Rotating compounds is important not just in keeping a population of worms around that will stay susceptible to chemical dewormers. It also helps control other species of internal parasites. The macrocyclic lactones are highly effective against brown stomach worms, which are the number-one internal parasite of concern to cattlemen.

“But none of them are very effective against the number-two worm in young cattle, and that’s Cooperia punctata,” Hollis says. “The white or drench dewormers have an advantage there. One of the things we have seen with continuous use of the avermectins is that we have selected for cattle that are loaded with Cooperia.”

Trouble is, you can’t tell. It’s very difficult to tell the difference between Ostertagia and Cooperia eggs when looking at them under a microscope, Hollis says.

But fecal egg counts can be diagnostic, at least in young cattle. “Pull a fecal sample at the time you treat, pull another fecal sample three weeks later, and look at the relative number of eggs.” Hollis says. “If it was high and went virtually to zero, it means you had Ostertagia and you did an effective job. If it was high and stays high, you’re probably dealing with Cooperia and you probably need to go back with one of the drench dewormers.”

But it’s only worthwhile to do fecal egg counts on young cattle. “In cows, it’s misleading because their immune system has kicked in and is trying to fight the worms. One way it fights the worms is to cause them to lower their egg-laying ability,” Hollis says. “On a cow, a low fecal egg count doesn’t mean much.”

That’s why, by and large, veterinarians don’t recommend doing fecal egg counts, because it’s an added cost that doesn’t always tell you all you need to know. “If you have a limited amount of money, put it into deworming the animal rather than testing,” Arnold advises.

But by all means, they say, put your money into deworming. You’ll be dollars and dimes ahead.


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