THE MERCK VETERINARY MANUAL
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Overview of Lice

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Numerous species of lice parasitize domestic animals. Lice are largely host specific, living on one species or several closely related species. Lice are obligate ectoparasites and depend on the host to complete their life cycle. Recent taxonomic changes have complicated the orders and suborders of lice. In general, lice are divided into two categories: bloodsucking (or sucking) lice (order Anoplura) and chewing (or biting) lice (formerly the order Mallophaga, now composed of three suborders). Bloodsucking lice are parasites of mammals, whereas chewing lice infest both mammals and birds. Lice live within the microenvironment provided by the skin and its hair or feathers, and are transmitted primarily by contact between hosts. All life stages occur on the host, although lice may survive off the host for a period of time. In temperate regions, lice are most abundant during the colder months and often are difficult to find in the summer. Infestations are most often seen on stressed animals, and husbandry and individual health are important in treatment and management of these parasites. (Also see Ectoparasites.)

Lice are wingless, flattened insects, usually 2–4 mm long, although the species infesting animals may range from 1–8 mm long. The claws of the legs are adapted for clinging to and moving among hairs or feathers; the size and shape of the claws tend to be specialized for the average width of a hair shaft of the host species. This characteristic plays an important role in host specificity. The mouthparts also aid in attachment to the host. Chewing lice have ventral chewing mandibles; they feed on epidermal debris, primarily skin scales, sebaceous secretions, and feathers, if applicable. The heads of chewing lice have a blunted appearance, with the head being wider than the thorax. As the name implies, bloodsucking lice have piercing mouthparts that allow them to feed on the blood of their host. In contrast to the heads of chewing lice, those of bloodsucking lice have a pointed appearance, and the heads are narrower than the thorax. When not in use, their mouthpart stylets are retracted within the head. The distinction between chewing lice and bloodsucking lice is important when choosing an effective therapy for treatment of pediculosis.

On mammalian hosts, louse eggs, sometimes called "nits," are glued to hairs near the skin surface and are pale, translucent, and suboval. The three nymphal stages, of increasing size, are smaller than adults but otherwise resemble adults in habits and appearance. Approximately 3–4 wk are required to complete one generation, but this varies by species. Adult lice are visible to the naked eye, but magnification is often required to identify the species.

Pediculosis can result in dermatologic disease, production loss, and occassionally anemia due to blood loss. Additionally, lice may be vectors of more arthropod-borne infections than previously believed. Lice have been shown to transmit viruses, bacteria, fungi, and protozoa. Most of the research has focused on agents transmitted by human lice species that are known to transmit a variety of pathogens to their hosts, including the causative agents of epidemic typhus (Rickettsia prowazekii), louse-borne relapsing fever (Borrelia recurrentis), and trench fever (Bartonella recurrentis), among others. Until recently, relatively few diseases were known to be vectored by domestic animal lice. A number of agents are now known or suspected to be vectored by lice.

Pediculosis is manifested 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 is common. In severe infestations, there may be loss of hair and local scarification. Extreme infestation with bloodsucking 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 may cause restless behavior 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 under good lighting conditions. 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 more 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. Diagnosis may be aided by use of a magnification device. Practitioners may find an otoscope, without the otoscopy cone, useful for this purpose.

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.

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.

Successful louse control is a multifactorial process. Factors to address include treatment of the affected animal(s), treatment of contact animals, environmental control, and properly addressing the stressors that either permitted initial infestation or exacerbated infestation. Effective treatment results in prompt improvement of clinical signs. Specific pediculosis treatments are discussed below.

It is the practitioner's duty to recommend a safe and effective treatment regimen. In the USA, ectoparasiticides are regulated by the Environmental Protection Agency (EPA) or the FDA Center for Veterinary Medicine. As a general rule, if a product is applied topically to an animal to treat ectoparasites, and the compound is not absorbed systemically, it likely falls under the jurisdiction of the EPA. If a product is administered parenterally, or if it is applied topically for systemic absorption, it likely falls under the jurisdiction of the FDA. This distinction is important for practitioners to recognize, because there is no legal extra-label use of products regulated by the EPA. Label directions must be followed regarding species treated, product concentration, product dosage, individuals allowed to administer application, and re-treatment interval. Because products regulated by the FDA are approved animal drugs, extra-label use may be allowed under the Animal Medicinal and Drug Use and Clarification Act (AMDUCA) (see Ectoparasiticides and see Anthelmintics). The Food Animal Residue Avoidance Databank (FARAD) can be consulted for extra-label drug use recommendations and calculated meat and milk withdrawal times.

Treatment of individual animals may be aided by debulking the infested coat if weather and coat type permit. Clipping an infested animal's long, heavily soiled, or matted coat can immediately reduce the parasite burden on an affected animal, allow topical products to be distributed evenly, and allow for treatment of secondary infections if present. Many compounds effectively kill adult and nymphal life stages, but few have been tested for ovicidal capabilities. Therefore, many sources recommend re-treatment at intervals of 7–10 days until the infestation has been controlled. Contact animals should also be treated to prevent spread of infestation within a herd, flock, or household. Animal equipment or bedding should be washed frequently with hot, soapy water until infestation is controlled. Finally, addressing animal overcrowding, poor feed quality, and underlying health issues are the final steps to manage pediculosis and prevent recurrence. New animals should be quarantined and inspected for infestation before herd or flock integration.

Last full review/revision May 2015 by Jennifer E. Thomas, DVM

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