Dear Heart: Canine Heartworm Disease is found in every state in America. The disease is transmitted when a mosquito bites an infected dog, drawing up microfilaria (heartworm eggs) into its body and then depositing them as it bites an unaffected dog. Once the microfilaria is in the dog, it grows to the larvae stage and makes it way to the heart and the pulmonary arteries that lead into the lungs, where it grows into the adult form. Adult females begin to reproduce and flood the blood with more microfilaria. It can be fatal if left undiagnosed. Adult heartworms, which can grow up to a foot long, can live in a dog’s heart for several years before symptoms appear, which include coughing, difficult breathing, fainting, swollen abdomen and fatigue.  

The threat is still present today, but I’d venture to guess it is not as high as before preventive medication hit the market. As more and more dogs are put on preventives as a result of aggressive marketing to pet owners by pharmaceutical companies, heartworm infection is becoming more of a thing of the past.

Preventive Treatment

In the beginning, a drug called Diethylcarbamazine (DEC), available in little white pills would kill the larvae in one stage of its life, when it was 48 hours old. Dog owners would give this medication on a daily basis (every 24 hours) to make sure the dog had the drug in its system when the larvae reached the age when the medication would kill it.

These pills evolved into the daily chewable Filarabits® tablets which were very popular for many years. Eventually, chewable tabs given monthly and injections given bi-annually were developed. Today, there are four FDA-approved products for heartworm control in dogs. They are ivermectin (Heartgard®), moxidectin (ProHeart®), selamectin (Revolution®) and milbemycin (Interceptor® and Sentinel®).

In fact, as a young dog owner back in the days when they were just coming out with daily preventives for this type of “worm,” I accidentally gave medication to a dog that was heavily invested (a street dog I had adopted from the pound several months earlier). He went into shock and cardiac arrest when tons of the parasites died suddenly clogging his blood stream and strangling his heart. This incident demonstrates the importance of testing before giving any heartworm medication.

Testing and Side Effects

As for testing, you must be off preventive medication for six months before a heartworm test will show results for adults. The blood test measures the female antigen present. Many owners stop giving preventive on November 1 and don’t start up until May 1, but test their dogs before resuming the medication. However, during a mild winter with warm days, mosquitoes can return in the off-season and infect your dog. Also, many boarding kennels require a negative HW test for boarding, so even if you have been on the preventive year-round it might be a good idea to blood test occasionally since humans can also get heartworm.

Pet owners’ biggest challenge is to remember to give their pets the monthly dose. If you forget you can still give a dose late since monthly medications rely on killing the larvae at 45 days old. It kills the month’s prior accumulation of larvae in the blood stream.

One word of warning: Earlier this month a television report on Proheart6® was aired citing possible side effects of this time-released heartworm preventive. Dogs are given an injection once every six months. You can read about the CBS investigative report through an Associated Press story at: http://kyw.com/consumer/local_story_063171726.html and for the manufacturer’s response to these allegations read a letter from Fort Dodge Animal Health at: http://www.proheart6.com/rumorletter.htm.

It is important to note that all dogs may be at risk for side effects from any medications. In a perfect world there would be no side effects, in fact, there would be no heartworm.

Lisa Peterson, a breeder and exhibitor of Norwegian Elkhounds, is a Delegate to the American Kennel Club. Ask Lisa questions at ask@lisa-peterson.com or Dogma Publishing, P.O. Box 307, Newtown, CT 06470.