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Why should we care about heartworms?

August 28, 2013
by Jami Stromberg, DVM
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Heartworm disease affects one in ten dogs and one in 20 cats in the US. Since the worm is transmitted by mosquitoes, you can bet that Minnesota is considered an area that sees a lot of this disease. Fortunately, the majority of our patients at BPPH are on heartworm preventative at least part of the year, so we only diagnose the disease in dogs about once a year. However, we work with several rescue organizations that receive animals that have had little to no veterinary care, and heartworm disease is fairly common in these dogs.

I am not going to go into the details of the life cycle, but suffice it to say that little tiny baby worms grow in a mammal (usually a dog or other canid, but heartworm has been seen in over 90 species of mammals), and then are picked up by a mosquito when it takes a blood meal. The baby worms (called microfilariae) mature a little in the mosquito and are injected into another mammal when the mosquito takes another blood meal. The “adolescent” worms migrate to the heart and vessels that come out of the heart and then grow into adults.  If there are male and female adult worms, they reproduce and make more little tiny baby worms, and the cycle repeats itself.

The adult worms can cause damage by both impeding normal functioning of the heart, and by causing an immune system reaction, which can cause inflammation in the blood vessels, kidneys, lungs, and other organs.

Left untreated, the adult worms eventually die, but they can cause a lot of damage in the meantime. Years ago, adult heartworms were killed by an IV injection of arsenic, which, as you can imagine, was a tricky endeavor. About 20 years ago a new product, Immiticide, was introduced. This drug was much safer, although the adverse effect rate was still around 10%.

However, last year the company that makes Immiticide (Merial) stopped making the product because they can no longer obtain the active ingredient. That leaves us with going back to arsenic, or trying a slow-kill method by treating adult heartworms with monthly heartworm preventative. The slow kill method works 95% of the time, but it can take years and the worms can continue to wreak havoc while they are alive.

What is a pet owner to do? It’s an easy choice. Give your dog or cat a heartworm preventive every month. It’s cheap, easy, and nearly 100% effective in preventing this disease.

Jami Stromberg, DVM