Lice are a common winter problem in cattle, especially in cold climates. Heavy infestations can rob valuable nutrition when cattle need it most, decreasing gain and leaving cattle more susceptible to disease.
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Doug Colwell, a livestock parasitologist in Lethbridge, Alberta, says two types of lice infest cattle, chewing lice and sucking lice. Chewing lice are probably the most common, but large populations of sucking lice are probably the most damaging.
All lice cause severe irritation and itching, causing cattle to react by rubbing, licking or chewing on themselves. Infested animals are often restless, focusing on their discomfort rather than eating. And, the broken skin that can result is an avenue for secondary infection, says Ralph Williams, a Purdue University entomologist.
Spread by direct contact
Lice are spread by direct contact; calves pick up lice from their mothers or herdmates. Lice can infest cattle all year round, but their numbers are typically low in summer because most lice shed off in spring with winter hair, says Jack Campbell, a University of Nebraska professor emeritus and veterinary entomologist.
Cattle’s winter hair coat provides lice protection and an ideal environment for reproduction, he says. The life cycle is 20-30 days and the entire cycle takes place on the host, which makes lice an easier parasite to kill.
Adult females attach their eggs to hairs, which hatch in 5-14 days. When the nymphs emerge, they look similar to adults, but smaller, and go through three molts within a week. They advance to egg-laying adults in about 14 days.
Campbell explains that adult lice don’t live very long apart from the host; nor do the eggs attached to hairs survive long in cold weather.
By March, longer days, more intense sunlight and the increased temperature will begin to crash lice populations, Colwell adds. “The lethal limit for lice is around 104°; the back of an animal in full sunlight gets warmer than that,” he says, so they retreat to cooler places on the animal.
Treatment and control
Maintaining cattle on a high plane of nutrition is the first step in lice control. “You need to ensure they’re healthy going into winter and well fed,” Williams says. Healthy cattle in good body condition have more resistance to lice and rarely carry heavy loads, he adds.
Most ranchers treat for lice at fall weaning; in some regions, this treatment also gets the last of the horn flies, as well as grubs. Campbell says the ivermectin pour-on formulation is fairly effective against both chewing and biting lice, whereas the injectable formulation is not.
Lee Townsend, University of Kentucky Extension entomologist, explains that pyrethroid pour-on insecticides are effective against all types of lice, whereas the avermectins, moxidectin and eprinomectin (systemic products) mainly kill sucking lice.
“Fall treatment using a systemic product is very effective for sucking lice,” Williams says. “For chewing lice, there are several brands of pyrethroid-based pour-ons that spread over the body via skin oil.”
Spraying is an option, too, but doing so on cold days can stress the animal, he adds. When using a spray, it’s important to wet the entire animal with high pressure to ensure both the skin and hair are thoroughly soaked.
Some pour-ons are systemic and are absorbed into the body to kill sucking lice, grubs and internal parasites at the same time. Systemic products that control external parasites (lice, ticks) and internal parasites (worms) are called parasiticides. Systemic products (either pour-on or injectable) must be used before winter to avoid adverse reactions due to death of migrating grubs, Williams says.
“If you fall-treat for lice and they recur later in winter, you can repeat the same treatment used in the fall,” he explains. “But, if you didn’t use a systemic product, you shouldn’t use it later in winter if the herd has any history of cattle grubs,” he adds. Dying grubs can create swelling around the esophagus or spinal cord. A non-systemic pour-on product for lice at that time would be safer.
Timing and diligence
Townsend says diligent lice control can effectively eliminate lice in most herds. But, if a producer doesn’t ensure that all the cattle on the farm get treated, or if he fails to follow up with the second treatment, there will be residual lice populations to reinfect the herd.
“Giving two treatments three weeks apart is an important key. Most products don’t work against the egg stage,” he says. Thus, the eggs on the hairs survive, and hatch later.
Lice control is also a matter of timing. Treat too early, and surviving lice can rebuild populations and return in large numbers. Treatment in October or November offers less opportunity for the population to rebuild, Colwell says. Many ranchers treat too early, as when they’re vaccinating and preg-checking in September.
“The avermectins, moxidectin, etc., won’t give winter-long control if applied too early. A few years back when we had severe drought, ranchers pulled cattle off pasture in August and September, put them in drylots and did fall treatments at that time. They had huge louse outbreaks in January and February,” Colwell says.
Retreatment for lice usually isn’t necessary if a few reappear in late March or early April, he says. “Even on untreated cattle, lice numbers are dropping rapidly by that time,” Colwell says.
The number-one rule, however, is that regardless of the product, never under-dose. “Always treat at maximum level. If you don’t kill all the lice, that animal can spread lice to the ones you just treated,” he says.
Colwell recommends re-treating cattle in February if lice become a problem, using an oil-based pyrethroid. Such products spread through the hair coat and their residual activity can last until spring.
Other useful methods include insecticide dust bags or backubbers, which allow animals to self-treat. “This can supplement lice control, without labor and handling cattle again,” Townsend says.
Signs of lice
Animals with heavy lice populations rub out patches of hair and may injure the skin. Jack Campbell, a University of Nebraska professor emeritus and veterinary entomologist, says cattle can be checked for lice by restraining them and looking closely at areas normally infested with lice.
“Part the hair and look at skin along the back, and the top of the neck.” Good light and/or a magnifying glass make them easier to see.”
Ralph Williams, Purdue University entomologist, says chewing lice are brown, while sucking lice have a blue tint because of blood inside them. “They also have elongated mouthparts that come to a point, for blood-sucking. By contrast, the head of a chewing louse is as wide as its body,” he says.
You can also check for eggs – tiny white, yellow or black specks attached to the hairs.
Young animals and older, undernourished cattle usually have the heaviest lice infestations, says Jack Campbell, a University of Nebraska professor emeritus and veterinary entomologist. But, often, there are a few carrier animals in a herd, usually older cows.
“Cattle normally develop some resistance after exposure. A young, naive animal will develop a high louse population upon first exposure, while the older, more immune animal will have fewer lice,” he says. Then there are immune-deficient “carrier animals” that are more susceptible to heavy infestations and difficult to fully delouse.
Campbell recommends culling carriers from the herd. “These animals harbor an unusually high number of lice that aren’t killed as readily by treatments. In addition, the animal carries so many lice that treatment doesn’t kill them all,” he says.
If carriers aren’t culled, lice in the herd can build up again quickly after treatment, necessitating further treatments through winter. Campbell cites a survey done in 1994 on cattle in North Dakota, Wyoming and Nebraska, in which some ranchers were performing up to five winter treatments for lice.
Conversely, Ralph Williams, a Purdue University entomologist, says some cattle have an innate resistance to lice and never become carriers.
“This depends on the individuals, and breed. Any animals with Zebu breeding (such as Brahman and Brahman crosses) have some resistance to insects and parasites,” William says.
Zebu cattle are a different species that originated in warmer climate where insects are a year-round condition. Thus, such cattle evolved with more resistance, he says.
Insecticide ear tags may help
Lice are winter pests, but insecticide ear tags can help control lice populations in summer, even though it’s a time when numbers tend to be insignificant, says Ralph Williams, a Purdue University entomologist. That’s because lice spend their entire lifecycle on the host, so anything that reduces their population or retards reproduction is helpful. Going into fall, cattle won’t have lice that would otherwise start to build up during colder weather.
“Insecticide ear tags by themselves are not considered lice control, but they do keep lice numbers down. We’ve treated cattle here at Purdue with insecticide ear tags containing organophosphates and every summer since 1977, and rarely see lice problems in our cattle,” Williams says.
Heather Smith Thomas is a rancher and freelance writer based in Salmon, ID.
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