Barnyard Basics: Lice control usually takes twice

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Lice are a common winter problem in cattle, especially in cold climates.

There are two kinds of lice that live on cattle: the sucking lice that live on blood and the biting, chewing lice that feed on the skin surface. Heavy infestations of sucking lice rob nutrition from cattle when they need it most. A lice-infested animal may lose weight, look unthrifty, and is more susceptible to disease.

Dr. Dave Barz, of Northwest Veterinary and Supply in Parkston, S.D., says there are three species of sucking lice that feed on blood, and mainly just one species of biting/chewing lice that parasitize cattle.

“There are also tail lice that are involved in cattle losing their switch, and there are some challenges involved in controlling those, as well,” he says. “When the avermectin class of dewormer-delousing drugs first came out, all we had was injectable products, and they don’t kill the biting lice. Those years were some of the worst lice infestations I’d ever seen.”

Even though producers treated their cattle, these treatments had no effect on the biting lice.

“There’s some argument within the cattle industry regarding how much economic loss or inefficiencies the biting lice actually cause,” he says.

They don’t make the animal anemic from blood loss, or cause weight loss directly, but they make cattle miserable and itchy. Affected animals may not eat as much because they are too busy itching and scratching. The cattle may rub out so much hair that they suffer more cold stress.

“You can really see their damage. We don’t see internal parasites, but we can see the lice and the damage that they do. This also doesn’t look good for our industry because it looks like we don’t take care of our livestock. There are plenty of people watching, and lousy cattle look terrible. Someone might see that those cattle looking scruffy and uncomfortable and think we are not treating them properly,” Barz says. “When the avermectin pour-on products came on the market, they worked much better. I can remember when they first came out, the feedlots were using just a quarter dose on the backs of cattle and it was controlling lice very well. We are our own enemy, however, because under-dosing can lead to resistance in the lice. Those products have now been on the market for at least 15 years so by now we’ve built up a lot of resistance in the current populations of lice.”

Cattle producers need to remember that lice have about a 28-day life cycle. The adult lice lay eggs, the eggs hatch and become nymphs and then they mature and become adults about 28 days after hatching from the eggs.

“If we use a product that merely kills the adults and some of the nymphs that have hatched, there’s still a good chance of more lice coming on from the eggs that have not yet hatched,” Barz says.

None of the products kill the eggs, and most of them don’t have a long enough residual effect to kill the lice that will hatch out later.

“When those avermectins came out, they were originally marketed as one pour, once a year, to take care of all your lice problems,” he says. “We started to have more problems 10 years ago, and the companies that made the delousing products realized they couldn’t say that one treatment would last all winter. They said we have to pour the cattle at least twice with the treatments about 28 to 30 days apart, to kill any lice that hatched after the first treatment.”

For most beef operations, however, the cattle are poured only when the producer is working them and putting them through the chute for some other purpose like pregnancy checking, vaccinating, etc. or getting the cattle in for some reason. It is very convenient to pour them once, but not always possible to give that second treatment.

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