The Hillcrest Animal Hospital in Poplar Bluff partnered with Dr. Peggy Fisher of the University of Missouri-College of Veterinary Medicine to study the prevalence of treatment-resistant heartworms in Southeast Missouri.

Fisher found nematodes with a near total resistance to ivermectin- and avermectin-based treatments such as Heartguard in the studied area.

The study included infected canines from Bollinger, Butler, Cape Girardeau, Dunklin, Mississippi, New Madrid, Pemiscot, Ripley, Scott, Stoddard and Wayne counties.

Fisher explained the treatment-resistant forms of Dirofilaria immitis, the parasite that causes heartworm disease, had been previously confirmed in the lower Mississippi Delta but were poorly studied in Southeast Missouri.

Spread primarily by mosquitoes, the study highlighted the swampy low-lying areas of the Missouri portion of the Delta as virulent breeding grounds of the carriers.

“The area has a unique environment for Dirofilaria immitis transmission and proliferation of the mosquito vector,” Fisher informed.

After an animal is bitten by a mosquito carrying the parasite, it may develop into heartworm disease causing fatigue and possibly death. Fisher also reported a severe deficiency in the suitability of milbemycins such as Interceptor Plus.

In the participating counties, Interceptor Plus only had a 35.9% effectiveness rate.

Fisher stated, “For registration studies, the Food and Drug Administration considers any preventive efficacy less than 100% as a failure.”

Veterinarian Mason E. Bell of Hillcrest wrote in a post on the animal hospital’s Facebook page, “Moxidectin (Simparic Trio, Proheart 6, and Proheart 12) are still 97-100% effective.” However, he noted Hillcrest will stop carrying Interceptor Plus because of the results of the study. The animal hospital is searching for a low-cost moxidectin substitute for this product. “We strongly recommend these patients switch to Simparica Trio or Proheart6/Proheart 12 during their annual exam,” Bell wrote.