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Schell: Parasite control becomes even more important during stressful periods

by Wyoming Livestock Roundup

When cattle are at their most vulnerable, that is when parasites are likely to appear, according to a scientist with the University of Wyoming (UW) Extension.

After a harsh Wyoming winter, some ranchers and scientists in the state found out first-hand how bad a cattle lice infestation can get.

“We had an issue last spring with some of the ranchers in the Laramie Valley,” explains Scott Schell. “Their cattle came down with a heavy infestation of lice.”

“Some of the heavy pregnant cows got down trying to scratch their backs and died,” he says.

These cattle were in the middle of late gestation and early lactation, which is considered two of the most stressful periods for cows. The ranchers that were impacted suffered both production and death losses.

Although the state doesn’t typically see heavy lice infestations, Schell attributes it to the harsh spring weather. This year, Schell is attempting to inform ranchers about what happened, so they can be better prepared.


Schell says, while it is possible for healthy cattle to have an immune response that will help suppress sucking lice, the key is having cattle that are not nutritionally stressed.

“If we have a rough winter, the cattle will be more stressed than usual, which makes them more susceptible,” he says.

Lice live in the microderm layer of the hair right next to the skin. If the skin gets too hot, they retreat to cooler areas, like the armpit or crotch of the animal.

Lice control

Lice can come in different types, including blood-feeders, chewing or biting lice.

“The blood-feeders feed on the blood of the animal and cause the animal to get anemia because of the nutrients they steal,” Schell explains.

Although injectable drugs will control sucking lice, they don’t have an impact on chewing or biting lice, Schell says.

“Chewing and biting lice are not feeding on any body parts that have a high enough level of the injectable to kill them,” he explains. “Producers will need a separate form of treatment to control them.”

Chewing lice have a three to four week life cycle, which is close to how long most parasite control methods are viable. Most pour-ons only last 10 days, which doesn’t cover the full period until the lice eggs hatch, he says.

“If the hair coat on the animal is thick, it is a perfect environment for lice,” the Extension educator explains. “Lice don’t need a male to reproduce, so they all produce eggs during this time.”

Producers planning to apply an insecticide need to realize they can’t use a pour-on and an injectable with the same active ingredients at the same time.

“If ranchers are using an injectable, they should find something to use that will be compatible with their injectable,” Schell tells producers. “Look at the labeling and know how long it is going to last.”

“Eggs and nits will be immune to whatever the producer applies, so we will have to wait until the eggs hatch to treat the cattle,” he explains. “The product needs to last longer than two weeks to get all the eggs.”

Novel products

Producers can check into a newer product on the market called Clean Up II. This product contains a pyrethoid, which is fast-acting and kills adult biting and sucking lice and louse eggs before they hatch.

Clean Up II also has a secondary ingredient, which extends the residual control for a long period of time.

Producers may also want to consider using back-rubbers, dust bags and ear tags, along with an injectable or pour-on.

“The key to back rubbers and dust bags is to place them where the cattle have to go through them to get to something they want,” Schell explains.

Selection decisions

Producers should also look at lice-resistance as a selection tool, Schell continues.

“There are studies out there that cattle can be selected for lice-resistance,” he explains. “It mostly applies to sucking lice because they generate an immune response from feeding on the blood. The immune system of the cattle can recognize that and develop resistance to them.”

“If it is within the scope of an operation, ranchers should select for animals resistant to parasites because that trait contributes to positive gain in the calves and the cows will breed back sooner,” adds UW Extension Educator Derek Scasta.

On the other hand, if female lice can develop resistance to insecticide, she can make clones of herself that will be genetically identical and will also be resistant to insecticide.

“Using the same product over and over without using resistance management could cause a problem,” Schell says.

Studies in Alberta, Canada have also shown some cows can also become chronic carriers of lice.  Chronic carriers can be caused by genetics or even be the older cows in the herd, Schell says.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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