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 preventative-resistant heartworms in Southeast Missouri.
She found nematodes with a near total resistance to ivermectin and avermectin-based preventatives in the studied area. Veterinarian Mason E. Bell of Hillcrest gave examples such as Heartguard.
The study included infected canines from Cape Girardeau, Bollinger, Wayne, Ripley, Butler, Stoddard, Dunklin, Pemiscot, New Madrid, Mississippi and Scott counties.
Fisher explained the preventative-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. Bell said the study reported a severe deficiency in the suitability of milbemycins. He listed Interceptor Plus as an example.
In the participating counties, Interceptor Plus only had a 35.9% effectiveness rate according to Bell.
Fisher stated, “For registration studies, the Food and Drug Administration considers any preventive efficacy less than 100% as a failure.”
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 due to the results of the study. The animal hospital is currently 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.
Editor's note: This story has been updated to correct attribution of claims regarding brand names.