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New drug-resistant heartworm strains identified

Louisiana veterinarian Cynthia Benbow urges “double protection” for dogs — a heartworm preventive and mosquito repellent.

New drug-resistant heartworm strains identified
Dr. Cynthia Benbow and her patient, Madie

A drug-resistant strain of heartworms infected a Louisiana dog whose owner was meticulous about administering a monthly heartworm preventive, according to a report published in the Nov. 9 issue of the journal Parasites and Vectors.

The finding demonstrates the need to not only administer heartworm preventives but also control the mosquito population, said Cynthia Benbow, DVM, CVA, of Benbow Veterinary Services in Metairie, Louisiana. Dr. Benbow’s patient, a 13-year-old Labrador retriever named Madie, contracted heartworm despite being on the popular preventive Trifexis (spinosad and milbemycin oxime).

“It is crucial for pet owners to understand the importance of being on consistent, monthly heartworm prevention and a mosquito repellent,” Dr. Benbow said. “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.”

The case began in March 2011 when Madie was diagnosed with and treated for heartworms. Dr. Benbow sent a blood sample to University of Georgia Professor Ray Kaplan, DVM, Ph.D., DACVM, DVM, whose laboratory confirmed a drug-resistant heartworm strain subsequently called Metairie-2014. A second dog, a Mississippi canine also on a regular heartworm preventive, was found to have a different drug-resistant strain, named Yazoo-2013.

The American Heartworm Society, which funded publication of the research, acknowledges that dogs can become infected with drug-resistant heartworms.

“However, there are no indications that resistance is common, nor is resistance increasing at a rapid rate,” the organization states on its website.

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