Photo courtesy of Boehringer Ingelheim
DULUTH, Ga. (October 14, 2025) — As temperatures drop and hair coats grow, external parasites like lice and mange start to become active in dairy herds. While these pests are easy to overlook in the early stages, their impact on cow comfort, and ultimately milk production, can be significant.“Rather than lying comfortably in the stall, ruminating and making milk, cows bothered with lice and mange are more restless, trying to scratch on posts and other parts of the environment to try and relieve that itchiness,” shared Jen Roberts, DVM, Boehringer Ingelheim.
To keep external parasite populations in control, fall is the time to evaluate deworming protocols and protect your herd before winter parasite pressure sets in.
External parasites have serious impact on production
Dairy cattle can be affected by two main types of lice: biting lice, which feed on skin and hair, and sucking lice, which draw blood. Clinical signs of lice include intense itching and hair loss, which can lead to irritation, weight loss and reduced milk production.
Mange mites, especially chorioptic mange (C. bovis), are also common in dairy herds. These mites cause thick, scabby skin around the tailhead, legs and udder. Like lice, mange infestations cause significant itching and discomfort, which disrupt normal behaviors, such as eating and resting. Sarcoptic mange (S. scabiei) is a less common type of mange mite but can lead to even more severe skin lesions.
Even small infestations of external parasites can lead to lost production, if left unchecked. In more severe cases, lice may cause immune suppression or anemia, which can further compromise cow health. While these symptoms are a clear sign of external parasites, Dr. Roberts shares that waiting for visual signs of infection isn’t the best option.
“Once hair loss is noted, a lot of the damage caused by lice has already occurred,” Dr. Roberts explained. “That’s why it’s so important to stay on top of regularly pouring the herd, even when we’re not seeing patchy hair loss or other clinical signs.”
Fall is the best timing for whole-herd deworming
Lice and mange mites have a rapid, host-specific life cycle that allows them to increase quickly. Lice complete their life cycle on the cow within three to six weeks, and the longer hair coats in winter allow them to stay close to the skin and avoid the damaging UV exposure. To prevent an infestation before it starts, choosing the right time to deworm is important.
“Ideally, we want to pour the entire herd during the fall, between October and December,” said Dr. Roberts. “A whole-herd pour is often more effective at knocking the entire lice or mange population down as much as we can, all at once.”
Treating all animals at one time also reduces the risk of reinfestation and ensures that both clinical and subclinical carriers are addressed.
Get the most out of your pour-on protocol
Consistent, accurate application is key to getting the full benefit from your pour-on dewormer. To ensure optimal control, dosing should be based on each animal’s body weight, and product should be applied in a clean, even line along the topline, from shoulders to hips.
“One of the most common mistakes is not accurately estimating the animal’s body weight,” noted Dr. Roberts. When possible, each animal should be weighed prior to application. If that’s not possible, the dose should be based on the largest animal in the group. “Improper application and mixing untreated animals with treated animals are also pitfalls in a fall deworming protocol,” she added.
Beyond the initial application, ongoing management plays a key role in maintaining parasite control. Animals should be re-treated as needed, and new herd members should be treated. “My recommendation would be to pour those new animals with an antiparasitic prior to introducing them to the rest of the herd,” said Dr. Roberts.
Some operations choose to pour cows at dry-off, taking advantage of cows in the lockups. While this is an option that works for some herds, there can be some downfalls. “The risk you run is never completely eliminating the external-pest issue,” stated Dr. Roberts. “A pour-on is not going to provide year-long protection and the duration of effect varies depending on the parasite being targeted.”
Choosing a proven pour-on product like EPRINEX® (eprinomectin), which offers broad-spectrum parasite control with zero milk withhold, can help operations maintain pressure on parasites without sacrificing productivity.
Dr. Roberts encourages producers to work closely with their herd veterinarian when evaluating and building a fall deworming program. For the best results, a program should be tailored to the current parasitic challenges and treatment needs of your herd.
“Try to be proactive about addressing external parasites, because anything we can do to maintain cow comfort is going to help those cows be more productive,” she concluded.
EPRINEX IMPORTANT SAFETY INFORMATION: No meat or milk withdrawal is required when used according to label. Do not use in calves intended for veal or unapproved animal species, as severe adverse reactions, including fatalities in dogs, may result.
