Horses occasionally get lice, and a horse owner needs to know what to look for and how to treat these irritating parasites. Bill Clymer, PhD, of Amarillo, Texas (now a livestock parasitologist on the professional services staff of Fort Dodge Animal Health), has worked with horses and lice for many years. Earlier in his career, he was an extension livestock specialist with Texas A&M University. We also talked to Jack Lloyd, PhD, professor of entomology at the University of Wyoming; and Sandy Gagnon, extension specialist at Montana State University, for this article.

Sucking louse

There are two types of lice that affect horses--sucking lice (above) and chewing lice (below)--and three species. All three live throughout the United States. The sucking lice are probably more damaging because they can create anemia due to blood loss (resulting in weakness or stunted growth in young animals), but chewing lice may be more irritating because horses have very sensitive skin.


Lice occur most often in horses that are stressed, Clymer says, by inadequate nutrition, a severe winter, illness, injury, etc. "For 20-some years I had my own research company, doing research on parasites. I'd buy cheap horses that were in poor condition for various research studies, and they'd be the ones that would get heavy lice infestations. Well-fed, well-groomed horses seldom get lice," says Clymer. They seem to have more resistance.

Lice are host-specific--cattle lice won't live on horses or vice versa, and poultry lice only live on poultry. They might crawl around for a while on another species of animal, but won't actually establish a colony, he says. Horse lice are brown and usually are found down next to the skin. They can be hard to see unless you part the hair and use a light and a magnifying glass, but if you watch for a moment, you can see them moving around.

There are two types of lice that affect horses--sucking lice and chewing lice--and three species. The blood-sucking louse is Haematopinus asini, and two species of chewing lice are Bovicola equi and Trichodectes pilosus. All three live throughout the United States.

"The sucking lice are probably more damaging because they can create anemia due to blood loss (resulting in weakness or stunted growth in young animals), but chewing lice may be more irritating because horses have very sensitive skin," he says. "The lice crawling around on the host cause great annoyance."

Horses with lice look rough and unthrifty, with a scruffy hair coat, and they continually rub and scratch. They might have open sores from the rubbing. Any horse that is itching, rolling, or rubbing a lot might have lice and should be checked. By the time you see hair loss, the horse has been itching for a while. Other things that can cause itching and hair loss are mites or dry skin, so be sure to identify the cause before treating the horse.

Horses that are on short feed rations in winter are more prone to both severe louse infestation and other diseases than horses on full feed and maintained in good body condition. "Horses with lice may be more susceptible to disease," Clymer says, "The immune response of the animal has a direct effect on lice numbers and efficacy of any control products."

The Life Cycle

The life cycle of lice varies with the species, with adult females laying between 20 and 40 eggs. "These hatch after a time, depending on external temperature and how close they are to the skin," says Clymer. "The closer they are to the skin, the warmer they are, and the faster they hatch. They generally hatch within two to three weeks, with 30 to 45 days from egg to egg-laying adult.

"Chewing lice are most prevalent on the head, mane, base of the tail, and shoulder, while the sucking louse is commonly found on the head, neck, back, and the inner surface of the upper legs," he notes.

On affected dark-colored horses, you might see the tiny, light-colored eggs on the hair. "Eggs are glued to the hair, and may still be there after the larvae hatch. If you don't look closely, you may think the horse still has a louse infestation, seeing the old eggs--even though the caps have broken off and the larvae already emerged," says Clymer. Empty egg shells stay there, glued to the hair--like an empty bot egg--until they break off or the hair comes out.

Many species of chewing lice have a unique life cycle. Nine out of 10 lice that hatch are reproductive females. The males don't have to fertilize the females for them to lay fertile eggs, says Clymer. "If you have one animal that gets one egg on it, and it hatches, nine out of 10 chances it would be a female. If she lays 40 eggs and 60% of those eggs hatch and nine out of 10 of those are females, in five generations you could have 300,000 lice (plus or minus) from that one female louse."


Lice are readily passed from one horse to another by physical contact, especially if those horses are confined together. "They (lice) can also be spread by brushes and equipment used on more than one horse, or in a horse trailer. In a trailer where horses are in close quarters, touching horse-to-horse may spread lice, or a louse may get off on a side panel and onto the next horse," says Clymer. "Horses put into stocks, stalls, or any other place where another horse was recently confined may pick them up."

Sharing brushes is not a good idea; nits (louse eggs) or lice from one horse might have come off in the brush and be ready to brush onto the next horse. Depending on climatic conditions, lice can live for a few days off the host, just waiting for a new host, he says.

"Horses are gregarious; there are lots of opportunities for contamination," Clymer adds. "Horses grooming one another in the pasture or over the fence, or standing next to one another swatting flies, may pass lice around. Lice don't spontaneously appear with the first cold rain in fall and disappear with warm weather in spring. There are a few carrier animals that have lice year-round because their immune systems are such that lice can thrive. These horses serve as a source (of lice) for other horses."

If lice suddenly show up on your horse and you wonder how he got them, think back to where the horse has been. You might have gone to a show, borrowed a horse trailer, or taken the horse to the vet or some other place where a louse-infested horse had recently been.


Lice populations increase in winter, partly because horses could be stressed more in winter, and partly because they have a longer hair coat then. "There is a direct relationship between hair length and lice populations. Hair protects lice from the horse licking and trying to bite them off. We can put animals in a stanchion where they can't groom themselves and can get as heavy a lice population in summer as we do in winter. The longer hair gives a lot of protection from the rubbing, licking, and biting," notes Clymer.

Lice hang onto the base of the hair as they feed and are only found in hairy areas of the horse. Wherever hair is missing (from being rubbed out) the lice quit biting and leave. Also, if you ride a lice-infested horse in winter and he becomes warm and sweaty, the lice come out to the ends of the hair and are easy to see.


Lice are not common on horses, and are fairly easy to control. There are numerous products available for louse control, including sprays, dusts, and wipe-ons. Horses dewormed regularly with one of the macrocyclic lactones (Quest, Quest Plus, ComboCare, Zimecterin, Rotation 1, Ivercare, etc.) might have less lice problems than untreated horses. Chewing lice are less affected, however, because they feed on skin and dander rather than blood.

Insecticide sprays (emulsifiable concentrates or wettable powders) are available for use on horses, but many horses don't like to be sprayed--especially with a high-pressure spray. You must accomplish complete wetting of the skin. If a horse protests, or weather is cold, you can use a dust to avoid wetting the whole horse. A brisk and thorough brushing of insecticide into the hair coat will work; it must get down to the base of the hairs and come into contact with the skin. Horses should be retreated in about two weeks to kill young lice that have just hatched and were not affected by the first treatment. Pyrethroid insecticides might control lice with just one treatment.

Always read and follow the label directions on any product. For advice on specific products to use on horses, consult your veterinarian.

Saddle blankets, brushes, and other equipment used on lice-infested horses should be treated with very hot water or rubbed with an insecticide solution. Bedding from a stall that housed an infested horse should be removed. The stall can be disinfected (or not used for awhile) to get rid of lice the horse left behind. If one horse in a group gets lice, they are all exposed and should all be treated. Otherwise they will keep passing the lice around.


Keeping horses well-fed, not stressed, and healthy in a clean environment is good prevention against lice, along with regular brushing and grooming. "If you are continually grooming the horse, you will probably scrape some of those lice off if a horse gets them. If horses get bathed periodically, this will also disrupt lice," Clymer says.

Horses should be carefully inspected at purchase before your bring them home. If they have lice, they should be treated before being introduced into your herd, says Clymer. Horses are often transported across the country for shows, breeding, etc., so there are a lot of opportunities to come into contact with other horses. They should be routinely inspected for lice, especially during winter months and particularly if they are kept in close confinement with other horses, he says.

There are two types of lice that affect horses--sucking lice and chewing lice--and three species. All three live throughout the United States. The sucking lice are probably more damaging because they can create anemia due to blood loss (resulting in weakness or stunted growth in young animals), but chewing lice may be more irritating because horses have very sensitive skin.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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