Use Grub and Louse Treatments Cautiously

– William P. Shulaw DVM MS, Extension Veterinarian, Beef/Sheep, The Ohio State University

Treatment for grubs and lice is a common procedure on both cow/calf operations as well as in feedlots. The life cycle of the cattle grub starts with a fly (the warble fly) that lays eggs on the skin of the cattle in the summer and fall. The eggs hatch larvae that penetrate the skin and migrate through the body during the fall and winter to eventually reach the tissues just under the skin of the back forming lumps. The grub larvae grow in size during migration and eventually penetrate the skin of the back. In the spring they drop out of the hole in the skin and burrow into the ground where they pupate and become adult flies. Generally we recommend that cattle not be treated for grubs after about November 1 in Ohio cattle. This is recommended because during their migration, the grub larvae tend to accumulate around the esophagus (gullet) and spine, before moving on. If they are killed while they are at those sites, the resulting inflammatory reaction may cause swelling that produces severe bloat problems or paralysis of the back legs. In Ohio, the larvae tend to be in the area of the esophagus or spine in the first few weeks after November 1. In recent years some very effective products for treatment of intestinal worms, lice, and all stages of the cattle grub have become available. Many of these, such as Ivomec, Dectomax, and Cydectin are available as pour-ons and are both effective and convenient.

Frequently, we get asked about grub treatment after November 1 or re-treatment for lice in January. Often producers wish to use the pour-ons like those mentioned because of their broad spectrum activity and convenience. Most of these products carry the warning that killing grub larvae when they are near the spine or esophagus may result in the problems cited above. If the cattle were not treated for grub larvae at the end of the warble fly season, if the previous grub treatment history of the cattle is unknown, or if the cattle are more recent arrivals from another part of the USA where the fly season is different, then there is a risk of reactions to dying larvae if systemic insecticides are used after November 1. It may be safer to use another product if re-treating for lice, such as a spray or pour-on product containing permectrin, and to skip, or postpone, treatment for grubs. If cattle were treated with one of the products listed above at the end of warble fly season, it is safe to re-treat the cattle in the winter if that is necessary. Treatment of cattle with grub larvae late in the winter or in early spring may kill the grubs without much danger to the animal, but the larval activity, or reaction to their death, can still cause muscle and subcutaneous tissue damage visible at slaughter. It is wise to plan ahead for grub and louse control. When you READ THE LABEL, read ALL the label, and ask your veterinarian for advice on specific questions of drug use.