Lice: Introduction - The Merck Veterinary Manual

Lice: Introduction
(Pediculosis)

Numerous species of biting or chewing lice (order Mallophaga) and sucking lice (order Anoplura) are obligate ectoparasites of domestic animals. 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 very 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 only. However, Mallophaga infest both mammals and birds. (See also ectoparasites of poultry, Ectoparasites.)

Etiology:

Lice are wingless, flattened insects, usually 2-4 mm long. The claws of the legs are adapted for clinging to hairs or feathers. Mallophaga have ventral chewing mandibles and they feed on epidermal products, primarily skin scales and scurf. 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.

Louse eggs or ""nits""

Louse eggs or nits are glued to hairs of mammallian hosts 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 (Bovicola) 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 2, 3, or all 4 species. These lice may be found on the head (including the ears), neck, topline, and brisket. In heavy infestations, they may be found over most of the body.

Cattle biting louse, Damalinia (Bovicola) bovis, skin

Damalinia bovis, female

Damalinia bovis, male

Long-nosed cattle louse, Linognathus vituli, skin

Linognathus vituli, female

Linognathus vituli, male

Short-nosed cattle louse, Haematopinus eurysternus, skin

Haematopinus eurysternus, female

Haematopinus eurysternus, male

Little blue cattle louse, Solenopotes capillatus, dewlap

Solenopotes capillatus, female

Solenopotes capillatus, male

Haematopinus quadripertusus , the cattle tail louse, is a tropical, sucking louse that has extended its distribution into subtopical 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

Horse sucking louse, Haematopinus asini

.

Horse biting louse, Damalinia (Bovicola) equi

Damalinia equi nits

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, close to the body; and on the inner flanks. All stages may be found under the scurf of the skin elsewhere on the body.

Sheep biting louse, Damalinia (Bovicola) ovis

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 coarse-wooled 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. Damalinia caprae , the goat biting louse, is most frequently found on short-haired goats, whereas chewing lice on Angora breeds are more likely to be D limbatus (the Angora goat biting louse) or D crassipes .

Dogs are occasionally infested with Linognathus setosus (the dog sucking louse), and the biting louse, Heterodoxus spiniger . Animals in poor health may become heavily infested. H spiniger , which may be quite rare in North America, serves as an intermediate host of the tapeworm Dipylidium 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.

Dog sucking louse, Linognathus setosus, female

Dog sucking louse, Linognathus setosus, male

Clinical Findings and Diagnosis:

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, particularly of 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.

Treatment:

Louse control requires treatment with an effective insecticide or drug (see ectoparasiticides, Ectoparasiticides, and anthelmintics, Anthelmintics: Introduction). Products that may be used are determined by government regulations, and users are required to read and follow product labels. Formulations classified for restricted use may be purchased and used only by certified applicators or by persons under their direct supervision. 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, coumaphos, tetrachlorvinphos plus dichlorvos, and amitraz. Nonlactating goats may be sprayed with several compounds, including fenvalerate, malathion, or methoxychlor. Beef cattle, sheep, and swine may be sprayed with coumaphos, malathion, methoxychlor, or permethrin. Tetrachlorvinphos, lindane, phosmet, and amitraz spray may be used on beef cattle and swine. Both swine and sheep may be sprayed with fenvalerate. A low-volume spray of fenvalerate is approved for sheep and nonlactating goats. Permethrin synergized with piperonyl butoxide, tetrachlorvinphos, and tetrachlorvinphos plus dichlorvos may be applied to both dairy and beef cattle. Diazinon spray is approved for control of lice on sheep. Horses may be treated with a permethrin spray, and horses not intended for slaughter may be sprayed with coumaphos or malathion.

Many, but not all compounds approved for beef cattle are approved for nonlactating dairy cattle. In lactating dairy cattle, appropriate milk withdrawal times must be observed.

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, and nonlactating goats may be treated with pour-on formulations of permethrin for louse control. A wipe-on formulation of permethrin is 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 the formulation is approved for the animals being treated. A permethrin pour-on that is synergized with piperonyl butoxide is also available for lice control on beef cattle, lactating and nonlactating dairy cattle, and sheep. Fenvalerate pour-on is approved for louse control on swine, sheep, and nonlactating goats. Cyfluthrin pour-on is approved for beef cattle and lactating and nonlactating dairy cattle, but l-cyhalothrin is approved only for beef cattle. Fenthion is approved as a pour-on for beef cattle and nonlactating dairy cattle, and amitraz pour-on is approved for swine.

Several systemic insecticides are available as pour-on formulations for control of cattle lice as well as a variety of other parasites. Because these products also control cattle grubs, precautions should be taken to avoid host-parasite reactions (see cattle grubs, Cattle Grubs: Introduction). Pour-on formulations of doramectin, eprinomectin, famphur, fenthion, ivermectin, and moxidectin are effective against both chewing and sucking lice of beef cattle. Lactating dairy cattle may be treated with eprinomectin, fenthion, and moxidectin pour-on. Doramectin and ivermectin are also formulated as injectables, and ivermectin is available as an oral paste; however, these are less effective against chewing lice than are the typical pour-ons. A paste formulation of famphur is approved for control of both chewing and sucking lice of cattle. Ivermectin, injectable and premix, is effective against the sucking louse of swine.

Lice on beef cattle can be controlled or suppressed by wintertime use of self-treatment devices, eg, back rubbers, dust bags, and insecticide ear tags, that are used for fly control in the summer. Insecticide ear tags containing a variety of active ingredients (eg, organophosphate insecticides, pyrethroid insecticides, piperonyl butoxide) control or aid in the control of biting and sucking cattle lice. Some tags contain a single active ingredient, while others contain a mixture. All of the tags are approved for use on beef cattle; however, not all are approved for lactating dairy cattle. Louse populations also can be reduced by hand-dusting with coumaphos, methoxychlor, tetrachlorvinphos, or permethrin on beef or dairy cattle; malathion or phosmet on beef cattle and swine; permethrin on swine; and malathion on sheep and goats. For severe infestations, dust formulations of permethrin, tetrachlorvinphos, and coumaphos can be used to treat bedding of swine.

Dogs can be treated with dips, washes, sprays, or dusts. Effective compounds include permethrin, pyrethrins, rotenone, methoxychlor, lindane, diazinon, malathion, or coumaphos. Doses of ivermectin high enough to be effective against lice are not recommended in dogs. On cats, only carbaryl, rotenone, or pyrethrins should be used.

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 labels, and all label precautions should be carefully observed.