Winning The Battle Against Lice

By: Heather Smith Thomas

Lice are a common winter problem. All lice cause discomfort and itching, and sucking lice rob essential nutrients when cattle need it most—in cold weather. Animals carrying a heavy population of sucking lice may become anemic, suffer weight loss and become susceptible to disease. Cattle that are continually rubbing can damage facilities.

Dr. Larry Hawkins, Technical Services Veterinarian at Bayer, says there are many species of lice but only three species of sucking lice and two species of biting lice commonly seen in the U.S. Many producers feel that some delousing products control sucking lice more effectively than biting lice. “We often hear about this problem. Injectable products are usually thought to be better at killing sucking lice,” he says.

“With pour-on products there is usually a fairly good kill of adults and nymphs because all lice are on the body surface, but our data shows it takes a little longer to control sucking lice than biting lice–probably due to their location on the body.”

In winter, life cycle of all lice takes about 21 days from egg to egg-laying adult. “Eggs hatch in seven days, and 14 days later they have matured and are ready to lay eggs again. If you start with just two lice in September, laying 40 eggs every three weeks (eggs that keep hatching and maturing to lay more eggs) by mid-January you could theoretically have a million lice on the animal,” says Hawkins.

Many products kill adult lice but not the eggs. They go ahead and hatch, and three weeks later the cattle have lice again. Producers often think the product didn’t work very well, but label directions recommend retreatment in two to three weeks to kill young lice that hatched after the first treatment.

Another problem is inadequate treatment. “Lice are found on the nose, dewlap, in the armpits and in the groin area where a pour-on may not reach very well. Lice move around, however, and are likely to come into contact with the pour-on if it was applied all along the back and on the poll. If we just put the dosage in the middle of the cow’s back, it’s a long ways to the nose, brisket or armpits from that one spot, so some lice may not contact the product. It’s best to spread it out and put a little on the poll and from withers to tail-head with the rest of it, to get better control,” says Hawkins.

All products must be applied properly, for best affect. “And with many products you have to repeat the treatment within two to three weeks to get the lice that hatched since the first treatment. This is the only way to ensure that the cattle won’t have another outbreak of lice in mid to late winter.”

One product called Clean-Up kills eggs as well as adults. “It contains an insect growth regulator proven to stop eggs from maturing and hatching, and a pyrethroid that kills adult lice. One treatment gives control,” says Hawkins. If a producer won’t have opportunity to run cattle through and treat again, this would be the best product to use.

After a pour-on application of Clean-Up, it takes about two weeks for sucking lice to die. By contrast, an injectable product that treats systemically kills sucking lice quicker because they encounter the product via blood they are sucking, but chewing lice do not feed on blood. With a pour-on placed along the back, it may take some time for the product to spread down through the hair in the oil layer on the skin surface to kill the lice.

“Another option after treatment in late fall (when preg-checking and vaccinating in a cow-calf herd) is supplemental treatments for later control. If cows are wintered on range and won’t be handled again, producers can hang dust bags or use other self-treating methods that cattle can rub on and be treated,” he says. These delousing devices must be located where cattle frequently congregate, so they have an opportunity to use them—but some animals never use them.

“Some producers think that after a treatment all lice are instantly dead, but it takes a couple weeks, in some cases, for those lice to all die. They may be impaired and not reproducing, but still on the animal; it takes some time for them to die,” says Hawkins.

People need to understand the importance of not mixing untreated cattle with the treated ones or lice will spread again to the treated cattle. “Also, if you mix cattle that you treated 30 days ago with a group of newly treated cattle, lice may be all gone from the animals you treated first but they could be re-infested from the recently-treated cattle that still have lice.” Some of the lice on the newly-treated animals have not yet traveled around enough to encounter the insecticide product and may transmit to other cattle if they have close contact.

“They don’t even have to be in the same group; nose-to-nose contact through the fence or at fence-line water tanks can be enough to spread lice. I was at a feedyard once time that had treated cattle on one side of the fence. The other side was the alley to the processing chute. Treated cattle were being exposed to animals that came past, and the treated herd was immediately re-infested. As soon as the insecticide wore off, those animals had lice again, from newly arrived animals,” explains Hawkins.