THE MERCK VETERINARY MANUAL
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Overview of Lice

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Numerous species of chewing or biting lice (order Mallophaga) and sucking lice (order Anoplura) are obligate ectoparasites of domestic animals. Many authors include both chewing and sucking lice in the order Phthiraptera. Lice live within the microenvironment provided by the skin and its hair or feathers, and are transmitted primarily by contact between hosts. In temperate regions, lice are most abundant during the colder months and often are difficult to find in the summer. Lice are largely host-specific, living on one species or several closely related species. Anoplura are parasites of mammals. However, Mallophaga infest both mammals and birds. (Also see Ectoparasites.)

Lice are wingless, flattened insects, usually 2–4 mm long. The claws of the legs are adapted for clinging to and moving among hairs or feathers. Mallophaga have ventral chewing mandibles and they feed on epidermal debris, primarily skin scales, secretions, and scurf. The mouthparts also aid in attachment to the host. The head of the mallophagan is wider than the prothorax. Anoplura are blood feeders. When not in use, their mouthpart stylets are retracted within the head.

On mammalian hosts, louse eggs or nits are glued to hairs near the skin surface and are pale, translucent, and suboval. The 3 nymphal stages, of increasing size, are smaller than adults but otherwise resemble them in habits and appearance. About 3–4 wk are required to complete one generation, but this varies with species.

In temperate climates, cattle may be infested with one species of Mallophaga, the cattle biting louse, Damalinia bovis, and 3 species of Anoplura: the longnosed cattle louse, Linognathus vituli; the little blue cattle louse, Solenopotes capillatus; and the shortnosed cattle louse, Haematopinus eurysternus. It is not uncommon for cattle, especially young animals, to be infested with multiple species. The cattle biting louse may be found on the top line, especially in the withers area extending to the head and tailhead. The little blue cattle louse is found in distinct clusters, mainly on the head and face, extending to the dewlap in heavy infestations. The shortnosed cattle louse is found on the front half of the host from the ears to the dewlap, tending to be heavier in the anterior portions of the body, including the ears, during warm weather. The longnosed cattle louse occurs widely on the animal, often among the other species. In early infestations, it may be found in clusters. In heavy infestations, this species may be found over most of the body.

Haematopinus quadripertusus, the cattle tail louse, is a tropical, sucking louse that has extended its distribution into subtropical areas (California, Florida, and Gulf Coast in the USA). The adults and ova are found in the tail switch; nymphs may be found on other parts of the body, including the perineum and vulva. The cattle tail louse is known to parasitize both European and Zebu breeds of cattle.

Haematopinus tuberculatus, the louse of the Asiatic water buffalo, appears to have transferred to cattle in various parts of the world, and is able to maintain itself on cattle in tropical climates. These lice are usually found on the back and hindlegs, although the eggs are usually deposited on the neck, shoulders, and forelegs of the host.

Horses and donkeys may be infested by 2 species of lice, Haematopinus asini, the horse sucking louse, and Damalinia equi, the horse biting louse. Both species are worldwide in distribution. Normally, H asini is found at the roots of the forelock and mane, around the base of the tail, and on the hairs just above the hoof. D equi prefers to oviposit on the finer hairs of the body and is found on the sides of the neck, the flanks, and the base of the tail.

Domestic pigs are infested with only one species of louse, Haematopinus suis, the hog louse. This very large (5–6 mm) sucking louse is common on domestic swine worldwide. Nymphal lice are normally found on the inside of the ears, often deep inside; on the skin behind the ears; in the folds of the neck; on the inside of the legs; and on the inner flanks. All stages may be found under the scurf of the skin elsewhere on the body.

Sheep may become infested with the sheep biting louse, Damalinia ovis, and 3 species of sucking louse: the sheep foot louse, Linognathus pedalis; the face and body louse, L ovillus; and the African blue louse, L africanus. Outside the USA, D ovis is also referred to as the sheep body louse. The foot louse of sheep is so named because, except in very heavy infestations, it is confined to the hairy parts of the foot. The face louse is usually found on hairy parts of the sheep's skin; as populations increase, they spread to other parts of the body. L africanus forms clusters, often on the flanks of sheep. Slippage of wool is common. L africanus has also been reported from a variety of hosts including goats and several species of deer.

Linognathus stenopsis, the goat sucking louse, is found on both shorthaired and Angora breeds of goats. It has been reported from sheep in various parts of the world. It is found mostly on the long-haired parts of the hindlegs and back. Severe infestations are rare. Damalinia caprae, the goat biting louse, is most frequently found on short-haired goats. Both D crassipes and Dlimbata (the Angora goat biting louse) are serious pests of Angora breeds.

Dogs are occasionally infested with Linognathus setosus (the dog sucking louse), Trichodectes canis (the dog biting louse), and rarely, Heterodoxus spiniger, another species of chewing louse. L setosus and T canis also parasitize a variety of wild canids. Dogs that are neglected or in poor health may become heavily infested with L setosus, which tends to prefer longhaired breeds. T canis prefers the head, neck, and tail of the host, and it may be found around wounds and body openings. Infestations of the dog biting louse may be heavy on very young and very old animals. Infested dogs rub, bite, and scratch the affected area and have a rough, matted coat. T canis serves as an intermediate host of the tapeworm Dipylidium caninum, a parasite of dogs, cats, foxes, and occasionally humans. H spiniger, originally a parasite of marsupials that transferred to the dingo then to the domestic dog, is considered rare in North America. Although worldwide in distribution, it appears to be more common in warmer environments and is heavier on animals in poor physical condition. H spiniger also serves as an intermediate host of D caninum and of the filarial worm Dipetalonema reconditum.

The cat louse, Felicola subrostrata, is a chewing louse that occasionally parasitizes cats. The louse may be seen more frequently on older, longhaired cats that are unable to groom themselves.

Pediculosis is manifest by pruritus and dermal irritation with resultant scratching, rubbing, and biting of infested areas. A generally unthrifty appearance, rough coat, and lowered production in farm animals are common. In severe infestations, there may be loss of hair and local scarification. Extreme infestation with sucking lice can cause anemia. In sheep and goats, rubbing and scratching often results in broken fibers, which gives the fleece a “pulled” appearance. In dogs, the coat becomes rough and dry and, if lice are numerous, the hair may be matted. Sucking lice cause small wounds that may become infected. The constant crawling and piercing or biting of the skin causes nervousness in hosts.

Diagnosis is based on the presence of lice. The hair should be parted, and the skin and proximal portion of the coat examined with the aid of light if indoors. The hair of large animals should be parted on the face, neck, ears, topline, dewlap, escutcheon, tail base, and tail switch. The head, legs, feet, and scrotum should not be overlooked, particularly in sheep. On small animals, the ova are readily seen. Occasionally, when the coat is matted, the lice can be seen when the mass is broken apart. Biting lice are active and can be seen moving through the hair. Sucking lice usually move more slowly and are often found with mouthparts embedded in the skin.

Pediculosis of livestock is most prevalent during the winter; severity is greatly reduced with the approach of summer. Infestations of both chewing and sucking lice may become severe. In dairy herds, the young stock, dry cows, and bulls may escape early diagnosis and suffer more severely. Young calves may die, and pregnant cows may abort. Effective treatment results in prompt improvement.

Transmission usually occurs by host contact. Lice dropped or pulled from the host die in a few days, but disengaged ova may continue to hatch over 2–3 wk in warm weather. Therefore, premises recently vacated by infested stock should be disinfected before being used for clean stock.

Louse control requires treatment with an effective insecticide or drug (see Ectoparasiticides and see Anthelmintics). Products that may be used are determined by government regulations, and users are required to read and follow product labels. Registrations of several products that have been listed in the past, especially some formulations of organophosphates, have been canceled and are no longer legally available for sale in the USA. It is possible that some products that are still available are not included in the following discussion. It is the users' responsibility to ensure that a particular treatment has not been canceled. Formulations classified for restricted use may be purchased and used only by certified applicators or by persons under their direct supervision. Label precautions regarding age and breed of animal and frequency of treatment should be observed. Some product labels direct retreatment in 2 wk to control a particularly refractory infestation.

A few compounds may be applied as a whole-body spray for lice control. A light, mist application of some formulations may be effective, while others may require soaking the hair to the skin.

Zero to very low residue tolerances for pesticides in milk limit the insecticides that may be applied to dairy cattle and dairy goats. Permethrin spray may be applied to these animals for control of lice. Additionally, dairy cattle may be sprayed with permethrin synergized with piperonyl butoxide (PBO), coumaphos, tetrachlorvinphos, tetrachlorvinphos plus dichlorvos, and amitraz. Some products approved for beef cattle may be used on dairy animals that have not yet reached breeding age. Appropriate milk withdrawal times must be observed for these animals. Beef cattle, sheep, and swine may be sprayed with coumaphos or permethrin. Amitraz, phosmet, and tetrachlorvinphos spray may be used on beef cattle and swine. Permethrin synergized with PBO, spinosad, tetrachlorvinphos, and tetrachlorvinphos plus dichlorvos may be applied to both dairy and beef cattle. Permethrin with PBO spray is approved for control of lice on sheep. Horses may be treated with a permethrin or pyrethrin spray.

Because of ease of application and reduced stress to the treated animal, the pour-on method has become a popular means of applying a variety of insecticides, both nonsystemic and systemic, for control of lice. Beef cattle, lactating and nonlactating dairy cattle, sheep, swine, and nonlactating goats may be treated with pour-on formulations of permethrin for louse control. Often permethrin is synergized with PBO. Wipe-on formulations of permethrin, permethrin plus diflubenzuron, and pyrethrins are available for lice control on horses. Because the percentage of active ingredient in commercial pour-on formulations varies from 1 to 10%, it is important that a formulation is approved for the animals being treated. A permethrin pour-on with PBO is also available for lice control on beef cattle, lactating and nonlactating dairy cattle, and sheep. Cyfluthrin pour-on, permethrin plus diflubenzuron, and spinosad pour-on are approved for beef cattle and lactating and nonlactating dairy cattle, but λ-cyhalothrin is approved only for beef cattle. Amitraz pour-on is approved for swine.

Several systemic antiparasitic compounds, the macrocyclic lactones, are available as pour-on formulations for control of cattle lice as well as a variety of other internal and external parasites. Because these products also control cattle grubs, precautions should be taken to avoid host-parasite reactions (see Cattle Grubs). Pour-on formulations of doramectin, eprinomectin, ivermectin, and moxidectin are effective against both chewing and sucking lice of beef cattle. Lactating dairy cattle may be treated with eprinomectin and moxidectin pour-on. Otherwise, use of systemic drugs for lice control is prohibited in dairy animals of breeding age. Doramectin and ivermectin are also available in injectable formulations, and ivermectin is available as an oral paste; however, these are less effective against chewing lice than are the typical pour-ons. Doramectin injectable and ivermectin injectable and premix are effective against the sucking louse of swine.

Lice on beef cattle can be suppressed by wintertime use of self-treatment devices, eg, insecticide ear tags, dust bags, and back rubbers or oilers. Insecticide ear tags containing a variety of active ingredients (eg, chlorpyrifos, diazinon, endosulfan, λ-cyhalothrin, permethrin, zeta-cypermethrin, often with PBO) control or aid in the control of chewing and sucking cattle lice. Some tags contain a single active ingredient, while others contain a mixture. Although many of the tags are approved for lice control on beef cattle, they may not be approved for lactating dairy cattle. Lice populations can be controlled on lactating and nonlactating dairy and beef cattle, lactating and nonlactating goats, sheep, and horses with dusts containing zeta-cypermethrin. Lice populations can also can be reduced by dusting with coumaphos, cyfluthrin, tetrachlorvinphos, or permethrin on beef or dairy cattle and coumaphos, permethrin, or tetrachlorvinphos on swine. Horses may also be dusted with coumaphos. For severe infestations in swine, dust formulations can be used to treat bedding.

Many products, such as shampoos, sprays, or dusts, are available for insect control on pets; however lice are seldom mentioned on the label. Dogs can be treated with permethrin or pyrethrin with PBO. Cats may also be treated with synergized pyrethrins. Doses of ivermectin high enough to be effective against lice are not recommended in dogs. Some topical spot-on treatments for flea control on dogs or cats are also labeled for control of lice. These treatments are generally applied to one or a few spots between the shoulder blades. Imidacloprid, imidacloprid plus permethrin, and imidacloprid plus moxidectin are approved for lice in dogs. Treatment with fipronil plus (S)-methoprene is approved for control of chewing lice on both dogs and cats. Imidacloprid and imidacloprid plus moxidectin are approved for cats.

In most countries, regulatory agencies specify tissue residue limits of insecticides and carefully regulate insecticide use on livestock. All such regulations are subject to change; pertinent current local laws and requirements should be determined. The treatment of meat and dairy animals must be restricted to uses specified on the product label, and all label precautions should be carefully observed.

Last full review/revision July 2011 by John E. Lloyd, BS, PhD

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