By: Lisa Malkin, Director of Hospital Administration for Humane Veterinary Hospitals — Information provided by the American Heartworm Society

• What is Heartworm?

Heartworm is a preventable, but serious and potentially deadly, parasite that primarily infects dogs, cats and ferrets.

• How is Heartworm transmitted?

Heartworms can only be transmitted from animal to animal by mosquitoes. When a mosquito bites an infected animal, young heartworms called microfilariae enter into that mosquito’s system. Within two weeks, the microfilariae develop into infective larvae inside the mosquito; these infective larvae can be transmitted to another animal when this mosquito takes its next blood meal.

The infective larvae mature into adult heartworms in approximately six months. During the first three months, the larvae migrate through the animal’s body, eventually reaching the blood vessels of the lungs. During the last three months, the immature worms continue to develop and grow into adults, with females growing to lengths of up to 14 inches. The worms damage the blood vessels, and reduce the heart’s pumping ability, resulting in severe lung and heart disease. When the animal shows signs of illness due to adult heartworm infection, it is called heartworm disease.

• How can I tell if my dog has Heartworm Disease?

If your dog has been recently or mildly infected with heartworms, he/she may show no signs of illness until the adult worms have developed in the lungs and signs of heartworm disease are observed. As the disease progresses, your dog may cough, become lethargic, lose his/her appetite or have difficulty breathing. You may notice that your dog seems to tire rapidly after only moderate exercise.

• Where are Heartworms found?

Geographically, heartworms are a potential threat in every state as well as in many other countries around the world. All dogs, regardless of age, sex, or living environment, are susceptible to heartworm infection.

• What pets should be tested for Heartworm Disease?

Because heartworms are spread by mosquitoes, any pet exposed to mosquitoes should be tested. This includes pets that only go outside occasionally. Remember that mosquitoes can also get into homes, putting indoor-only pets at risk as well.

• How the Veterinarian tests for Heartworm Disease?

Blood tests are performed by your veterinarian to detect the presence of adult heartworm infection (>6 month old infections) in your dog. The antigen test is most commonly performed, and is very accurate in dogs. Antigen tests detect the presence of adult female heartworms, and antibody tests determine if your pet has been exposed to heartworms.

Further tests, such as chest radiographs (x-rays), a blood profile and an echocardiogram (an ultrasound of the heart), may be necessary to confirm the diagnosis, to evaluate the severity of the disease, and to determine the best treatment plan for your dog.

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit for preventive care.

Following are guidelines on testing and timing:

• Puppies under 7 months of age can be started on heartworm prevention without a heartworm test (it takes at least 6 months for a dog to test positive after it has been infected), but should be tested 6 months after your initial visit, tested again 6 months later and yearly after that to ensure they are heartworm-free.
• Adult dogs over 7 months of age and previously not on a preventive need to be tested prior to starting heartworm prevention.  They, too, need to be tested 6 months and 12 months later and annually after that.
• You need to consult your veterinarian, and immediately re-start your dog on monthly preventive—then retest your dog 6 months later. The reason for re-testing is that heartworms must be approximately 7 months old before the infection can be diagnosed.

Annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working. Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication—or give it late—it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill—or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog tested, you won’t know your dog needs treatment.

• If my dog tests positive, how can my dog be treated?

Heartworm is a progressive, life-threatening disease. The earlier it is detected and treated, the better the chances that your pet will recover and have less complications.

As with most medical problems, it is much better to prevent heartworm infection than to treat it. However, if your dog does become infected with heartworms there is an FDA-approved treatment available. There is substantial risk involved in treating a dog for heartworms. However, serious complications are much less likely in dogs that are in good health and when you carefully follow your veterinarian’s instructions.

The American Heartworm Society recommends testing pets every 12 months for heartworm and giving your pet a heartworm preventive 12 months a year.

For more information, visit: www.avma.org or

American Heartworm Society   www.heartwormsociety.org