But in an exam room last week, I gave my lesson on heartworms and heartworm preventative to a client. Then I listened to the client discuss that they did not want to put their pet on heartworm preventative. I know that there are different levels of care that people want for different pet and that money is often an issue and I let it go. But later that night, I realized that many clients don’t really understand why their dog needs heartworm medication.
Heartworm disease starts when a mosquito bites an infected dog. The microfilaria are taken up with the blood and move to the mosquito kidneys for two weeks of growth. The slightly larger larvae are then deposited on the skin when the mosquito bites again. The larvae quickly burrow into the skin and the capillaries of the skin. (By the way, over seventy species of mosquitos can transmit heartworm larvae.)
Once back into a dog (or ferret or cat), the larvae grow and molt and grow and molt again. They outgrow the capillaries and start moving to the heart and lungs. The worms can be ten to fourteen inches long by the time they are living in the ventricles of the heart. The foot-long worms then get together, mate and have babies. (The males can be differentiated from the females by a corkscrew-shaped tail that holds on to the female to assist in mating.) These babies are picked up by a mosquito and the cycle starts all over again.
Cats, ferrets, foxes and some other animals all can get heartworms, but they may have some natural immunity that makes them not as susceptible. Coyotes get heartworms about as often as dogs. (When I was in vet school, I concurrently worked on my masters degree and studied over 500 coyotes and foxes for heartworm prevalence. I received research awards and presented at national conferences.)
A foot-long worm in a small pet heart is bad news. Heartworms damage the lining of the lungs, thrombotic events (clots) and granulomas due to dead or dying worms. The constant cracking the whip motion of the worms in the blood vessels cause the pulmonary vessels to become thickened and tortuous. The worms cause the valves to leak causing a reduction in cardiac output and pulmonary hypertension. The right side of the heart enlarges and can fail. Although in the early stages a pet can look normal, severe or long-term infections can cause coughing, difficult breathing, weight loss, abdominal fluid retention, exercise intolerance and arrhythmias. It can also damage the kidneys by a secondary antigen-antibody complex deposition.
Sometimes the worms get lost and are in strange places in the body. I removed a heartworm from the anterior chamber of an eye once. Although it is very rare in our area, sometimes there are so many worms that the pet goes into caval syndrome. Worms cause enough problems with circulation that the heart beats too rapidly for the heart to fill and blood does not circulated. Sudden collapse and death can occur.
But most of the time, we find dogs are positive on a simple blood test at the annual exam. And to be fair more often we see tick-borne diseases than heartworm disease. So, owners think that it is not important. After all, whipworms and hookworms can kill pets and fleas are bad. When we notice a problem, we treat the animal and they are all better.
But heartworms are different. When intestinal parasites die, they just move out with the poop. When fleas die, we give the pet a bath or vacuum. When heartworms die, their graveyard is the lungs. The tiny white blood cells must clean up the foot-long worms. This is not an easy or quick procedure. If not done carefully, the patient may still die even after (or during) treatment. Not to mention, the drug is somewhat expensive (think hundreds).
So, while it is okay to wait until you have fleas to treat them, heartworms really need to be prevented or killed while they are under the skin and tiny. Prevention, not treatment. Especially, this peak mosquito year. Get your dog tested, get them on preventative and miss the expense and scare of heartworm treatment.