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A calf’s inflamed flank shows signs of a lice infestation.

Lice: A Wintertime Threat

Wednesday, February 3, 2021

When we think about external parasites that affect livestock, we typically think of battling flies and ticks through the summer and consider the winter to be a welcome respite. We do, however, have lice to contend with in the winter.

Lice are wingless insects that live on the skin of various species, including horses, cattle, sheep, goats and, of course, humans. Lice spread via direct contact between animals but are species-specific, meaning they cannot be transmitted across species. Lice that affect cattle cannot affect a horse, sheep or goat or vice versa. And the best news: lice that affect livestock cannot infest humans. 

There are two main families of lice: biting (or chewing) and sucking lice. Biting lice feed on skin and skin secretions, while sucking lice have a long, piercing mouthpiece that allows them to draw and feed on blood.

Lice infestations cause intense itching. Livestock can spend a tremendous amount of time rubbing and licking in an effort to alleviate their discomfort. They can spend so much time doing this that it decreases their feed intake, feed efficiency, weight gain and growth, which results in a significant financial hit for the enterprise. Further, sucking lice, because of their ability to drain blood, can cause severe anemia and devastate young calves.

Lice thrive in winter. Their survival and transmission is further enhanced by other factors at play in the wintertime, such as long haircoats and huddling behavior.

The first indications of lice infestations in livestock are excessive rubbing (on things such as fenceposts and buildings) and licking. Remember that healthy cattle naturally groom daily, licking their sides and upsweeping the hair. Lice, however, will induce rubbing and licking to the point of removing the hair and damaging the skin beneath. Hairballs that cause obstructions in the stomach and intestines have occurred in animals due to the extreme grooming that lice can induce. Patchy hair loss typically starts on the neck and back and extends down the sides of the body and legs. Examine animals along the topline to look for the lice or their eggs, which are most easily seen on black hair. As veterinarians, we then do the “Scotch tape test,” where we stick a piece of tape onto the animal, picking up any lice that are present. Examining the lice on that tape under a microscope lets us look at the mouthpiece, identifying the offending lice as biting or sucking. Classifying the lice helps guide treatment. 

Lice cannot survive off the animal for more than a day or so. For this reason, our primary focus for control of these parasites is the animal rather than the environment. Caveats to this include situations where cattle may be sharing tack or bedding, as happens with exhibition animals. A halter taken from one animal and placed on another or an animal placed immediately in a stall just evacuated by another animal are examples of how transmission can occur aside from animal-to-animal contact.

Many products are available to control lice, including dusts, sprays, pour-ons and charges for backrubbers. Selection of these products is based on number of animals, facilities, labor and cost. Regardless of the product used, the eggs are not killed, so a repeat treatment is necessary once those eggs have a chance to hatch. This second treatment, done two to three weeks after the first, helps break the life cycle and stops further generations from coming along. All animals in the group need to be treated, regardless of which ones are showing signs of infestation. Be sure to follow the manufacturer’s instructions for product use and follow all withdrawal times. 

Some injectable and pour-on dewormers, such as ivermectin and its cousins doramectin and moxidectin, also kill lice. In the winter, worm control is not a high priority because the conditions are not right for transmission. Frequent treatment with dewormers at low transmission times of year encourages the development of populations of worms that are resistant to the drugs. In addition, injectable products only kill sucking lice; biting lice do not ingest blood, which contains the drug. 

Several years ago, I was called to look at a group of stocker calves who were losing their hair and scratching constantly. The accompanying photo is of one of those calves. On arrival, this group had been given an injectable dewormer with activity against sucking lice and yet they were showing clear signs of lice. We ran a few through the chute, did the Scotch tape test, and when we looked under the microscope, they were all biting lice — and not accessing the anti-louse medication in the blood. We recommended that the calves all be put through the chute and poured with an insecticide, and the problem cleared. I generally recommend wintertime lice control using pure insecticide products applied topically, saving your dewormers (and their associated cost) for use in the spring and fall.

About the author: Dr. Meredyth Jones is an associate professor in the food animal medicine section at Oklahoma State University’s Veterinary Medical Hospital at the College of Veterinary Medicine. She earned her DVM degree from Oklahoma State University and is a Diplomate of the American College of Veterinary Internal Medicine (Large Animal).

Veterinary Viewpoints is provided by the faculty of the OSU Veterinary Medical Hospital. Certified by the American Animal Hospital Association, the hospital is open to the public providing routine and specialized care for all species and 24-hour emergency care, 365 days a year. Call 405-744-7000 for an appointment or more information.

MEDIA CONTACT: Derinda Blakeney, APR | OSU College of Veterinary Medicine | 405-744-6740 | derinda@okstate.edu

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