Confirmed: Some Heartworms Have Become Resistant to Prevention Drugs

What does that mean for dog owners concerned about this blood parasite?

Last Updated on November 18, 2022 by

Unfortunately I was not able to attend the recently ended triennial symposium of the American Heartworm Society in new Orleans. But I have a hunch what the hot topic was this year at the symposium: resistance to heartworm preventatives.

Although New Orleans might not be a terribly conducive learning environment, the location of the symposium was appropriate: New Orleans is the hotbed of heartworm disease (and resistance) in America. And the timing was also appropriate, because last month it was formally announced that experts have reached a consensus: Some heartworms are resistant to preventatives.

What does this mean for dog owners? Read on for some insight.

The basics

Heartworm is a blood parasite of dogs. It is spread by mosquitoes. As the name implies, it is a worm that lives in the heart and the arteries of the lungs. The worms damage the arteries and in some cases the heart. Severe infestations can lead to heart failure. Worms that dislodge from the arteries can cause pulmonary emboli — a condition in which blood supply to the lungs is cut off.

The pathology of heartworm is influenced strongly by worm burden. A large dog infested with one or two worms might not show any symptoms. However, the same dog with 50 worms would be in very big trouble. Smaller dogs require smaller infestations to suffer symptoms.

Symptoms of heartworm include decreased exercise tolerance (tiring out easily), coughing, and difficulty breathing. Severe infestations can cause fainting, liver enlargement, and death.

Diagnosing heartworm

Heathyr recently posted a question on my Facebook page (which is an excellent way to connect with me). She is fostering an adult dog who initially tested positive for heartworm, and then later tested negative. Why would such a thing happen?

It turns out that testing for heartworm is somewhat complicated. The screening test for dogs is the heartworm antigen test. It is a blood test that checks for products released by heartworms into the dog’s bloodstream. However, the test is imperfect: Only adult female heartworms release significant quantities of the antigen. If a dog is infested only with male heartworms, or if the heartworms are immature, a false negative will occur. And the test, like any test, might occasionally not work properly, leading to other false negatives and false positives.

Other common ways to assess for heartworm include radiology (X-rays), echocardiography (ultrasound), and blood work to assess for circulating larvae called microfilaria. However, these tests generally only detect the presence of severe infestations. (Heathyr, I would recommend repeating the blood tests as well as performing radiology and echocardiography on your dog. I would not recommend treating him unless the infestation is confirmed.)

Treating heartworm

Treatment of dogs with heartworm is difficult and complicated. The most effective (and only FDA-approved) method involves use of an arsenic-containing product called melarsomine. This product has been in chronically short supply for years.

Complications from melarsomine treatment are not uncommon. Significant pain might occur at the injection site. And the dying worms can cause pulmonary emboli that can cause severe respiratory problems.

Because of its expense, short supply, and complication rate, some vets have advocated for an alternative to melarsomine treatment: the “slow kill” treatment. In this treatment, dogs receive regular heartworm preventatives and also receive an antibiotic (which is currently hard to obtain) called doxycycline, which in theory kills a bacteria (Wolbachia) upon which heartworms depend. Slow kill treatments can take up to two years to be effective, and dogs must be strictly confined during treatment.

I have never been a fan of slow kill methods. I mentioned that they take up to two years to be effective. During that time, the worms will continue to cause damage to the arteries, heart, and lungs.

Finally, physical removal of worms might be possible in some limited instances using advanced equipment at referral specialty centers.

Preventing heartworm

It should be obvious by now that the best bet by far is to keep dogs from ever being infested with this terrible parasite. When it comes to heartworm, 272 micrograms of prevention is worth a ton of cure.

There are a number of heartworm preventatives on the market; all of them fall into a category of drugs called macrocyclic lactones, which are administered at regular intervals.

Heartworm has a complex lifecycle involving numerous larval stages. Heartworm preventatives kill only some of those stages. Here is how heartworm preventatives work: Dogs become infested with larvae after being bitten by infested mosquitoes. The preventatives kill the larvae before they develop into adult worms. The preventatives do not prevent larval infections — only avoidance of mosquitoes can do that. Heartworm preventatives are recommended in all dogs.

Resistance to heartworm preventatives

It was bound to happen. Anybody who understands natural selection knows that bacteria will eventually become resistant to antibiotics, fleas will become resistant to flea preventatives, malaria will become resistant to chloroquine, and heartworms will become resistant to heartworm preventatives.

Sadly, resistant heartworm has now been confirmed. The resistant strains have been found right in the area where the Heartworm Society Symposium is taking place: the Mississippi Delta. The full geographical extent of the resistant strains is unknown.

Resistance was inevitable, but many experts believe it was hastened by widespread use of the slow kill method. This subject is controversial; some vets still stand by the method. However, researchers and heartworm experts appear to have reached a consensus that the slow kill method should not be used.

Although resistance has developed to all heartworm preventatives, one of them (Advantage Multi) appears at this time to be the least impacted (and therefore the most effective). However, some experts have cautioned that this might be an artifact of the product’s relatively low market share rather than a truly greater efficacy.

What should dog owners do?

Emphatically, dogs should stay on their heartworm preventatives. Although resistance exists, remember that the overwhelming majority of worms are not resistant. And remember that the number of worms infesting a dog dramatically impacts the course of disease. The preventatives might not be able to stop all worms from developing to adulthood, but they will still stop most of them.

Also, don’t panic. If you do not live in the southern United States, heartworm resistance is not likely to be a problem in your area — yet. At this time resistance is a very limited problem. Hopefully new and better preventatives will be developed before resistance becomes common.

Finally, remember that mosquitoes are the bad guys (or rather girls, since only female mosquitoes spread the disease). Follow the advice that is offered to people traveling in areas where malaria, Dengue, and Yellow Fever are common: avoid mosquitoes. Mosquitoes are most active at night, but they are especially bad at dawn and dusk. Keep your dog indoors when mosquitoes are active. Do not, however, apply insect repellent to your dog.

Got a question for Dr. Barchas? Ask our vet in the comments below and your topic 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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