Lice are an external parasite found on the hair of mammalian hosts, including horses. A lice infestation is also referred to as pediculosis. There are many species of these small, wingless insects, and horses are usually infested with either the “sucking” or “biting” type.

Lice are typically transmitted by direct contact with an infected animal. Since lice can live in the environment without a host for a few weeks, they can also be transmitted indirectly by sharing space or equipment with infected horses.

Symptoms include itching, a patchy coat, inflamed skin, and in severe cases, loss of body condition. Diagnosis is based on the presence of lice and nits (eggs) and may be confirmed with skin scraping or grooming.

Anti-parasitic topical medication is the primary treatment for lice infestations in horses. In severe cases, secondary skin infections may also require antibiotics and wound care.

Lice (Pediculosis) in Horses

Lice are flat, wingless, parasitic insects that live and feed on the body of mammalian hosts, such as horses. [1] Pediciulosis is the medical term for a lice infestation in any animal. [1]

Horses are most often infected during the winter and early spring. Horses with thick coats, young foals, senior horses, pregnant mares, and those who are ill or weak are are at particular risk of pediculosis.

There are over 4000 species of parasitic lice worldwide. Two types of lice can infest horses: sucking lice and biting lice. [1][2][3]

Sucking Lice

The sucking louse species is also known as Haematopinus asini. [3] It feeds on the blood, fluids, and tissues of the horse. [2]

The sucking louse is between 3 and 5 millimeters long. [2][4] It moves slowly and can sometimes be observed with the sucking parts of its mouth embedded in the skin. [2]

Its head and thorax are grey-blue or yellow-brown and it has a wide, pale yellow abdomen ringed with dots. [2][3] The thorax of the sucking louse is much larger than its head and it has distinct mouth parts that are adapted for biting. [2] Its legs are thick and bulbous. [2]

Sucking lice are most likely to be found on the mane, tail, and fetlocks of the horse. [3]

Biting Lice

The biting louse is called Werneckiella equi (previously referred to as Damalina equi and Bovicola equi). [1][2] This species is also known as a chewing louse. [2] W. equi feeds on the skin flakes and hair of the horse, only occasionally taking a blood meal. [2]

The biting louse is between 1 and 2 millimeters long and is more active than other species of lice. [4] Heightened activity makes it easy to spot this species as it moves across the horse’s skin, especially in cases of widespread infestation. [1][4]

In biting lice, the head and thorax are brown with a pale yellow, striped abdomen. [2] The head is square or rounded and broader than the thorax. [2][3] The legs of biting lice are fine and tapered. [2]

Chewing lice can be differentiated from sucking lice by the shape of their body: the sucking lice’s head is narrower than its thorax, whereas the biting louse has a narrow thorax and a larger head. [2]

Biting lice are most likely to be found on the sides of the neck, back, and the abdomen of the horse. [2][3] In cases of heavy infestation they can be found everywhere on the horse’s body. [2]

Parasitic Life Cycle

The life cycle for both species of lice is the same. [2] The louse starts as an nit (egg), develops into a nymph (larva), and then becomes an adult louse. [2]

All stages of the life cycle occur on or just under the skin of the horse. [2] The full cycle takes between 20 and 40 days. [2][3][4]

The nits are small, oval, and translucent. [2] The adult female lays the eggs onto hair follicles near the skin of the horse. [2] As the eggs are laid, they are attached to the individual hairs with a sticky, cement-like substance secreted by the female. [2]

Each female lays one egg per day for the 30 to 35 days of adult life. [2] Eggs take between 5 and 20 days to hatch into nymphs. [2]

The nymphs have a body composition similar to the adult with a head, thorax, and abdomen but they are smaller. [2] It takes a nymph between 14 and 28 days to mature into a louse. [2] The nymphs of sucking lice feed on blood immediately. [2]

Adult lice are wingless and flat which allows them to hide in the horse’s coat. [2] Like the nymphs, they have a head, thorax, and abdomen. [2]

Transmission

Lice are transmitted through direct contact with an infected host when an adult louse crawls from one horse to another. [1][2]

Lice can live for a few weeks off the host. [3] Therefore, transmission can also occur by sharing infected grooming tools, tack, and other equipment or by rubbing against a shared fence post. [2][3][6]

Sharing stall space or using stall space after an infected horse are also possible routes of transmission. [3]

Risk Factors

Lice are ubiquitous, with species found in every part of the world, and are active year-round. [2][3] Lice are more commonly found on horses during the early spring and winter months. [1][2]

Contributing factors to seasonal fluctuations in lice infestation rates may include: [1][2][4]

  • Indoor housing: horses usually have closer contact when kept indoors during cold and rainy weather
  • Increased humidity: most species of insect prolifierate during humid seasons
  • Seasonal changes in coat condition: horses’ coats are longer and thicker during the colder months, providing a more habitable environment for lice to live

General risk factors increasing susceptibility to lice infestation in horses include: [1][2][3][4][6]

  • Inadequate stabling: horses that live in close contact or overcrowded environments are more likely to be infested with lice
  • Pre-existing health conditions: horses that have underlying health issues, are under-conditioned, or have poor nutritional support are at increased risk
  • Poor immune response: horses who are immunosuppressed, either from medication or another cause, are more at risk of parasitic infections overall
  • Natural coat condition: breeds of horse with longer body hair or feathers are also more susceptible to lice infestation; they can remain dormant in the fetlock feathers and hair on the submaxillary space
  • Insufficient grooming: horses that are not regularly or thoroughly groomed are more at risk to lice infestation
  • Age and reproductive status: horses that are young, old, or pregnant are also more susceptible to lice infestation

Inter-species Infestation and Disease Vectors

Typically, lice are species-specific, meaning they are adapted to live on one type of animal. However, there have been reports of poultry lice infecting horses, particularly when chickens live in the same premises as the horses. [4]

The risk of cross-species lice infestation is more likely if infected poultry inhabit the environment and then are removed from the area. Any poultry lice left behind in this scenario will turn to horses in the environment as the only available host. [4]

Unlike other types of external parasites, lice do not carry any disease that can further affect a horse beyond the potential negative effects of the infestation itself. [4]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn USA

Symptoms

Horses with lice have nits, larvae, or adult lice living on their skin and in their coat. [2]

Horses infested with lice are itchy, and are likely to rub, bite, or scratch themselves. [1][3] Persistent scratching can lead to development of a rash, inflammation, and redness on the skin. [1][3]

Excessive scratching can lead to hair loss and a patchy looking coat. [3] This is most likely to occur on the sides of the neck and at the base of the tail. [3]

In severe cases infested horses can lose weight and body condition. [1] In cases of sucking lice infestation, the horse may develop anemia due to blood loss. [3]

If itching is severe, the horse can tear its own skin trying to scratch and cause weeping sores, which can lead to secondary bacterial skin infections. [1][3]

Diagnosis

Diagnosis is generally made by visual confirmation of the lice themselves or their nits on the horse’s skin or hair. [3][1]

The biting louse is more likely to be seen moving across the surface of the skin, while the sucking louse often has its mouth embedded in the horse’s skin when they are detected. [2]

Additionally, skin scrapings or skin grooming are also used to determine the presence and species of lice. [3][5]

Some of the horse’s hair and dander are scraped or brushed from the body and examined under a microscope, loupe (lens), or by the naked eye to confirm the presence of lice and to determine the species. [5]

Treatment

Treatment is primarily by use of anti-parasitic medication applied to the skin and coat. [3] These medications are available in a shampoo, powder, or spray form. [4]

As with all types of parasites, for treatment to be effective, it must break the parasite’s life cycle. The products used to treat lice are only effective at killing adults and larvae, while the nits remain viable and attached to the hair shaft.

Therefore the treatment must be repeated after 10 to 14 days once the nits have hatched and before they have matured, preventing them from laying new eggs. [1][3]

Spot application is not effective, so the whole horse must be treated for effective relief. [3] Certain formulations require that the hair is soaked with medication while others require only light misting. [4]

Oral formats of the lice medication are not as effective as topical formulations. [3]

The horse’s coat may need to be clipped in severe cases, especially where the horse has feathers, or a winter coat, or suffers from hirsutism. [4]

All horses that share tack or stable space must be treated at the same time to prevent reinfestation. [3]

All tack, trailers, stalls, wash racks, other equipment, and premises must be treated with an anti-parasitic spray to reduce the risk of reinfection. [4]

Mares with young foals must be treated judiciously as the foal can easily be overexposed to treatment, especially if delivered in powdered form, by rubbing against the mother. [4]

Always inform your veterinarian of any animal species that may be exposed to insecticides before treating your horse. Some animals are highly sensitive to certain pesticides, especially cats.

Prognosis

With appropriate treatment, symptoms usually begin to improve within a day or two. [3] Some horses are allergic to anti-parasitic medications and develop hives or other reactions during treatment. [3] Contact your vet if your horse develops additional skin irritation symptoms after beginning medication.

Lice are not known to carry any diseases that could be transmitted to the horse. [4]

Prevention

Prevention of lice infestation requires proper biosecurity protocols and disinfection measures when adding a new horse to the group or using equipment or premises used by infested horses.

New horses that are arriving from other barns or facilities, especially premises that have a history of lice infestation, should be checked for lice before being stabled with uninfected horses. [2]

Tack, equipment, and premises used for infected horses should be treated before they are used with uninfected horses.[2]

Ensuring good health, maintaining internal parasite control, performing regular grooming, and providing adequate nutrition may help in reducing a horse’s susceptibility to lice. [2]

Summary

Lice are a common parasite found worldwide that live on the skin of mammals including horses.

  • The main symptom of lice infestation is persistent itching; scratching can cause skin damage and lead to secondary bacterial skin infection
  • Transmission occurs through direct contact between horses or through shared use of infected equipment or premises
  • Lice are diagnosed by the presence of the nits, nymphs, or adults on the horse’s skin and may be confirmed with skin scrapings
  • Treatment involves repeated rounds of antiparasitic medication and disinfection of the horse’s environment, including tack and grooming equipment

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Payne, P. A. and Carter, G. R. Ticks, Mange, Pediculosis (Lice), Chiggers / Harvest Mites (Trombiculidiasis) and Straw Itch Mites (Forage Mites), in Microbial and Parasitic Diseases in Horses. IVIS, 2007.
  2. Unknown. Lice. American Association of Equine Practitioners.
  3. Sloet van Oldruitenborgh-Oosterbaan, M. M. Parasitic Skin Problems in Horses, in European Veterinary Conference Voorjaarsdagen, Amsterdam, 2010.
  4. Ketzis, J. Lice in Horses and Donkey. MSD Veterinary Manual, 2023.
  5. van Oldruitenborgh-Oosterbaan, M. S. and Koeman, J. P. How to make a thorough dermatological diagnosis, in European Veterinary Conference Voorjaarsdagen, 2007.
  6. Paterson, S. Senter, D. Funiciello, B. Pediculosis in Horses (Equis). Vetlexicon. Accessed: April 17, 2024.