We are a Royal College of Veterinary Surgeons (RCVS) approved equine hospital.
Louse infestation is one of the most common causes of itchiness, properly called pruritus, in horses. Lice are most common in cool damp climates in late winter and early spring. So, in short, if your horse or pony is itching, always rule out lice and indeed other parasites, such as mites, before thinking of other cause of itchiness, such as allergic skin disease, which is the second most common reason why horses and ponies itch.
Never dismiss lice as a reason for itchiness. Lice are not just a problem that affects horses in poor condition, kept crowded together and/or rarely groomed. Although horses that are neglected may be more likely to have lice, they can affect any horse worldwide. Low level of lice infestations may not actually produce many signs of skin irritation and as such, they may go unnoticed certainly in the early stages. A heavy infestation can cause intense itching, hair loss and skin infections. Skin that is rubbed raw by persistent itching is extremely unpleasant for the poor animal and a cause of concern for the owner too.
Horse owners tend to be more concerned about internal parasites, namely worms; yet external parasites, like lice and mites can be a major nuisance as well as having an impact on equine health. Horses that are crawling with lice must be uncomfortable and may fail to thrive. A severe infestation can actually cause anaemia due to the raw skin sores that are caused and because some varieties of lice actually are blood sucking.
It is not unusual for groups of young horses housed together during winter months to become infested with lice, as large numbers multiply unnoticed in their long winter coats, so check them carefully come spring.
There are two varietis of lice: bloodsucking (Haematopinus asini) and biting (Damalinia equi). Both species are six-legged, tiny, wingless and usually light brown in colour.
The sucking variety is the larger of the two and is quite easy to spot during routine inspection. They have pointed heads and penetrate the skin with their mouth parts. They are commonly found on the longer hairs of the mare, tail and fetlocks, where their eggs (nits) are visibly attached to the hair.
The biting lice live close to the skin on scurf and dead cells, so are harder to see. They are generally found along the back and sides of the horse, but may spread over the entire body if untreated.
Lice have a life cycle of between 10 and 21 days. The adults lay eggs on hair, which hatch and develop into adults.
Infected animals typically look rather moth-eaten, particularly under the mare, along their back and around their tails as they rub and bite themselves in response to the irritation. This can cause bald patches and sore areas that may release serum, while the horse’s coat is often dull and scurfy.
Severely affected horses often lose condition and may become restless. It is possible for a secondary infection to take hold around the areas of damaged skin.
Both the lice and their eggs (known as 'nits'), can be seen on close inspection of a horse’s hair coat. Often live lice are seen as a rug is removed and they dive for cover by burrowing into the horse’s hair coat, being just big enough to see with the naked eye in good light…if you have good vision. The eggs or nits are tiny white oval blobs, which are almost more obvious as they literally stick to the hairs. Not a pleasant comparison, but horse lice are very similar to human lice, and the old-fashioned nit combs will help to find lice in horses, just as with people.
If your eyesight is not sharp enough to spot the actual lice, severely affected lousy horses are obvious as they simply look dishevelled with rough, dull coats with patchy irregular areas of hair loss and rubbed sore skin, but mildly affected horses can look surprisingly normal. If lice are found on one animal, it should be assumed that all in contact horses are infected, even if lice are not seen and the skin appears normal.
Lice seem to be worse, if the horses in question are malnourished or immunocompromised, for instance, when affected by Equine Cushing’s Disease. Stressed or otherwise unwell horses may be more prone to louse infestations, but even apparently healthy horses may carry lice - some may be more irritated or possibly even allergic to the lice bites or irritation. This may be similar to some ponies developing sweet itch as an allergic response to midge bites.
Lice are generally transmitted by direct contact between horses. They will only survive for a few days in rugs, grooming brushes and tack or on paddock fences and trees that horses rub against, so may also be spread in this manner, but most transmission is by direct animal contact or spread via items such as grooming kit.
Lice are host specific, which means horse lice stick to horses and are not found on other animals, whilst donkeys have their own types of lice.
Treatment for equine lice involves anti-parasitic powders and liquids, as well as appropriate improvements in nutrition and management. Grooming and clipping off the contaminated coat will help too. It may be unpopular, but it is worth treating all in-contact horses at the same time to ensure all the lice are eliminated. Louse eggs are hard to remove and are resistant to most chemicals, hence the use of nit combs in people historically, and also it is thought to be a reason for ironing of human clothes. The logic being that the heat killed the lice and their eggs, which may be one of the few sensible reasons for ironing, but hopefully it is redundant nowadays!
As neither ironing nor nit-combs will solve the problem of lice in horses, it is important to repeat medical treatment at 3-week intervals to eliminate the lice that hatch out. Theoretically the lice should not persist in the environment, however, where there is a seriously heavy infestation, it will do no harm to clean grooming kit and rugs as well.
Obviously, lice treatment requires an effective product which will actually eliminate them. It is best to consult your vet as to what works best. There has been both scientific research and anecdotal stories, that not every product works quite as well as one might think. There has to be a balance between medication that is toxic enough to kill the lice, but not harmful to humans. Some over-the-counter louse powders are not always that effective, but there has also been some research showing that donkey lice have been developing resistance to some insecticide products, namely cypermethrin and permethrin used in a pour-on form. Tests showed that the product was not evenly distributed throughout the donkeys’ hair coats, resulting in lice being exposed to sub-lethal doses, which encouraged the development of resistance, so always ensure all lice treatment is spread throughout the coat.
The same study showed organophosphates were more effective, but they are recognised as harmful to people. Interestingly another study raised the possibility of using essential and non-essential oils in the control of biting lice. Researchers examined the toxicity of six plant essential oils to chewing lice, collected from donkeys. The six oils assessed were: tea-tree, lavender, peppermint, eucalyptus, clove bud and camphor. All oils except camphor showed high levels of toxicity to the lice. Fifty per cent mortality was achieved at concentrations below 2 per cent. Concentrations of 5-10 per cent resulted in 100 per cent mortality. Two essential oil components, also showed similar levels of toxicity. This suggested that these botanical products may offer environmentally and toxicologically safe, alternative veterinary treatments for lice control, but more information is required as to how this works in the actual animal.
Finally, the wormers ivermectin or moxidectin used to treat parasitic worms may also help kill lice, especially the blood sucking lice that will take up the toxic wormer, whereas the chewing lice that eat skin scale, are less likely to be exposed to a louse lethal dose of wormer. These days, such wormers are being used less often, again due to concerns about worm resistance as well as environmental hazards. Instead other worm control measures are in use, such as worm egg counts to see if worming is really needed, rather than regular treatment. Typical of the law of unintended consequences, it would seem the reduction in use of routine wormer medication may mean an increase in lice infestations, so keep looking out for the louse, particularly in any itchy horse, pony or donkey.
It is recommended that three treatments are given at 10-day intervals to allow fresh eggs to hatch. Protective gloves and clothing should be worn.
Although not licensed for the treatment of lice in horses, the following have been shown to be effective and may be used under instruction from your vet:
Tack, rugs and brushes should also be treated, either with a suitable topical application or by steam cleaning.
Ideally treatment should be applied simultaneously to all horses in contact, and repeated at the recommended intervals to kill off the hatching eggs or nits. The eggs are resistant to treatment, so repeated treatments are needed as the eggs hatch
Please see this article on Control of lice infestation in horses using a 10 mg/mL deltamethrin topical application (June 2017)