Dewormer resistance real

Nov 12, 2010
by DTN

Ted Dyer didn’t want to keep guessing about the effectiveness of commonly used dewormers in his cattle herd. So last year, the University of Georgia animal scientist conducted a trial on the Northwest Georgia Research and Education Center’s weaned calves. His verdict? “It looks like there are resistance problems with some dewormers.”

In Dyer’s study, 80 head of weaned calves were divided into four groups. The first group received no dewormer. The second group was treated with fenbendazole oral drench (Safeguard); the third group got injectable Cydectin; the fourth group got both Safeguard and injectable ivermectin (name brand Ivomec, not a generic).

Calves were dewormed Sept. 8, and fecal samples were taken so fecal egg counts (FEC) could be done later. All the calves were then placed in the same pasture. Sixteen days later, more fecal samples were taken, along with the follow-up FEC. “The Cydectin had a much lower reduction in FEC—42.4 percent,” says Dyer.

University of Georgia veterinarian Ray Kaplan says, “The worms appear to have resistance to the macrocyclic lactone drug group. This group includes both Cydectin and Ivomec.”

Kaplan, whose lab did the FEC work, says, “We didn’t expect to see resistance in a cow/calf operation because cows do not get treated very frequently.” But, he adds, “these drugs have been used for almost 30 years.”

Weight gain differences minimal

Another surprise was the small difference in weight gain between the groups. The group that wasn’t dewormed had a 1.68-pound average daily gain (ADG) in the 57 days following deworming. The Safeguard group had an ADG of 1.83 pounds; the Cydectin group had an ADG of 1.74 pounds; the Safeguard plus Ivomec treated group had an ADG of 1.79 pounds.

Dyer’s research followed the calves to the feedlot. “There was not much difference in their weight per day of age there either,” he adds. “These cattle were from a well-managed herd. There weren’t many parasites for them to pick up. I think you would have seen more of a difference if the herd weren’t so well-managed.”

Kaplan says, “Part of it [the small difference in ADG] was because of the short time of the study. Most likely it was because Cooperia [the type of parasite that showed resistance to the dewormer] doesn’t have as big an impact on production as Ostertagia.”

Doug Ensley, veterinarian with Boehringer-Ingelheim, the manufacturer for Cydectin, says, “The Cydectin group contained primarily Cooperia ssp. and no Ostertagia, while in the samples from the other groups, they found

Ostertagia and Cooperia. Cydectin continues to show excellent results against Ostertagia, as shown in this study, even in a herd where macrocyclic lactones have been used for years.”

Customize for control

That still leaves the question of choice of dewormers. Which is best?

Which is least likely to fail due to resistance? Kaplan says the only way to know if you have a resistance problem is to do fecal egg counts on your farm.

“Just because a drug works at the center doesn’t mean it will work in your herd,” he says. “Just because the parasites at the center are resistant to certain parasites doesn’t mean the parasites will be resistant at your farm. Each farm has different circumstances.”

Kaplan recommends working with your veterinarian to do the FEC. He or she can tell you how to prepare the manure samples and when to do them.

They can either do the FEC in their office or send them off for both an egg count and a parasite ID. By doing FEC before and after deworming, you can get an idea as to which dewormers are working on the particular internal parasites present on your operation.

“If you don’t want to test [FECs], the other option is to use a combination of drugs from different classes,” adds Kaplan. He emphasizes that the drugs do need to be from two different classes, though, not just different names or brands.

“If a parasite is resistant to one drug in that class, it will be resistant to all the drugs in that class,” Kaplan explains. For example, Ivomec, Cydectin and Dectomax are all macrocyclic lactones. The white dewormers like Safeguard, Panacur, Valbazen and Synanthic are all in the benzimidazole class.

In the study at the Calhoun center, there wasn’t much of an advantage in combining treatment with Safeguard and Ivomec as opposed to treatment with just Ivomec, even though they are in two different drug classes. With Safeguard, there was a 99.2 percent drop in FEC after treatment while there was a 99.6 percent drop with the combination of Ivomec and Safeguard.

Dyer says the bottom line is producers need to take a hard look at their deworming program. “You can’t just go out and deworm your cattle like you used to. It looks like producers may be overusing some of the lower-cost dewormers. And it appears that over long periods of time, when you use the same family of dewormers, they aren’t as effective. Some producers are even using them not only for deworming, but for fly control as well.”

He agrees with Kaplan on a deworming strategy. “Take samples to find out the FEC in your herd and what parasites are in those counts, then deworm accordingly.” — DTN