Heavy lice infestations are a common winter problem in cattle, increasing dramatically during cold weather. Drain on the animal from these parasites can cause weight loss, reduced feed efficiency and general unthriftiness, making the host more susceptible to disease. Cattle lice are one of the most costly and underrated parasites of cattle, accounting for millions of dollars lost each year. According to Dr. Robert Cope, veterinarian at Salmon, Idaho, lice are the most harmful parasite of cattle during the last months of winter and into early spring; the irritation from lice puts stress on cattle and drains energy reserves.

This calf has become very unthriffty due to a heavy lice infestation.

Both biting and sucking lice infest cattle. Biting lice are most common, while sucking lice are most damaging, says Cope. Both cause severe irritation and itching; infested cattle rub, and chew at themselves, scratching against feed bunks, gates, posts and any other objects in pens or pasture. The constant crawling and biting (or piercing of skin by sucking lice) causes infested animals to be restless and nervous; normal feeding activities are disrupted.

Sucking lice congregate around the head, neck and shoulders of the host animal. They are the most harmful because they can cause anemia. They feed by piercing the skin with sharp mouthparts to suck blood. Continued heavy infestations can weaken an animal to the point that stress from disease or cold weather can cause death. Sucking lice can extract so much blood that red-cell blood counts can be lowered as much as 75 percent. An animal with lice may become anemic, leading to poor feed conversion and vulnerability to disease, according to Dr. Cope.

Lice infest cattle all year but reach a peak in late fall and winter, multiplying dramatically in cold weather. Increased body contact between animals aids the spread of lice (as when cattle are congregated in pens for feeding, or brought into corrals for weaning and routine working). Winter hair gives lice increased protection and an ideal environment for reproduction. The eggs are attached to the hair and hatch in 5 to 14 days.

A loss of hair is a good indication that cattle may have lice. This cow has rubbed much of her hair off due to a lice infestation.

Dislodged eggs (from when hair is rubbed off on a fence or feeder) can still hatch, and often keep a pen or pasture infective for susceptible animals that come in contact with them. If you put new cattle into a pen where lousy animals have rubbed, lice can be readily transmitted to the healthy cattle. Though direct contact is the primary means of spread, eggs and nymphs can be transmitted by contact with feeders and fences where infested animals have rubbed. These can remain viable for up to two weeks.

Certain animals harbor abnormally high numbers of lice, even in summer. These "carriers" serve as a continuing source of infestation for other cattle. It helps to treat cattle in early fall before lice populations build up, and all animals should be treated in late fall before infestation becomes severe. It is very important to treat cattle for lice before winter, says Cope.

Several products are available. Lice can be controlled with insecticides applied by spray, backrubbers, dust bags or pour-ons but sprays do not kill eggs and a second spray no later than 18 days afterward is necessary. If using back rubbers or dust bags, don't put them where spills might contaminate water. Follow label directions when applying insecticides and do not use them in conjunction with other insecticides at the same time (such as ear tags). If using pour-ons, do not exceed maximum recommended dosage, and apply along the topline from shoulders to hips.

Some insecticides also kill grubs and must be used before winter to avoid toxic reactions due to grubs being killed while migrating through the animal's esophagus or spinal nerve canal, says Cope. Dying grubs release substances that cause swelling and inflammation in the tissues, which occasionally may lead to death of the animal. Check with your veterinarian for advice on using insecticides and which products might be best for your particular situation or time of year.

Do not mix treated and untreated animals, or the untreated ones will reinfest the treated ones. Put treated cattle in a separate (and uncontaminated) pen or pasture. For best louse control, treat cattle in late fall and again in mid-winter or early spring, since lice populations may start to build up again before winter is over. ©