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Assess herd in March for lice control needs

Don’t mix treated and untreated cattle, and apply products at the proper dosage and at the proper timing

Lice are a common winter problem in cattle, especially in northern climates with cold weather and short winter days. Their life cycle speeds up and numbers increase dramatically when weather is cold and they have long winter hair to hide in.

Doug Colwell, a livestock parasitologist with Agriculture Canada in Lethbridge, Alta., says sucking and chewing lice can both be a problem for cattle.

“We ran a serological survey in Western Canada (southern Alberta, Saskatchewan and a few herds from Manitoba) for several years, checking for lice, and found 75 to 80 per cent of beef calves coming off range in the fall tested positive for sucking lice,” he says. “This is consistent with results we found a few years earlier, checking calves coming into a feedlot that time of year — prior to the time that they would be treated for lice.”

Calves pick up lice from their mothers or herdmates, and most have lice by end of summer.

Many cattle also have chewing/biting lice, but it’s harder to test for these. “We don’t have a serological test for them,” says Colwell. Many cattle harbour both types.

He did studies on lice populations, using a counting technique to determine typical numbers from January through May on untreated cows.

“By May we couldn’t find lice on the animals using this method,” says Colwell. “There may be a few in areas on the animal where they can hide, such as between the hind legs, but otherwise the animal is relatively free of lice during summer. By mid-October we start seeing lice again.”

Treatment is common

Most ranchers in northern climates treat for lice in late fall or early winter.

“Our basic treatment program is to knock the top off the population growth curve so that by January we don’t have a massive outbreak,” says Colwell.

Today, chewing lice may be more common than biting lice. “There is some indication the products we’re using are not as effective as they once were,” he says. “Also the macrocyclic lactones used as injectables only kill sucking lice.” The chewing lice aren’t sucking blood and therefore are not affected by a systemic product.

The pour-ons have more effect on chewing lice, especially if the products get distributed fairly well over the body. For a while after treatment, this will reduce chewing lice population on the animal.

“We don’t know how effectively these products spread over the body, or whether there may be locations the drug never reaches such as between the hind legs and up around the armpit area of the front legs. These may be areas lice can retreat into and survive,” Colwell explains. This residual population could proliferate and eventually cover the body again after the drug no longer has an effect.

Many herds experience a resurgence in lice before spring — cattle start rubbing and itching again by February or March, and may need another treatment. “There are also some thought that drug resistance may have developed,” says Colwell.

Effective lice control depends on timing. “If you treat too early in the fall, this gives hiding lice a chance to rebuild populations and come back in large numbers,” he says. “If you wait until early winter, there’s less opportunity for them to rebuild.”

Treatment options

The macrocyclic lactones (which include Ivomec, Dectomax, Cydectin and now the generics) do a good job when applied at the right time, Colwell says. October/November is a good period for treatment, if weather is already cold, but not any earlier. If ranchers are preg-checking and vaccinating in early fall they often apply a delousing product, especially if they won’t have the cattle in again until spring. Then they may have serious lice problems in February. Winter-long control can be obtained only if cattle are treated late in the season when lice are starting to build up.

“When macrocyclic lactone products first came on the market, some drug companies gave a guarantee, saying one treatment would last through winter and if you found lice on any of your animals they would pay for retreatment,” says Colwell. “We learned that you can’t mix treated and untreated cattle, use improper dosage, or treat too early. “

It’s usually not necessary to re-treat for lice if a few show up in late March or early April, because lice populations won’t proliferate at that late date. “Lice don’t survive in heat,” says Colwell. “If the cow is standing in bright sunlight in summer, temperature on her skin may go up above the thermal tolerance of a louse or a louse egg. Adult lice are dying and not reproducing, so the population crashes when weather warms up.”

Colwell says no matter what you are using, never underdose. Always treat at the maximum level. If you don’t kill all the lice on an animal, that animal serves as a source of lice to spread to the rest of the herd. Then you may see high levels of lice again before winter is over.

Worth the expense

Colwell recommends retreating later in winter, like February and early March, if lice become a problem. It used to be relatively inexpensive to do this, using a topical oil-based pyrethroid such as Cylence or Boss. These products spread through the hair coat and have enough residual activity to last awhile and get through to spring. Unfortunately they are now more expensive than some of the generic macrocyclic lactones. “I still think they are the best type of follow-up treatment,” he says.

Other methods are useful, such as insecticides on back-rubbers that allow animals to self-treat. “Self-grooming — licking, rubbing and scratching — helps keep lice populations down,” says Colwell. “But long winter hair reduces effectiveness of the tongue to pull lice off. The tongue is a great grooming device, but with a thick hair coat it can’t get to the lice that are right down on the skin. Insecticide back-rubbers and other structures for cattle to scratch on can be a help.”

This also minimizes damage to facilities if cattle rub on these instead of fences.

By March, days are getting longer with more intense sunlight, and most lice populations start dwindling — and retreat to cooler places on the animal.

“Some animals act as carriers; lice don’t leave them during summer. The most effective lice control is culling the carriers. Any animal that is chronically infested will keep spreading lice to the others. There are a few animals in the herd that are highly susceptible to certain parasites. This may be due to ineffectiveness of their immune response, and their genetics.

“You probably shouldn’t make your entire culling decision based on louse populations, but it should be a factor to consider,” says Colwell.

Lack of immune response may be genetic. Some animals have stronger immune systems than others. Carriers usually have some deficiency in their immune system that makes them more susceptible to heavy lice infestation, since cattle normally develop some resistance to lice after exposure.

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