Lice in Horses

Courtesy of American Association of Equine Practitioners

Lice are members of the Phylum Arthropoda, Class Insecta, and Orders Anoplura (sucking Lice) and Mallophaga (chewing biting lice). Horses, donkeys, mules and other equids may be parasitized by an Anopluran or sucking louse, Haematopinus asini, and a mallophagan louse, Werneckiella (Damalinia, Bovicola) equi. These species of lice are capable of parasitizing the skin and/or subcutaneous tissues of horses.

Lifecycle/Biology As a sucking lice, Haematopinus asini feeds on tissue, fluids, and blood from horses, whereas Bovicola (Werneckiella) equi ingests skin and the occasional blood meal from horses. Lice undergo a life cycle referred to as a simple metamorphosis, consisting of the egg (nit), larvae (nymph), and an adult stage. All stages of the louse lifecycle may be found among the body hair coat of the infested horse. Lice are typically host specific insects, and thus, horse lice are permanent ectoparasites of horses. The entire lifecycle of the horse lice species are spent on the horse (or other equids). The eggs are oval, pale, and translucent. Adult females oviposit their eggs or nits on hairs, near the skin, using a ‘glue-like’ substance as a means of attachment. A female will deposit 1 egg/day and will usually live 30-35 days. Eggs hatch in 5 to 20 days, into small, pale nymphs, which are the same general body configurations – head, thorax, and abdomen. Sucking lice nymphs begin taking blood meals immediately, reaching maturity in 2-4 wks. Lice breed in a horse’s thick hair coat, and can be found throughout the year, but the total population numbers tend to diminish during the spring to summer months. Transmission of lice is by direct contact between horses, via infested brushes, blankets and other tack.

Adult lice are dorsal-ventrally flattened insects, a configuration that allows them to position themselves under the hair coat. As insects, the typical louse possesses a body with three distinct divisions: the head, the thorax, and the abdomen. Sucking and chewing/biting lice can be differentiated by a comparison between the widest part of the head (minus the antennae) with the widest part of the thorax. Sucking lice possess a head which is narrower than the widest part of the thorax, whereas chewing lice will have a head that is wider than the widest part of the their thorax.

The sucking louse, Haematopinus asini is approximately 3-5 mm long, grey to yellowbrown in color, with a thorax which is about half the width of it’s the abdomen. The head of H. asini is narrowed anteriorly, and less than one-third of it’s abdominal width. Since it is a sucking louse, this louse possesses distinct piercing mouth parts. H asini is often found on the head, mane and tail, and tend to move slowly. This specie may be observed with its mouthparts embedded in the skin.

The chewing louse, Werneckiella equi, is approximately 1-2mm long, flat with broad, rounded head. This louse has an arc, anterior to the antennae, a ventral chewing mandible, and thin legs. These chewing lice also are brown in color, and have yellow abdomens with contrasting dark bands. W. equi lice are usually found on the back and flanks, are more mobile and tend to move faster than Hematopinus asini. However, when heavy infestations occur, lice can be found everywhere on the body.

Lice infestations are more common in sick, debilitated, possibly under conditioned, immunosuppressed animals. Pediculosis in horses is characterized by pruritus (scratching, rubbing, biting, etc), skin irritation, unthrifty appearance, a rough ‘unkept’ hair coat, and possibly a loss of body condition. In severe infestations, hair loss and skin scarification are often seen, and in the case of sucking lice, anemia may be present. Although both types of lice can be found anywhere on the horse, the chewing lice are more common on the head, mane, tail base, and shoulders; whereas the sucking lice are more commonly found in shorter haired regions of the horse (eg. head, neck, back, and inner thigh). Lice infestations and subsequent clinical signs are more common in late winter and early spring. Horses with thick hair coats appear to be more commonly infested. Some heavily infested horses may exhibit a nervous behavior which is associated with the constant irritation of feeding lice.

A diagnosis can be made based the presence of lice on the horse and possibly clinical signs. Use of a pen light and magnifying glass may be helpful, in a well lighted environment, the hair around affected areas of the horse can be carefully parted, and the layers of the hair coat and skin carefully examined. Fast moving chewing lice are more easily observed, while sucking lice tend to move more slowly. Louse eggs can also be detected as small white eggs ‘cemented’ to the hair shafts. The mane, forelock, lower neck and base of tail are common sites of infestations and clinical signs.

Infested horses should be thoroughly washed with a shampoo that contains an approved insecticide (permethrins , coumaphos, dichlorvos, etc), insuring adequate skin contact to all affected areas. The use of a shampoo containing 1% selenium sulphide, just prior to the application of the insecticide containing shampoo, may help remove dead skin and scale, and allow for better insecticide contact. Selenium sulfide shampoos may also have a direct antiparasitic action. To maximize the effectiveness of lice control, the shampooing/cleaning should be repeated in two weeks in order to kill any nits that hatch after the first treatment. If shampooing is impractical, wetable powders or dusts containing insecticides (eg. carbaryl, coumaphos, fenthion, pyrethroids synergized with piperonyl butoxoide, rotenone, etc) can be used. Regardless of the method used (washing or dusting) the horse should be completely covered and the treatment should come in contact with the lice near or on the skin. The treatments may also be delivered by spraying, but care should be taken to insure sufficient skin contact. Gloves, and other protective gear should be worn in order to minimize human skin, eye, etc contact with the lice killing chemicals. The person treating the horse should also make every attempt to avoid the horse’s eyes, mucosa of the mouth, nostrils, prepuce and vulva when applying insecticides or other chemicals. When using organophosphates such as coumaphos, you may be required to obtain a special application license—depending upon the formulation and the specific requirements of your state. These products should not be used on horses intended for slaughter.

The routine use of a macrocyclic lactone de worming products (ivermectin, moxidectin) may also aid in the control of sucking lice. A possible treatment protocol may include the use of deworming product, particularly if the horses are concurrently parasitized with susceptible nematodes. It is preferable that all product labels be carefully read, the instructions followed and only approved lice control products be used. Where possible, the extra label use of any insecticide, or other pharmaceutical should be avoided.

All fomites (eg.,tack, brushes, saddles, etc) should be treated with an effective insecticide. Blankets used on infested horses should also be washed in hot water, carefully rinsed, then dried at the hottest possible clothes dryer setting.