A lousy deal: Controlling lice in beef cattle
January 11, 2018
Lice are a common problem in winter. Heavy infestations of sucking lice rob nutrition from cattle when they need it most, and chewing lice cause discomfort and itching. A lice-infested animal may lose weight and become susceptible to disease.
Dr. Bill Lias, Interstate Vet Clinic, Brandon, South Dakota, says winter is prime time for lice to explode in numbers. "When it's warm, lice don't want to be on the warm back of a black cow with sunshine on it. They thrive in colder weather," he says.
Two types of lice infest cattle—sucking lice, which suck blood, and biting lice, which feed on dander and debris in the skin surface. Tail lice can cause cattle to lose their switch.
"There's some argument within the industry regarding how much economic loss or inefficiency the biting lice actually cause," says Dr. Dave Barz, Northwest Veterinary and Supply, Parkston, South Dakota. They don't usually directly make the animal anemic or cause weight loss, but they can make the cattle miserably and itchy, and cause them to rub out so much hair they suffer cold stress, and can even reduce their immunity.
"If an animal is weak, and parasites are taking blood, that animal is more susceptible to pneumonia, scours and other secondary infections. This is why lice control is so important and not just because the cattle are scratching/rubbing the fence down. Lice are nibbling away at the potential of your herd," says Barz.
"You can really see their damage. We don't see internal parasites, but we can see lice and the damage they do. This doesn't look good for our industry because it looks like we don't take care of our livestock. There are plenty of people watching, and lousy cattle look terrible. Someone might see that those cattle look uncomfortable and think we are not treating them properly," he says.
"When the avermectins first came out, all we had was injectable products, and they don't kill the biting lice. Those years were some of the worst lice infestations I'd ever seen," says Barz. Even though producers treated their cattle, there was no effect on the biting lice, since the biting lice don't ingest blood.
"The pour-ons are considered effective against both kinds, but there seems to be some increase in lice resistance to these products over the years; they may not be as effective as they once were," says Lais. "In this region, many producers like to use pour-ons in the avermectin class (ivermectins, cydecton, dexomax, etc.) because they want to kill the internal parasites as well. For convenience it's nice to be able to hit them with just one product and feel we've covered everything."
Myxydectins, like cydectin, have touted longer residual effects, but don't kill the eggs. The ivermectin pour-ons were originally thought to have enough residual effect to handle the new hatch, but lice may be developing resistance, like internal parasites have.
"This is inevitable. When we keep using the same products year after year we eventually select for the resistant strains of parasites," says Lias.
"When the avermectin pour-on products came on the market, they worked very well," says Barz. "Feedlots were using just a quarter dose on the backs of cattle and it was controlling lice." While that seemed like all that was necessary, it contributed to resistance development.
"Those products have now been on the market for at least 15 years so we've built up resistance in populations of lice," says Barz.
Russ Daly, DVM, extension veterinarian/professor, South Dakota State University, says lice control can be challenging and producers often see a resurgence in lice populations after cattle are treated. "There are several things that play a role, including the fact that most products for killing lice often don't last as long as needed."
Most products will kill the adult lice but not the eggs, so within three to four weeks there can be a newly hatched population of lice on the animal.
"Some products are able to last long enough to control a couple cycles of lice emergence, but we can't expect to have lice coverage for more than a couple months at the most," Daly says.
"Regarding resistance issues, often I think the problem is not so much with the product than with timing, or just a heavy infestation. We've used some of these products for a number of years, but unless we are using the same product frequently throughout the year, for deworming as well as for lice control, I am not sure resistance is building that quickly. We are beginning to see more resistance to deworming products, particularly in parts of the country where they are used more often," says Daly.
He thinks that often it's a case of the product not lasting as long as you'd hoped, bad timing for the treatment, inadequate dose, or not delousing every animal in the herd, rather than ineffective treatment.
Timing the treatment to when the lice are most active—later in the winter–rather than when it's most convenient, like at preconditioning or preg-checking, may provide better coverage.
"Your treatment will cover more of the winter months. In addition, more of the adult lice are active by then. You have more chance of killing them later rather than earlier in the fall when the adult lice are fewer and more hidden on the animal," Daly says.
"The other issue is the hatch that occurs after we treat the cattle," says Lais. "If a person could re-treat the cattle two to three weeks after using an avermectin pour-on product (following that with a pyrethroid pour-on like Boss or DeLice or others) this would provide a more complete kill." The second treatment would kill the newly hatched lice before they mature enough to start laying eggs.
Lice have about a 28-day life cycle. Adult lice lay eggs, the eggs hatch and become nymphs and then mature and become adults about 28 days after hatching. None of the products kill the eggs, and most of them don't have a long enough residual effect to kill the lice that will hatch out later, Barz said.
"When the avermectins came out, they were originally marketed as one pour once a year, to take care of all your lice problems. We started to have problems 10 years ago, and the companies that made them realized they couldn't say that one treatment would last all winter. They said we have to pour the cattle at least twice—with the treatments about 28 to 30 days apart, to kill any lice that hatched after the first treatment," says Barz.
For most beef operations, however, cattle are poured when the producer is working them (pregnancy checking, vaccinating, etc.) or getting cattle in for some reason. It is very convenient to pour them once, but not always possible to give that second treatment.
A new product called Clean-Up II seems to be more effective in a one-dose treatment. It contains a pyrethroid, which kills adult lice, and an insect growth regulator that keeps nymphs and newly-hatched lice from maturing, says Barz. It has enough residual effect to thwart lice that hatch out after the treatment.
"We've been using that for a couple of years and are seeing better results. In our area, if you start to have problems, the company you bought the product from will usually give you more, so you can get them re-poured. That's been their guarantee, at this point in time," he says.
Lias said the fact that the entire life cycle of lice happens on the host animal makes control a little easier. "With lice, everything happens on the cow, so if you are judicious in the products you use and the timing of treatments, you should have good control," says Lias.
This means delousing every animal, and not letting treated animals mingle with untreated animals that might re-infest the treated cattle. "When I see flare-ups of lice again in the winter, it is often because the producer was not careful about making sure that all cattle got treated at the same time," he says.
Exposure to an untreated animal is all it takes, and the lice start all over again. This is especially true in winter when cattle are more confined—grouped for feeding or calving. "This may negate all the good you did in treating just part of the herd," he says.
"The choice for fall treatment is often one of the avermectins because they control internal parasites," Lias says.
Many ranchers in the Dakotas pour cattle with an avermectin product at turnout time (to kill internal as well as external parasites) and again in the fall at roundup, to kill lice.
If you have to treat again during winter, it's best to use a pour-on pyrethroid product. "Otherwise, if you are in a region that has cattle grubs, you'd have to be careful on the timing in late winter (to not kill the grubs at the phase of their life cycle that might cause a reaction in the esophagus or along the spinal cord). If you have grubs, you'd want to avoid the avermectins," he says.
Other tactics for late winter control include the oils, dusters, back-rubbers, etc. that can be installed in the pen or pasture for cattle to rub on and self-treat for the lice that are causing itching. Each producer needs to figure out a strategy that works best for their own situation and management.
"Any time you are handling the animals you can think about using a pour-on," Barz says.
"We've talked about rotating the pour-on products, using different ones different years, but we are seeing resistance to all of them. I don't know that certain products are any better than the others because we're not seeing much difference. The only one that is really helping us right now (in terms of thwarting a new hatch of lice) is the Clean-Up II," Barz says.
"In some of our feedlots we've been using injectable and pour-on products at the same time–a full dose of each–and in those groups of cattle we haven't seen as much problem with lice recurring. Hopefully we are getting a better kill, and maybe more residual effect."
Before insecticides, ranchers used natural methods of lice control, such as feeding more protein and using back-rubbers with oil on them. The oiliness tends to deter lice, Barz said. "But these are only spot treatments. Also, in every herd it seems like there's a cow or two that act as carriers; they have heavier infestations and may have lice even after treatment, spreading lice to the other cattle."
Calves are even more susceptible to the effects of lice than cows, and lice are easily transferred from cows to calves. "This is why controlling lice on the cows is so important, so they won't spread lice to their calves," Barz said.
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