I want to address some of the misconceptions that I deal with every day.

-“I only give heartworm prevention during the summer months.”
-“It’s so hard to remember to give that prevention once a month, I forget to give it sometimes.”
-“My dog doesn’t get bit by mosquitoes.”
-“My dog never goes outside.”
-“I know I last purchased 6 months of heartworm prevention a year and a half ago, but I have plenty at home.”
-“Cats get heartworms?”

Here are the facts on heartworm disease. Heartworm disease is a deadly but preventable disease of both dogs and cats. Heartworms are spaghetti shaped worms that as adults grow to 8-10 inches in length. They are transmitted by a mosquito bite. The mosquito bites an infected dog and sucks up blood and baby worms called microfilaria that are in the infected dog’s blood. These baby worms undergo further development in the salivary glands of the mosquito. They are transferred to another animal when the mosquito feeds again. These microscopic microfilaria are then deposited under the skin of that animal. They migrate through the body as they grow. Over a 4-6 month period they reach adult size and end up in the animal’s pulmonary arteries. These arteries are the vessels that lead from the right side of the heart and carry blood to the lungs for oxygenation. Depending on the size of the animal, as little as one worm can lead to vessel blockage and acute death. I have personally done post mortem examinations on animals with over 200 worms in the heart. To give you a visual of the problem, I use the analogy of hair clogging a plumbing drain. This blockage of the pulmonary arteries leads to congestive heart failure and a whole host of issues with the cardiorespiratory system.
This blog represents a daily conversation in my exam room. Usually at this point of the story I get asked, “How would I know if my pet has this problem?” “What clinical signs would they show? “ Herein lies the problem, most animals don’t demonstrate any recognizable signs until very late in the course of the disease. In fact, if there are clinical signs that develop, it may be too late for treatment. The treatment is much more risky if clinical signs have become apparent. The key to success with heartworm disease lies in preventing the disease. In our worst case scenario, we would certainly want to diagnose the disease before signs develop.

I think that I have heard every possible reason why people’s dogs are not on prevention. These reasons range from:

“We don’t have mosquitoes”,
“My dog is inside only and doesn’t get bit by mosquitoes”,
“I don’t give it in the wintertime because there are no mosquitoes then”,
“It’s too expensive”,
And the most shocking of all, “heartworm disease is not a real disease it is just something vets made up to sell a product”.

Well here are the facts; at our hospital alone, we diagnose about 12 cases every year. Mosquitoes are everywhere. They are in Oklahoma 12 months of the year. Even in December through February we have warm enough temperatures to have mosquitoes. Don’t believe me? You only need to go outside around dawn and dusk on any day the temps are over 60 F. Mosquitoes come indoors just like other flies and bugs, through open doors and windows. As one of my mentors once said “Unless your dog lives on the 100th floor of a skyscraper and never goes outdoors it is not truly an indoor only dog and is most likely exposed to mosquitoes and thus heartworm disease”. The cost of treating heartworm disease ranges from $1500 to $2000. This does not include the cost of managing the long term effects the disease causes. Just because an animal has had heartworm disease once there is no acquired immune response preventing a second infection. Unfortunately this has happened in my practice, a dog has actually been diagnosed with heartworm disease more than twice.

Cats also get heartworm disease. They are not the natural host of the parasite, and thus the disease looks different than it does in dogs. Due to the small size of the cat’s heart and pulmonary arteries compared to the large size of the heartworm parasite, the most common sign associated with an adult heartworm infection is sudden unexplained death.
As I said above cats are not the natural host for the heartworm. Approximately 75% of the parasite’s larval stage is killed during their migration through the lung toward the heart. This rapid death of migrating worms causes a severe inflammatory response in the lungs. This is manifested as lethargy, coughing, vomiting, or asthma like signs. Heartworm disease in many cases is more of a respiratory disease than a heart disease in the cat.

Diagnosis can also be problematic in the cat. The current tests for heartworm disease catch heartworm disease months after exposure. They are not always accurate in the cat. Unfortunately that timing and inaccuracy of the tests are enough for severe clinical signs to have developed in the cat prior to diagnosis. The chemicals that we use to treat heartworm disease in the dog are fatal to the cat. Currently there exists no treatment for cats with heartworm disease other than symptomatic therapy. Thus heartworm disease is often summarized as very challenging to diagnose and impossible to treat. Prevention is the key!!!

That just seems repetitive, but like everything else in this world prevention is the solution. If prevention fails then early detection provides the next best option for a favorable outcome. There are many good preventative options for both dogs and cats. In dogs there are monthly chewable tablets and topical solutions as well as a shot that is given every 6 months. In cats there are topical solutions and oral tablets that should be used monthly. All cats and dogs in our area should be on a heartworm preventative year round. Dogs should have a yearly screening test for any preventative failures.