Lice In Dairy Cattle

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Lice are the most important winter parasites of cattle. The two clinical signs of lice are hair loss and scratching. Lice have been considered by many to be more of a nuisance parasite than an important health problem in cattle. While not a lot of study has gone into the milk production and body condition loss issues associated with lice in dairy cattle I believe that severe lice infestations do cause some milk production and BCS loss in dairy cattle.

There are two types of lice: biting and sucking lice. The only importance in knowing this fact is that ivermectin, Dectomax®, and Cydectin® injectable are only effective against sucking lice.

Lice spend their entire life cycle on cattle. Lice have 3 stages of their life cycle. These 3 stages are egg (nymph), larvae, and adult. It takes about 3 weeks for an egg to hatch and develop into an egg laying adult. No lice treatment products are effective against eggs. Winter weather has a significant impact on the severity of lice problems in cattle. The longer the winter and the more winter moisture the more severe the winter weather conditions.

Lice treatment is best applied to cows around the first of the year before lice numbers have started to explode on cattle. Once the clinical signs of hair loss are seen the numbers of lice on the cattle have exploded and the number of eggs present have risen significantly making it more difficult to break the life cycle. Macrcocylic lactones (ivermectins, Dectomax, Eprinex, and Cydectin) have persistent activity. These compounds stay in the body long enough to kill the larvae as they hatch out and thus break the lice life cycle with one application. All other products only kill the larvae and adults on the cattle at the time of application. They must be reapplied in 2-3 weeks to kill off the larvae that have hatched out since the first application. Lice treatment is often applied in the fall when cattle are traditionally worked. If the winter weather is not too severe then application in the fall with macrocyclic lactones should provide winter long lice control. Cydectin®, Eprinex®, and most pyrethrins are labeled for lactating dairy cows with no milk withdrawal. Be sure and check the label on the product before using on lactating dairy cows. The table above details the products available for lice control and their important facts.