Moxidectin Implicated As Ostertagi Resistance Driver in Sheep

June 20, 2008 - A recently published Australian study found that sheep farms using only moxidectin drench for parasite control were 2.72 times more likely to have resistance to brown stomach worms than those using ivermectin.

The study collected fecal egg counts of brown stomach worm and parasite control histories from 103 commercial sheep farms in southeastern Australia. Resistance in this study is defined as less than 95 percent reduction of Ostertagia spp. using a fecal egg count 10 days to 14 days post treatment with ivermectin at half the labeled dose. Resistance was evident on 66 percent of sheep farms that used only moxidectin in the preceding five years. In comparison, only 11 percent of sheep farms using only ivermectin showed evidence of resistance. Additionally, prevalence of resistance was significantly higher - 64 percent to 77 percent - on sheep farms where moxidectin had been used for more than two of the preceding five years.

"Ivermectin and moxidectin are both macrocyclic lactones used for parasite control in cattle, sheep and other livestock. While their mode of action is the same, there are differences in their pharmacokinetic and potency profiles that can affect the likelihood of resistance occurring,"1 said James Hawkins, DVM, parasitologist and consultant for Merial Veterinary Services.

Brown stomach worm is considered to be the most harmful and economically important parasite of cattle in temperate areas of the world. Brown stomach worm resistance to all anthelmintics and endectocides is well recognized in sheep. However, in cattle, there are only two well-documented instances of resistance in the United States, a finding consistent with other countries. No other reports of brown stomach worm resistance in the United States have currently held up to scientific scrutiny.

"Resistance has been a buzzword recently," Hawkins says. "It's important that cattle producers know that based on the fact that there have only been two documented cases among the 750,000 herds in the United States, the risk of developing brown stomach worm resistance in cattle is low."

One of the cases of resistance in cattle documented in the United States was with stocker cattle that were managed with a very high stocking rate in a rotational grazing program (75 head per acre or more) with multiple endectocide treatments per year (five to eight times or more). This operation has used this approach for many years on the same pastures and only recently had a problem. In this case, all endectocides were ineffective: doramectin, eprinomectin, ivermectin and moxidectin. Benzimidazoles, fenbendazole and albendazole also were ineffective.

In the other case, doramectin and ivermectin pour-ons were found to be ineffective. Cow/calf herds and stocker operations with typical stocking rates and proper treatment strategies have not experienced problems with resistance in the United States.
Reprinted in part from CattleNetwork.com

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