Lice infest cattle all year, but numbers are usually low in summer because most of them are shed off in the spring with winter hair. Lice also tend to leave the hottest areas of the animal in summer and don’t reproduce as fast. So generally, producers generally don’t need to treat cattle for lice in the spring if a few show up in late March or early April, because the population won’t grow at that late date. 

“Lice don’t survive very well in heat. If the cow is standing in bright sunlight in summer, the temperature on the skin may go up to {115 to 120 degrees F} and this is outside the thermal tolerance of a louse; a louse or a louse egg can’t survive at those temperatures. Adult lice are dying off and not reproducing, so the population crashes when weather warms up,” explains Doug Colwell, livestock parasitologist with Agriculture and Agri-Food Canada in Lethbridge, Alberta.

Winter, however, is a different story. “We ran a serological survey here in western Canada [southern Alberta, Saskatchewan and a few herds from Manitoba] for several years, checking for lice,” says Colwell.  “We found that 75% to 80% of beef calves coming off range in the fall tested positive for sucking lice. This is fairly consistent with results we found a few years earlier, checking calves coming into a feedlot about the same time of year.” Calves pick up lice from their mothers or herdmates, and most of them have lice by end of summer.

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Many cattle also have chewing/biting lice, but it’s harder to test for these. “We don’t have recent information on prevalence because we don’t have a serological test for them. The counting survey we did about 15 years ago indicated a fairly high proportion of calves had chewing lice,” says Colwell. Many cattle harbor both types.

“By mid-October we started seeing lice again. Our basic treatment program has been to knock the top off the population growth curve so that by January we don’t have a massive outbreak,” he explains.

Some animals, due to poor immune function or some other factors, are more vulnerable to extensive lice populations and transmit lice to the other cattle in the herd. “This is the old 80-20 rule; about 80% of the cattle don’t have lice and 20% do, for some reason. The carrier animals always have heavy loads, and general recommendation is to cull those,” says Colwell. Cattle with lice readily pass them to herdmates through direct contact, since cattle are social animals. 

Smith Thomas is a rancher and freelance writer based in Salmon, Idaho.

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