Ask a Vet: Why Are Heartworm Preventatives Starting to Fail?

Photo of Dr. Eric Barchas by Liz Acosta.

August is high season for heartworm transmission. That is unfortunate, because heartworm disease is serious business.

Heartworms, as the name implies, are worms that live in the circulatory system. The name is a bit of a misnomer — heartworms prefer to live not in the heart, but in the pulmonary arteries (which are major blood vessels leading out of the heart). This matter really boils down to semantics, because worms living in those arteries can cause heart failure as well as severe lung damage. And if there are enough worms, they will be found in the heart as well.

Photo of Dr. Eric Barchas by Liz Acosta.
Photo of Dr. Eric Barchas by Liz Acosta.

Heartworm disease is spread by mosquitoes. The worms have many life stages, which are the bane of freshman vet students in parasitology classes. However, just a few of the life stages are truly relevant in order to understand the disease and its treatment and prevention.

Here are the basics: Adult worms average five to 12 inches in length. They live in dogs and produce microscopic juveniles called microfilaria. Heartworms are prolific breeders; a dog with only a few adult worms may have hundreds of millions of microfilaria in his system.

When a mosquito bites a dog with microfilaria in its bloodstream, the mosquito becomes a vector. The microfilaria go through a few molts in the mosquito and ultimately turn into infective (L3) larva. The L3 are then spread to the next dogs that are bitten by the mosquito.

Mosquito by Shutterstock.
Mosquito by Shutterstock.

The worms go through several more molts in newly infected dogs, and ultimately mature into adults.

Here is the key thing: The L3 larva and the subsequent L4 larva are (or rather were) exquisitely sensitive to heartworm preventatives. Microfilaria also can be killed by the preventatives. All of the other stages are not as sensitive to the drugs.

Preventing heartworm disease used to be very simple and easy. A monthly dose of preventative would kill the L3 and L4 larva before they could mature into the adults that wreak havoc upon dogs.

I have never understood why some people don’t take heartworm disease more seriously. Why would anyone tolerate the idea of worms slithering around in their dog’s heart and major arteries?

Photo of Dr. Eric Barchas by Liz Acosta.

For whatever reason, some people are less bothered by that idea than I am. It is possible to treat heartworm infection in dogs with a product called melarsamine. Melarsamine contains arsenic (which creeps some people out), and treatment complications can occur.

Therefore, for a period of time there have been people who have advocated for a so-called slow kill method, which uses regular doses of preventatives instead of melarsamine. The worms typically die within two years when the slow kill method is employed. It is my opinion that the potential complications of a two-year infection with worms are more worrisome than the potential complications of treatment with melarsamine, but not everyone sees it that way.

For years, parasitology experts have warned that slow-kill methods might lead to heartworms that are resistant to preventatives. Their logic was based upon evolution. When a population (in this case, heartworms) is exposed to a toxin, selective pressure will exist to create resistant individuals. This process will inevitably lead to resistance, especially if large populations are exposed to the toxin (this is how antibiotic resistance develops as well).

When heartworm preventatives are used for their intended purpose, a relatively small number of L3 and L4 larvae are exposed to the drugs. Survivors are not likely to occur. However, when hundreds of millions of microfilaria are exposed, one or two might have a random natural immunity and therefore may slip through. If this happens often enough, a resistant population may develop. Resistant microfilaria might develop into resistant L3 and L4 larvae, and then into adult worms that produce nothing but resistant microfilaria.

Mosquito larvae by Shutterstock.
Mosquito larvae by Shutterstock.

Alas, this doomsday scenario appears to have played out. Resistance to heartworm preventatives has now been reported in the Mississippi River basin. The resistant worms surely will spread.

But did the slow-kill technique truly cause resistance to develop? Recent biomarker studies (scroll down to abstract 31) suggest that it has.

Resistant heartworms mean that until new preventatives are found, more dogs will be infected with heartworm. And it gets worse: Humans can contract heartworm, too, with pathology possible in the lungs, brain, eye, and testicle. As more dogs are infected, the risk to humans will grow.

What should a dedicated dog owner do about this situation? For now, remember that most strains of heartworm are not resistant to the preventatives, so keep giving them every month. And if for some reason your dog contracts heartworm, do not let him suffer for years with worms slithering around in his chest. Talk to your vet about melarsamine instead.

Read more about heartworm-preventative resistance on, and also read the Companion Animal Parasite Council’s rundown on heartworm and guidelines for treatment.

Read more about heartworm on Dogster:

Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)

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