Horses are susceptible to two types of lice — one that bites and one that sucks. Both, however, are unlikely to infest you, your cat or your dog. Both are difficult to see with the naked eye, but in good light you will find them. They are pale, nearly translucent.
Biting lice are active and move through the hair, while sucking lice are usually found with their mouth parts embedded in the skin. Biting lice live off the skin but also feed on blood and skin exudate. Sucking lice live off blood.
Both have elongated oval bodies, and under magnification you can see their legs — if you really want to. The eggs are also pale and translucent, flattened oval in shape. They’ll be glued onto hairs near the skin. Both lice and their eggs look like little flakes of dandruff, except the lice move and the eggs are fixed to hairs.
In severe infestations, the hair will be matted and the skin irritated and itchy. The small wounds, particularly of the sucking lice, may become infected. The constant crawling and piercing or biting of the skin can make the horse nervous and edgy. There may be hair loss, skin scarring, and even anemia (in the case of sucking lice). In less severe infestations, the coat will look rough and the horse will scratch/rub against fence posts and trees. It is possible, however, for the horse to have a normal-appearing coat without any signs of discomfort.
If you suspect lice, part the hair of the forelock, mane and base of the tail. Lice are most commonly seen on the top line, though they do travel throughout the hair coat. Be patient and watch for movement.
In northern areas, lice infestations increase in the winter when the hair coat is thicker and the parasites can escape detection. Horses also tend to congregate in small areas and stay together for warmth in winter, making spread easier. There is no seasonal pattern for lice in the southern states.
Other mammals and birds don’t carry horse lice. They are probably harbored in small numbers on a lot of horses and don’t take hold or cause any problems unless the horse is debilitated, develops immune system deficiencies, or otherwise becomes susceptible.
If you detect lice, don’t panic — quarantine. Lice dropped or pulled from the host will die in less than a week, but eggs may continue to hatch in the environment for two to three weeks in warm weather. Tack and grooming equipment can carry lice from one horse to another. Use separate equipment for the infested horse(s), and store that equipment well away from other brushes, pads, etc.
Use latex gloves when working with the affected horse. Palpation sleeves (plastic shoulder-length gloves, available from your vet or livestock supply store) are great in the winter when you are wearing layers of long-sleeved clothing as they help prevent you from carrying lice around on your sleeves. If you have a pair of coveralls, hang them near your isolation area and wear them when working on the infested horse to help keep your clothing lice-free.
Do not use lice/flea/tick formulations designed for other animal species. Safety of these products for horses has not been well studied. Organochloride or organophosphate pesticides are the most commonly used remedy for lice on livestock and don’t require application as frequently as other agents.
Pesticides are often prescribed as much by habit as anything else. Malathion is readily available and reasonably priced. It is safe at a 0.5% concentration but should not be used indiscriminately. Older, young and debilitated animals are more prone to toxicity, as are animals with open sores or breaks in the skin from rubbing. Don’t use on foals, pregnant mares or lactating mares.
There are better alternatives. One of the simplest is to deworm/delouse the horse with a standard dose of ivermectin. Although careful efficacy studies for treating lice in horses this way have not been done, it works well in other species, and you basically get to kill two birds with one stone.
Give ivermectin in the standard dose on the tube for the horse’s weight and repeat in two weeks. Do a careful check for lice at the four-week mark to be sure the problem is resolved.
Use caution in administering to horses that are debilitated and/or suspected of having a heavy parasite burden. Be careful not to overestimate the horse’s weight, especially if he or she has a long winter coat.
Perhaps the safest option of all, especially in debilitated horses, is to use pyrethrin fly sprays, which have low toxicity. Choose a fly-spray brand with pyrethrin that is marked to specifically kill, not just repel, flies. Pyrethrin potency can be boosted chemically, such as in piperonyl butoxide, also of much lower potential toxicity than organophosphate pesticides.