Lice are small insects (1 to 5 mm) without wings. Some lice species suck blood (sucking lice) and belong to the order called Anoplura. Other species do not suck blood (chewing or biting lice) and belong to the order called Mallophaga.
Lice are found worldwide, without a preference for particular climatic regions.
Lice are obligate parasites, i.e. they can neither survive nor complete their life cycle without parasitizing their hosts.
Most lice species, also those affecting horses are species-specific, and consequently there is no risk of transmission from one animal species to the other (e.g. from cattle to horses, from dogs to cats or humans, etc.).
Are horses infected with lice contagious for humans?
- No, because lice species that affect horses are horse-specific.
Infestations of animals with lice are medically called pediculosis.
Biology and life cycle of lice on horses
Lice undergo an incomplete metamophosis. The life cycle takes about 1 month to complete. Each female deposits a few dozen eggs (nits) during her lifetime. She deposits them one by one to single hairs. Incubation lasts 4 to 20 days. Young nymphs look like adults but are smaller. Adult life lasts 3 to 6 weeks. Off the host most lice survive only for a few days.
Lice spend their whole life on the same hosts: transmission from one host to another one is by contact, occasionally through contaminated equipment. Transmission from herd to herd is usually through introduction of an infested animal, but flies may also occasionally transport lice.
Chewing lice feed on skin and hair debris as well as on skin secretions. The other species have mouthparts adapted for piercing the skin to suck blood.
Lice infestations develop mostly in the colder season and peak in late winter and early spring. Usually they decline during the hotter season. Stabling the animals during the winter season favors overcrowding, which makes contact transmission easier. And the usually poorer diet during winter weakens their natural defenses of horses against lice infestations. The denser and more humid hair coat in winter offers an excellent environment for lice development as well.
In spring exposure to the sun reduce skin humidity, and free grazing ends overcrowding in the winter quarters, which also diminishes transmission. As a consequence lice infestation usually recede spontaneously during the summer season. However, a few lice usually manage to survive in some animals that will re-infest the whole herd when it comes back to the winter quarters for the next winter.
Click here to learn more about the general biology of insects.
Main species of lice on horses
The main lice species the affect horses and donkeys in Europe and the Americas are:
- Werneckiella equi equi, the horse biting louse
- Werneckiella equi asini, the donkey biting louse
- Werneckiella ocellatus, the donkey chewing louse
- Haematopinus asini, the horse sucking louse
Werneckiella (also called Damalinia, or Bovicola) species are chewing lice that infest horses, donkeys and mules worldwide. They are found mainly on the neck and around the base of the tail. In case of massive infestations they can be found everywhere in the animal's body. Adult lice are rather small, not longer than 2 mm, and have a flattened body. As all chewing lice these species feed on skin and hair debris. Development from egg to adult goes through three subsequent nymph stages and takes about 3 weeks to be completed. Development takes place on the same host. Adult females lay eggs that are attached to the host's hairs. The adult lifespan does not exceed 2 to 3 months.
Haematopinus asini is the largest species among horse lice (3 to 4 mm long). It is a bloodsucking louse that infests horses, donkeys and mules all around the world. It is mostly found on the head, neck, back, brisket and between the legs. All stages have piercing mouthparts that allow them to feed on blood and other body fluids of the host. Oviposition and development are comparable to those of Werneckiella spp. Some authors consider it as more damaging than Werneckiella spp.
Damage, harm and economic importance of lice on horses
Lice are usually not a major parasitological problem on healthy horses, and low to moderate infestation are mostly not very harmful.
However, lice can be very annoying to horses. Bites from bloodsucking and chewing lice are rather irritating and affected animals suffer from severe itching. They rub, scratch and even bite themselves. This can result in hair loss and skin injuries that can be infected with secondary bacteria and attract screwworm flies and other flies that deposit their eggs on the wounds.
In case of heavy infestations, particularly in foals, this results in rough hair coat, hair loss, weight loss, etc. and makes affected animals more susceptible to other diseases. Large numbers of Haematopinus asini can also cause anemia due to blood loss.
Werneckiella spp can act as vectors of Equine Infectious Anemia, a viral disease of horses and other equids.
Diagnosis is based on identification of lice found in the hair coat. In contrast with mites, lice infestations usually do not lead to the appearance of scabs and crusts on the skin of infected animals.
Prevention and control of horse lice
The best way to prevent lice infestations is to avoid overcrowding, especially during confinement in winter, and to keep the animals well fed and in good health conditions. Whatever weakens the animals makes them more susceptible for louse infestations, and animals in good condition are less likely to develop severe lice infestations.
To avoid introducing lice in clean herds, all incoming animals must be preventatively treated.
Frequent grooming may reduce the number of lice, but it is never enough to control them. But it can be also a way to transmit lice among horses in the same stud if combs or brushes are not throughly cleaned.
Whereas there are numberless topical lousicides approved for use on livestock in most countries, only a fraction are also approved for use on horses or other equids. The reason is often that the horse market is smaller than the livestock market and thus not attractive enough for most Animal Health companies to invest in a claim for use on horses.
If an outbreak happens in confined animals, transmission among animals can be very fast. To prevent epidemic outbreaks it is essential to treat all the animals in the herd, and not only those that already show clinical symptoms. Many already infested animals may not yet show symptoms and the infection will progress if left untreated.
Classical concentrates for dipping, spraying or dusting, containing traditional contact insecticides such as organophosphates (chlorfenvinphos, coumaphos, diazinon, etc.) and synthetic pyrethroids (e.g. cypermethrin, deltamethrin, permethrin, etc.) are quite effective lousicides for horses. A few ones contain mixtures, typically organophosphate + pyrethroid.
Ready-to-use pour-ons (backliners) that control lice and other external parasites on horses are also available in most countries. They contain mainly synthetic pyrethroids (e.g. cypermethrin, deltamethrin, permethrin, etc.).
However, all these topical insecticides do not kill lice eggs (nits) and their residual effect is usually not long enough to ensure that immature lice that hatch out of the eggs a few days later are killed as well. Therefore it is highly recommended to repeat the lousicide treatment 2 to 3 weeks later.
Macrocyclic lactones (e.g. ivermectin, moxidectin) administered orally to horses are not effective against chewing lice. They may provide some control blood sucking lice, but most commercial products do not include such an efficacy claim.
For the time being there are no vaccines that will protect horses by making them immune to lice. There are no repellents, natural or synthetic that will keep lice away from cattle. And there are no traps for catching cattle lice, for the simple reason that they spend their whole life on the animals and therefore there are no stages in the environment searching or waiting for a host.
Click here if you are interested in medicinal plants for controlling lice and other external parasites of livestock and pets.
Insecticide resistance of lice on horses
So far there are a few reports on tolerance of Werneckiella (Bovicola) ocellatus to synthetic pyrethroids in the UK, suggesting that resistance of horse lice is not yet a widespread problem. However, research on horse lice resistance to insecticides is rather scarce.
This means that if a particular product has not achieved the expected control, it is most likely because the product is not adequate or it was not used correctly, not because horse lice have become resistant.
|If available, follow more specific national or regional recommendations or regulations for horse lice control.|