on the market, some drug companies gave a guarantee, saying one treatment would last through winter and if you found lice on any of your animals they would pay for retreatment. There were some useful lessons learned. You can’t mix treated and untreated cattle, use improper dosage, or treat too early,” says Colwell. It’s usually not necessary to re-treat for lice if a few show up in late March or early April, because lice populations won’t proliferate at that late date. “Lice don’t survive in heat. If the cow is standing in bright sunlight in summer, temperature on her skin may go up to 115 to 120 degrees F and this is outside the thermal tolerance of a louse or a louse egg. Adult lice are dying and not reproducing, so the population crashes when weather warms up,” explains Colwell. Tips for proper treatments: No matter what you are using, never underdose. Always treat at the maximum level. If you don’t kill all the lice on an animal, that animal serves as a source of lice for the rest of the herd and spreads lice to the ones you just treated. Then you may see high levels of lice again before winter is over. “We recommend re-treating later in winter, like February, if lice become a problem. It used to be cheap to do this, using a topical oil-based pyrethroid such as Cylence or Boss. These spread through the hair coat and have enough residual activity to last awhile and get the cattle through to spring. Unfortunately they are now more expensive than some of the generic macrocyclic lactones but I still think they are the best type of followup treatment,” Colwell says. “Products containing synthetic pyrethroids are very effective for lice control. Many folks don’t use the endectocides even in the late fall/early winter; they just use a purge dewormer for internal parasites and a pyrethroid pour-on for lice,” says Campbell. If you are unsure about what to use and when, in your own situation, this can be discussed with your herd health veterinarian. Sometimes you have to do something a little different because of a bad year, or in a situation where fenceline contact or a break in a fence allowed untreated cattle to mingle with treated cattle. Other methods are useful, such as insecticides on back-rubbers that allow animals to self-treat, according to Colwell. “Self-grooming—licking, rubbing and scratching—helps keep the lice population down. In northern climates, however, long winter hair reduces effectiveness of the tongue to pull lice off. The tongue is a great grooming device, but with a thick hair coat
it can’t get to the lice that are right down on the skin. Thus insecticide back-rubbers and other structures for cattle to scratch on can be a help,” says Colwell. This also minimizes damage to facilities if cattle rub on these instead of the fences. By March, days are getting longer with more intense sunlight, and most lice populations start dwindling--and retreat to cooler places on the animal. “Some animals act as carriers; lice don’t leave them during summer. The most effective lice control is culling the carriers. Any animal that is chronically infested will keep spreading lice to the others. There are a few animals in the herd that are highly susceptible to one kind of parasite or another. This may be due to ineffectiveness of their immune response, and their genetics. You probably shouldn’t make your entire culling decision based on louse populations, but it should be a factor to consider,” says Colwell. Lack of immune response can be linked to genetics. Some animals have stronger immune systems than others. Carriers usually have some deficiency in their immune system that makes them more susceptible to heavy lice infestation, since cattle normally develop some resistance to lice after exposure.
LIFE CYCLE AND SPREAD –
Lice infest cattle all year but numbers are usually low in summer because most of them are shed off in the spring with winter hair. Lice also tend to leave the hottest areas of the animal in summer and don’t reproduce as fast, according to Colwell. They increase their reproductive rate in cold weather, with more generations in a shorter time span. Lice populations build as the animals grow winter hair, and reach a peak in winter, dramatically multiplying when weather turns cold. The long winter hair gives lice added protection and an ideal environment for reproduction.
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