As temperatures drop and cattle begin experiencing the physiological stressors of winter, external parasites, especially lice, move from being a background nuisance to economically meaningful threats. In a recent discussion on “DocTalk,” Kansas State Beef Veterinarian Dr. A.J. Tarpoff outlined what practitioners need to know about lice biology, diagnosis, treatment timing and client communication. Winter-specific parasite management is critical to prevent losses that occur when herds remain untreated or undertreated.
Lice: The Dominant Winter Parasite
Lice remain the most impactful external parasite of cattle during winter months. As Tarpoff emphasizes, lice are highly species specific. Cattle lice do not infest humans, horses or other livestock.
“If your cattle have lice, don’t worry about your kids having lice going to school,” Tarpoff explains. “There are very specific cattle lice that affect our animals.”
This simple clarification can be valuable in client conversations, especially when producers worry about cross-species transmission.
Cattle host two functional groups of lice: sucking lice, which pierce the skin and feed on blood, and biting lice, which feed on skin debris and hair. Both groups spend their entire life cycle on the animal, being mostly dormant in the summer.
“Adult lice will actually hide out in different areas around the ears and eyes,” Tarpoff says. “They really don’t like hot weather, so they go into a dormant state. Now, once the cooler weather starts, that’s where they really jump-start their reproductive cycles.”
Once winter sets in, egg laying accelerates. Eggs, or nits, are cemented to hair shafts with a glue-like substance, hatch into nymphs and develop into egg-laying adults in approximately 28 to 30 days. This predictable cycle can be used to inform treatment expectations and retreatment intervals.
Clinical and Economic Consequences
By the time cattle are visibly rubbing against posts and fences, the infestation is well advanced. The clinical hallmarks of a lice infestations are:
- Patchy hair loss over the neck, withers and rump
- Intense apparent itchiness and restless behavior
- Fence post, building and equipment damage from rubbing
- Reduced performance, including decreased average daily gain (by up to 0.2 lb. per day), lower milk production and worsening body condition
“You know when you pull into the feedyard and just kind of zone out and don’t focus on anything,” Tarpoff says. “You look across the yard and all of a sudden say, ‘holy moley, everything’s up against the fence rubbing.’ It’s pretty obvious that we need to have a talk about lice.”
Industrywide, lice contribute to an estimated $125 million in annual losses, a figure driven by reduced productivity and infrastructure damage. Practitioners often struggle to quantify the latter, but any producer who has replaced a stretch of barbed wire knows the cost.
Diagnosing Infestation Before It Escalates
Waiting for obvious rubbing means the opportunity for early intervention has already passed. Tarpoff recommends a practical chute-side evaluation.
“We can part the hair on the neck, the withers, the rump and actually see the little lice,” he says. “They’re about the size of a grain of sand. We can see them scurrying around and if there’s roughly 10 lice per square inch, they definitely need to be treated.”
Fall Deworming Isn’t Enough
Many fall processing protocols include a macrocyclic lactone (pour-on or injectable) intended to clean up internal parasites after summer grazing. While these products provide some lice control, they are imperfect winter solutions. Injectable macrocyclic lactones only kill sucking lice, not biting lice. Pour-ons kill both, but residual activity rarely extends into peak winter months.
This leaves a winter window where live populations surge despite fall treatments. This is often mistaken for product failure rather than an issue of timing and louse biology.
Winter Topical Products for Active Outbreaks
For true winter control, contact-based topical insecticides are the most appropriate tools. These products require direct contact with adult lice to be effective. They do not kill eggs, making retreatment crucial. A second application in two to three weeks ensures newly hatched nymphs are eliminated.
“I see people that are have issues in their cows, and they’ll say, ‘Well, I put it on twice already, and it didn’t do anything,’” Tarpoff says. More often than not, these are issues with the treatment application not covering the entirety of the animal. “The majority of these products … don’t absorb into the bloodstream. They work on contact, and they have to come in physical contact with those lice.”
Don’t Overlook Biosecurity
Even a perfectly executed treatment protocol fails if every animal is not treated. A single untreated bull or newly purchased replacement heifer can reinfest the whole herd within weeks. Tarpoll recommends treating new arrivals upon entry and maintaining a short quarantine period until their lice load is confirmed cleared. This biosecurity step prevents chronic reinfestation cycles.
The key messages for veterinarians to reinforce with their clients are:
- Winter lice require winter treatments. Fall dewormers are not a full-season solution.
- Coverage matters more than chemistry. Application technique determines outcomes.
- Plan for retreatment. One round rarely solves the problem.
- Treat the whole herd. Biosecurity is essential for long-term control.
