Drug-resistant heartworm strain found in Louisiana Labrador

The strain has been used in several research projects to help understand what leads to drug-resistant heartworms

A drug-resistant strain of heartworm was discovered in a dog in Metairie, La., according to local veterinarian, Cynthia Benbow, DVM, CVA.

Maddie, a yellow lab from Metairie, was a patient of Dr. Benbow.

According to the news release issued by Benbow’s office, Maddie contracted heartworm while on a monthly preventive. The heartworms had resurfaced in Benbow’s long-time patient after a bout three years prior even though Maddie’s owner had, as Benbow described, “religiously” stuck with the prescribed prevention regimen.

Testing revealed the dog was infected with a drug-resistant strain of heartworm.

A sample of Maddie’s blood was sent to Ray Kaplan, DVM, at the Department of Infectious Diseases at University of Georgia (UGA) College of Veterinary Medicine, according to Benbow.

Dr. Kaplan’s lab confirmed that the heartworms were indeed drug-resistant, and established the strain for research purposes, naming it Metairie-2014.

Louisiana hosts “by far the most cases” of suspected drug-resistant heartworm strains, according to Cassan Pulaski, a doctoral candidate at Louisiana State University (LSU). Southern states show higher incidences of likely drug-resistant strains, particularly those surrounding the Mississippi River Delta, he said.

It’s unknown how many drug-resistant heartworm strains exist because local veterinarians do not have the research tools to verify resistant strains, according to Pulaski. Instead, vets determine instances of “suspected” strain resistance when prevention drugs fail. Pulaski said LSU receives reports about apparent drug-resistant heartworms from vets about every other month.

The heartworm strain found in Maddie marks one of three lab-verified resistant strains in Louisiana, Pulaski said. One was discovered in Baton Rouge; the other in Lafayette.

Kaplan said his lab has used the Metairie-2014 strain in several research projects to help understand what leads to drug-resistant heartworms. Maddie has since passed away from old age, but Kaplan said her blood sample could help researchers in creating a faster test to confirm resistant heartworms.

Preventive is crucial

Kaplan, Pulaski, and Benbow all stressed that pets always should remain on heartworm preventive. Even in pets with resistant strains, the medication managed to kill off all but only a few of the heartworms, Kaplan said.

“Those drugs are still preventing the animal from becoming more infected,” he said.

“It is crucial for pet owners to understand the importance of being on consistent, monthly heartworm prevention and a mosquito repellent,” said Benbow. “Since this resistant strain was found in Metairie, the double-protection approach is the best way for owners to protect their pets from now on, no matter what time of the year.”

Benbow urged pet owners in the area to not panic, as it’s extremely rare for dogs to contract the drug-resistant strain of heartworm, she said.

“It’s not common, but it is out there,” she said.

Benbow said thanks to Maddie’s owners, this strain of heartworm is helping researchers at UGA better understand and help solve the problem of drug resistance in heartworm.

Kaplan’s findings were published in the journal, Parasites & Vectors last month.

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14 thoughts on “Drug-resistant heartworm strain found in Louisiana Labrador

    1. Drug resistant means that the parasite will still grow in the presence of a chemical (drug) that would normally kill it. What I would like to no is how do you treat the parasite and or the dog to prevent the heartworm from killing your pet since nothing kills the parasite.

    1. Adult heartworms are naturally resistant to ivermectin to begin with, not immune but resistant. Its the larva that is acquired by being bitten by a mosquito that is susceptible to the ivermectin or other preventatives. They are saying that the larval stage is also resistant to what ever preventative the dog was on but they did not say what kind it was so we are left to speculate.

  1. This form of resistance is to the monthly preventive. The adults can still be killed with the typical adulticides, for now, anyway.
    This just emphasizes the importance of using an appropriate insect repellent on your dog
    to decrease the chance of mosquitoes biting.
    Resistance to he monthly heartworm preventives is definitely on the rise.

  2. 1) So, what about the Katrina dogs who were sent throughout the US? Any follow-up ref this strain? What about changes to protocols for future dispersion of dogs due to natural disasters? Mandatory 1 year HWP for dogs with retesting?
    2) What about heat-treated sample testing? Are we missing dispersion of this type of HWD if not heat-treated testing?? If I wish to test a sample by heat treating, where do I send the sample?
    Thank you

    1. It’s not just Katrina dogs. Every time there is a Hurricane in the south multiple rescue organizations at the behest of national animal foundations swoop in and “rescue” the dogs and disperse them across the country. There are regulations and rules but when it comes to pets they get tossed out the window. Heartworm is just one of the issues.

  3. This news seems so old. As a practicing veterinarian in northeast Arkansas I remember well the summer of 2006. It was my first year out of school in a busy multi doctor practice. Ivermectin containing drugs were the HW prevention of choice at this practice. Around the time of my hire an exponential increase in HW + dogs (that had good prevention history) began to surface. Prior to 2006 this practice would see a handful of dogs that were considered “breaks”. The number rose sharply in 2006 to well over 100! Adjustments were made regarding the HW prevention recommendation (sorry no infomercial here) and the number of breaks sharply declined over the next couple years. I’m still practicing in northeast Arkansas and we do not have an issue with drug resistance / breaks ever since changing preventions.

    Given my “data” is anecdotal the blatant obvious idea of drug resistance has been known for over a decade. These experiences are not confined to my practice but rather are the norm for practices in my region.

  4. So the research paper states Macrocyclic lactones but only lists Ivermectin in its testing. Are they resistant to Macrocyclic Lactones in general or are they only resistant to Ivermectin?

  5. As I recall from something a heartworm researcher told me a few years back: this has already been looked at, and turns out the problem isn’t drug resistance; it’s that at the tiny doses being used for heartworm control, 30 days is too long of a treatment interval, and allows a few individual heartworms that are near the end of the larval stage to survive and mature — and then you get a surprise infection.

    He said the problem was easily fixed by reducing the treatment interval to 15 days. (I imagine it could also be fixed by using a higher dosage that would kill heartworms at all stages.)

  6. While mosquito repellent seems to be universally recommended, are there studies that show a treatment efficacy differential comparing topical repellents to systemic? The best topical repellents are somewhat toxic, especially if used daily over the entire warm weather mosquito season.

    Ideally, but probably not likely given that our patient population are canine instead of human, a genetically modified mosquito akin to that used vs. Zika would be the best solution.