Mites are tiny parasites (< 1 mm) that live in the skin of horses, donkeys and many other mammals and vertebrates worldwide. Mites are not insects but belong to the group called Acarina, together with ticks.
They are obligate parasites, i.e. they can neither survive nor complete their life cycle without parasitizing their hosts.
The most important parasitic mite species of horses and other equids are:
- Chorioptes equi, the itchy leg mite, the causative agent of chorioptic mange. Affects horses, donkey and mules worldwide.
- Demodex equi, the horse follicle mite, the causative agent of demodectic mange. Affects horses worldwide.
- Psoroptes equi, the scab mite or equine body mite, the causative agent of psoroptic mange. Affects horses and mules worldwide.
- Sarcoptes scabiei var. equi, the common mange mite and causative agent of sarcoptic mange. Affects horses, donkeys and mules worldwide.
Are horses infected with mites contagious for humans?
- Basically no, because most mite species (Psoroptes, Demodex, Chorioptes) are quite host-specific and those that affect horses do not affect humans. But it cannot be excluded that persons in very close and repeated contact with horses infected with some mite species (e.g. Sarcoptis scabiei) may become transiently infected as well. However, the infection won't be as severe as in horses and it should receed spontaneously.
Infestations with mites are technically called acariosis or acariasis, both on animals and humans.
Horse mites are not vectors of other pathogens, i.e. they do not transmit microbial diseases as many other livestock parasites do.
Click here to learn more about the general biology of mites.
Biology and life cycle of horse mites
Adult Psoroptes mites are oval in shape, 0.5-0.6 mm long, usually only recognizable under the microscope. As for all mites, development goes through various larval and nymphal stages. A female mite lays a few eggs a day, a total of about 50 to 100 eggs in her lifetime. Adult life lasts for about 30-40 days. The life cycle takes about 8 days to complete.
Psoroptes mites produce typical scabs on the skin of affected animals, thus their common name scab mites. Psoroptes mites do not dig tunnels in the skin. In the past it was thought that they pierce the skin of their hosts. Today it is believed that they do not pierce the skin, but that the mite feces cause an allergic reaction of the host's skin, which reacts producing exudations and skin thickening and hardening (lichenification) with formation of papules, scales and crusts (excoriations), often with hair loss. The mites than suck the exudates and secretions produced.
As all mite species, Psoroptes mites spend their whole life on the same host. Transmission from one to another animal is mostly by physical contact. The mites do not actively jump or crawl from one host to another one, but are passively transmitted when animals come in close contact. Nevertheless, Psoroptic mites and eggs can survive 2 to 3 weeks off the host by suitable conditions, i.e. animals can pick mites or eggs from their environment, and they can also be passively transmitted by saddlery or tools and equipment in the stable. But there are no external vectors that transmit the mites, e.g. insects, worms, rats, birds, etc., as it happens with many other parasites.
In regions with a cold winter (Canada, most of the USA and Europe, etc.), psoroptic mange is a typical winter pest, favored by crowding due to indoor confinement during the cold season.
Psoroptic mange is not very frequent in horses. It develops mainly in areas with thick hair (neck, mane, base of the tail, etc.), but may affect the whole body surface. Psoroptic mites are not as harmful for horses as they are for sheep. Nevertheless, affected animals suffer from intense itching (pruritus) and react vigorously scratching, biting and rubbing against objects, which can cause injuries that can be infected with secondary bacteria or attract other parasites (e.g. screwworm flies). All this leads to hair loss, weight loss, and general weakness that make the affected animals more susceptible to other diseases. Massive infestations can cause severe weakening.
Diagnosis has to be confirmed examining skin scrappings of affected parts under the microscope for visualization of the mites.
Psoroptic mites of horses are usually not infectious for humans, dogs and cats.
Psoroptes cuniculi, a species that affects rabbits may occasionally infect horses and affect mainly the ears.
The classification in different Psoroptes species (Psoroptes ovis, Psoroptes bovis, Psoroptes cuniculi, etc.) is still discussed among specialists. Many authors believe that they are only varieties of a single species.
Sarcoptic mites of horses are a species-specific strain of Sarcoptes scabiei, a mite species that infests also sheep, cattle, pigs, other livestock and also humans. This means that it can be transmitted to humans.
Sarcoptic mites are very small (0.3 to 0.5 mm) and can be seen only under the microscope. As all mite species, sarcoptic mange mites spend their whole life on the same host. The mites do not actively jump or crawl from one host to another one, but are passively transmitted when animals come in close physical contact. However, horses can pick mites from the immediate environment and they can also be passively transmitted by saddlery or tools and equipment in the stable. But there are no external vectors that transmit the mites, e.g. insects, worms, rats, mites, birds, etc., as it happens with many other parasites.
Sarcoptes mites dig tunnels beneath the skin. Their saliva has potent digestive enzymes that dissolve the skin tissues. The mites feed on the resulting liquids. They do not suck blood. Adult females deposit their eggs in tunnels, which hatch in 3 to 5 days. The whole development through several larval and nymphal stages can be completed in less than 2 weeks. Adults live for 2 to 3 weeks. Off the host the mites survive only a few days because they are very susceptible to dryness.
Sarcoptic mange is not very common in horses, but when it happens it is the most harmful of all mite infestations. It is more frequent in large herds than in animals hold individually. Harm to horses can be substantial. Mite digging causes skin irritation, which is enhanced by allergic reactions to the saliva that develop a few weeks after infestation. The affected skin develops pimples and papules that become crusty, with massive hair loss, progressive hardening and thickening, and building of large folds. Infestations start often on the head, spread to the neck and the shoulders and may cover the whole body within a few weeks.
Affected animals suffer intense itching (pruritus) and react vigorously scratching, biting and rubbing the affected parts against whatever object. This causes injuries that can become infected with secondary bacteria and attract other parasites (e.g. screwworm flies). All this results in reduced weight gains and general weakness that makes the affected animals more susceptible to other diseases. Extended skin damage may impair the regulation of the body temperature. Severe infestations can lead to emaciation. Fatalities are not excluded if left untreated.
Sarcoptic mange is also a typical winter pest in regions wilt a cold winter. Outbreaks usually peak in late winter and early spring.
Diagnosis has to be confirmed examining skin scrappings under the microscope for visualization of the mites.
Sarcoptes mites of most domestic mammals (and other mite species too) can be contagious for humans, causing the so-called pseudoscabies, characterized by intense itch. However, the mites cannot complete development on humans and the infestation recedes spontaneously.
Chorioptic mites of horses (the itchy leg mite) are less harmful than psoroptic or sarcoptic mites. They are not transmitted to humans.
These mites are also very small (0.4 to 0.6 mm) and can only be seen under the microscope. They have chewing mouthparts and neither suck blood, nor dig tunnels as sarcoptic mites, but bite the outer skin layers and feed on skin debris, fat, lymph or exudates. The whole development through several larval and nymphal stages can be completed in about 2 weeks. Off the host the mites survive only a few days.
Chorioptic mange is also a typical winter pest in regions wilt a cold winter. Outbreaks usually peak in late winter and early spring. Once the animals leave the winter confinement, chorioptic mange mostly regresses spontaneously.
Preferential sites or chorioptic mites are the hoofs and the lower part of the legs, mainly on the hind legs. Horses with long hairs on the fetlocks are especially at risk. Initially, infestations result in more or less extended eczema. If left untreated, scabs and crusts are build, later even large callus and warts.
Chorioptic mange also causes itching, but usually not as intense as with psoroptic and sarcoptic mange, and the reactions of affected animals (biting, scratching, rubbing, etc.) are also less vigorous. Secondary bacterial infections are seldom. Harm is seldom severe, but infestations are difficult to heal, become chronic and may persist for a long time.
Diagnosis has to be confirmed examining skin scrappings under the microscope for visualization of the mites.
Horse demodectic mange mites have an elongated shape and are even smaller (~0.25 mm) than psoroptic or sarcoptic mites. They get into the hair follicles and sebaceous glands and build nodules and papules that can become infected with secondary bacteria. The life cycle can be completed in about 2 weeks, but is poorly understood. Demodex mites can survive up to 4 months off the host.
Lesions of the host's skin consist of papules and nodules that often appear first on the forehead and around the eyes, later on the shoulders. They may infest the whole body surface. Affected skin is often covered with scales.
Demodectic mange can develop year-round, not only during the cold season but is rather unusual in horses. It is more common in large herds than in animals kept individually.
Unlike other mites, demodectic mites do not cause itching and the affected animals are not severely affected. Therefore demodectic mange is usually rather benign, often without clinical symptoms. If the affected horses are otherwise weakened infestations may become harmful and cause more or less severe skin lesions.
Diagnosis has to be confirmed by microscopic examination of samples taken from hair follicles and sebaceous glands.
Prevention and control of horse mites
Most mite infestations are usually a winter problem in regions with a cold season (e.g. Canada, most of the US and Europe, etc.). Crowding during indoor confinement means closer physical contact between the animals, which makes mite transmission easier. It also means a more humid hair coat, which is also favorable for mite development. Feeding is often deficient during the cold season, and confinement stresses the animals. These factors make the animals less resistant to the disease, especially to sarcoptic and prosoptic mange.
For all these reasons, keeping the animals well fed and in good health and hygienic conditions is crucial to reduce the risk of winter outbreaks, or at least to limit the harm that such outbreaks can cause.
As soon as the animals go back to pasture in spring symptoms regress quickly. Exposure to sun reduces the humidity in the hair coat, which slows down mite development, and without crowding mite transmission is significantly reduced.
For reasons yet unknown, a few mites often survive such unfavorable summer conditions in a few animals within a herd, without showing any clinical signs. Once the herd goes back to the winter quarters in fall, these animals will transmit the mites to the rest of the herd if preventive measures are not taken.
Therefore, where mange is known to be a problem, it is highly recommended to preventively treat animals kept in herds in late autumn. All animals in a herd must be treated, because it is impossible to know which are the carrier animals.
If a herd is free of mites, contamination can only come from animals brought in. Consequently, to avoid contamination treat all incoming animals against mites, especially during the winter months.
For the time being there are no vaccines that will protect horse by making them immune to the mites. There are no repellents, natural or synthetic that will keep mites away from horses. And there are no traps for catching mites, for the simple reason that they spend their whole life on the animals.
There are no biological control means for controlling horse mites (or any other mites of livestock and pets). Learn more about biological control of ticks and mites.
Whereas there are numerous topical and systemic acaricides that control mites 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 it is not attractive enough for most Animal Health companies to invest in a claim for use on horses.
Some 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.) can control mites on horses. A few ones contain mixtures, typically organophosphate + pyrethroid.
Ready-to-use pour-ons (backliners) that control mites, lice and other external parasites on horses are also available in most countries. They contain mainly synthetic pyrethroids (e.g. cypermethrin, deltamethrin, permethrin, etc.).
Topical acaricides have a short residual effect, i.e. there efficacy does not last more than a week, if ever. And they often do not kill mites remaining deeply inside thick scabs and crusts. Therefore it is highly recommended to repeat the treatment once or twice with an interval of 2 to 3 weeks. This is also important because some mite species can survive off the hosts for several weeks and re-infest the animals, and because acaricides do not kill the eggs, which will hatch and re-start the infestation. If thick scabs and crusts are present the product should be vigorously brushed on the affected skin for the acaricide to be able to penetrate the thickened skin and reach the mites.
Macrocyclic lactones (e.g. ivermectin, moxidectin) administered orally (pastes, gels, suspensions, etc.) to horses can work more or less against Psoroptes and/or Sarcoptes mites. However most commercial brands containing macrocyclic lactones for horses do not include a label claim for mite control. They will certainly don't work against Chorioptes and Demodex mites.
For the time being there are no vaccines that will protect horses by making them immune to mites. There are no repellents, natural or synthetic that will keep mites away from horses. And there are no traps for catching horse mites, 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 mites and other external parasites of livestock and pets.
WARNING. NEVER USE AMITRAZ products ON HORSES because amitraz is toxic to horses. Such products containing amitraz are often used in many countries for mite control on livestock and dogs.
Resistance of horse mites to parasiticides
So far there are no reports on resistance of horse mites to parasiticides.
This means that if a product fails to achieve the expected efficacy, it is extremely unlikely that it may be due to resistance of the mites to the product. Either the product was used incorrectly, or it was not appropriate for mite control.
|If available, follow more specific national or regional recommendations or regulations for mite control.|