if any blood tests are needed prior to starting the preventative medicine.

This information, prepared as a public service by the Rhode Island Veterinary Medical Association, answers problems Rhode Island veterinarians currently are seeing in their practices, as well as new developments in animal care.