Unsuccessful lice control: Poor timing or insecticide resistance?

Russ Daly Special to the Farm Forum
Farm Forum

We’re at that time of year when lice infestations in cattle become rear their ugly heads. It’s hard to see lice on cattle, but easy to see their effects. Lice cause skin irritation in cattle, inciting them to scratch themselves incessantly. This causes patchy hair loss over their rumps, sides, and shoulders. These bugs are mostly mere nuisances to the animal with few long-term consequences. The more time a critter spends scratching a fence post, however, means less time they’re eating or taking care of their calves. For some cattle (show animals or those to be sold at production sales), big bald patches don’t exactly enhance their appearance.

In wintertime, the woolliness of the bovine hair coat creates favorable conditions for these bugs. Additionally, closer quarters for animals during cold months helps lice move between animals. When springtime hits, lice numbers drop but never completely go away, hiding out in protected areas like the flank to survive the warmer months.

Seeing the effects of lice on their cattle is frustrating for producers, but it’s particularly vexing to see problems when they’ve done what they think are all the right things to prevent such problems.

Most cattle producers in this part of the world give their animals pour-on lice treatments in the fall. Yet, treatment failures seem to be more common. Treatment failures can occur due to several different reasons.

One possible reason is poor treatment timing. Pour-ons given at pregnancy testing time might be given too early some years. Even the best lice control products don’t last all that long: two months is about tops. In the fall, lice are still relatively low in numbers and more “sheltered” on the body. A lot of lice treatments have worn out by the time winter conditions cause lice populations to explode.

Another reason for lice treatment failure could be re-infestation. Could previously-treated animals have had contact with animals that weren’t treated at the same time? Lice from untreated animals could infect treated animals, especially when significant time has passed since they were poured.

The third reason is the scariest. What if our lice products aren’t working anymore? That question isn’t easy to answer in an individual herd. Insecticide-resistant lice infestations don’t physically look different than other lice infestations.

Insecticide resistance would most likely occur in a herd using the same lice product consistently for many years. While a herd may have had good results for many years using Product X, it’s reasonable to expect that it didn’t kill every bug on every critter: some were naturally resistant to the effects of the medication. These surviving bugs hide out through the summertime, only to reproduce and burgeon come winter. Repeated treatments with Product X mean an increasing number of surviving resistant lice over time. One winter, enough of them will be present to cause an observable lice problem.

If parasiticide resistance is a real issue, what can producers do to avoid it? In the short term, producers need to provide their animals some relief by using a different class of insecticide. Some experts advocate to rotate lice product classes from year to year, but another view is that that only encourages resistance to the second product! The future doesn’t look all that great, unfortunately: there aren’t many new types of lice products coming out.

For now, producers would be well-served to work with their veterinarian to make sure the existing products are given at the right time (probably later than we commonly give them now) and that re-infestation is not a problem. Some of our perceived cattle lice treatment failures might not be the fault of the product, they might be more a fault of their timing.

Russ Daly, DVM, is the Extension veterinarian at South Dakota State University. He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.