Lice are most abundant in livestock during the cold months of the winter and early spring. They typically infest animals that are stressed due to cold weather, inadequate nutrition, suppressed immune system, or that are impacted with internal parasites. These issues compounded by moderate to heavy infestations of lice can cause lower weigh gains, lower milk production, reduced feed efficiency, a slower recovery time from disease, anemia, and general unthriftiness. Livestock infested with lice can become very agitated and will relentlessly rub, scratch, and lick to try and find relief. Other signs that livestock are impacted by lice included rubbed off or blood-stained hair and faces with a bluish/gray appearance.
There are four predominant species of lice in Montana, three sucking lice species, long-nosed, little blue, and short-nosed, and one biting lice species that is the most common of all species in the state. Lice spend their entire life on the host, and their life cycle is three to four weeks long. They cement nits to the animal’s hairs and are spread by direct contact. The long-nosed lice typically infests calves and are located on the shoulders, back, neck, and dewlap. The little blue lice are typically found on the face. The short-nosed species is usually found on the neck, dewlap, and brisket. The biting lice are found on the back, withers, poll, and tail head and feed on the skin and hair.
Control options for lice include back rubbers and dust bags, but they will only provide control and not eradication. These options work best when placed where cattle will use them daily. There are ear tag options for control of biting lice and must be applied in the fall. There are two pour-on options including pyrethroids and avermectins. Pyrethroids do not kill eggs, and therefore, require two applications 14 days apart to ensure optimal lice control. This second application will kill the newly hatched nymphs. Common pyrethroids include cypermethrin, permethrin, and cyfluthrin. Avermectins including ivermectin, doramectin, abamectin, and eprinomectin, control both sucking and biting lice and require only one application. There are injectable avermectins but they are only registered for use on beef cattle and are only affective for sucking lice, which may require an additional application for biting lice.
No matter the control method you choose, it is always important to read and follow the label directions. Pour-ons and injectable doses will be determined by weight and will also have withdrawal times/days to slaughter. These will vary by product, making it important to consult the label before applying. Reading the label to note the active ingredient and mode of action of the active ingredient is important. Rotating modes of action every season is the key to managing resistance.
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