Vet’s Voice: Treating lice in cattle
March 17, 2017
Calving season has arrived and it brought a blizzard. The winds and the eight inches of snow made things tough. It only lasted a day and the weather warmed when the wind went down. Hopefully the weather will warm up and we can enjoy an uneventful spring and calving season.
This winter seems to be a great year for lice. The cows and calves moving through the auction market show large hairless areas indicative of louse infestations. Most of these animals have been poured at least once, but the lice persist. Through the last twenty years we have seen increasing problems. Most believe they are developing resistance to the macrocyclic lactones (ivermectin, dectomax etc.). When these products were introduced they were very expensive and worked very well. I remember feedlot owners telling me they were pouring with one-tenth of the dose and getting good protection. Now some pour with twice the recommended does with no control. Originally these products were advertised as dewormers, but absorption and concentration are variable and results are unpredictable.
If lice populations are high on a lactating cow, we have seen them move to the calves and cause anemia. The calves become weak and may even die. It is important to minimize the crossover from mother to calf. Sometimes we pour both the cow and calf after birth to prevent problems.
There are two types of lice causing problems in cattle. It is important to know which type is causing the problem to use appropriate treatment. The most common lice are biting lice. There are two species found in our area. They spend their entire life on the skin of the animal eating dander. They are found in the dewlap, armpits and groin of the animal, but do travel over the entire body. When we pour on the critters back, the lice most travel to the area and make direct contact with the pour-on. It is very important to distribute the pour-on all along the back, rather than spraying it all in one spot on the back. We have three species of sucking lice. These burrow into the skin and spend time beneath the surface of the skin. Generally, we think injectable products do better on sucking lice. It enters the bloodstream and is consumed by the louse when it has a meal. We have had good success at several feedlots using injectable and pour-ons at the same time.
The lifecycle of the louse makes it difficult to control. The life cycle from egg, to nymph, to adult requires about thirty days. Most pour-ons eliminate adult lice and some nymphs, but new hatching nymphs grow to re-infest the herd. We have had good success with a new pour-on product which has residual and lasts for thirty days. It slowly eliminates the more mature forms, but also attacks the eggs. Strangely, there is not withholding on the product. Consult with your veterinarian and determine which protocol works best for your operation.
It is very important that all animals are poured to eliminate re-infestation. Nose to nose contact and fence line exposure are very common. Dust bags and back rubbers are useful n decreasing populations, but they require continuous use as re-infestation is common.
Lice are becoming a tough problem for feedlots and cow herds in our area. Theproducts we have been using in the past are becoming less effective. New products and careful application of existing products will generate better control and minimize losses to your livestock and abuse to your fences.