New research shows that ivermectin and moxidectin dewormers are losing their efficacy again small strongyles. This is particularly troublesome as these drugs are the last lines of defense against the worms and no new dewormers are in the research pipeline.
The study was the first in the world to confirm small strongyle resistance through repeated testing. Resistance to two of the three deworming drug classes was confirmed years ago and it was predicted that small stronglyes would become resistant to macrocyclic lactones, a class of dewormers of which ivermectin and moxidectin are a part.
The study team included Dr. Martin Nielsen, Professor of Equine Infectious Disease at the M.H. Gluck Equine Research Center, Michael Banahan of Godolphin's Jonabell Farm in Kentucky, and Dr. Ray Kaplan, parasitologist in the Department of Infectious Diseases at the College of Veterinary Medicine at the University of Georgia.
The team found that the efficacy of both ivermectin and moxidectin were reduced in a group of 50 imported Irish Thoroughbreds, though 50 US-bred horses on the same farm had no such resistance. The farm rigorously followed current guidelines for deworming and pulled fecal egg counts on all horses on the farm multiple times.
The horses were dewormed and tested multiple times over an eight-month period. The team concluded that the resistance to ivermectin and moxidectin was imported with the Irish horses. They note that this demonstrates how quickly resistant parasites can spread across the globe. They encourage farm and horse owners to utilize fecal egg count tests and to stringently follow deworming guidelines to attempt to increase the longevity of the efficacy of ivermectin and moxidectin.
Read the study here.
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