Most lice are host-specific, obligate ectoparasites that depend on the host to complete their life cycle. In general, they 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; chewing lice infest both mammals and birds.
Lice live within the microenvironment provided by a host's skin and its hair or feathers, and they are transmitted primarily by contact between hosts. The predilection site depends on the lice species. All life stages occur on the host, but 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 most often occur on stressed animals, and husbandry and individual health are important in the treatment and management of these parasites. (Also see Ectoparasites Ectoparasites .) Lice infestations can lead to secondary dermal infections, and lice can transmit diseases and be intermediate hosts for other parasites.
Etiology and Pathogenesis of Lice in Animals
Lice are wingless, flattened insects, ranging from 1 to 8 mm long and visible to the naked eye; however, magnification is often required to identify the species. 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; in habits and appearance, however, they resemble adults. Nymphs and adults use their claws and mouthparts to attach to the host.
Completion of one generation takes ~3–4 weeks. Usually, lice are transmitted by direct host contact. Lice dropped or pulled from the host die in a few days, and few, if any, nits develop after being removed from the host. However, fomites can be involved in transmission; therefore, the bedding of small animals and the premises of large animals that have been recently vacated by infested animals should be disinfected before being used again for uninfested animals.
Approaches to treating lice differ depending on whether the lice are chewing or sucking lice. Head shape, size and shape of claws on the legs, predilection site, and host are useful characteristics for identification.
infest mammals and birds
have a head with a blunted appearance that is wider than the thorax
have ventral chewing mandibles
feed on epidermal debris, primarily skin scales, sebaceous secretions, and feathers, if applicable
Are active and can sometimes be observed moving through the hair
infest only mammals
have a head with a pointed appearance and narrower than the thorax
have piercing mouthparts
feed on blood
move slowly and are often found with mouthparts embedded in the skin
Pediculosis can result in dermatologic disease, production loss, and occasionally anemia due to blood loss. In addition, lice may be vectors of viruses, bacteria, fungi, and protozoa. Some lice species also serve as intermediate hosts of other parasites, such as cestodes.
Clinical Findings and Diagnosis of Lice in Animals
Presence of lice and/or nits
Pediculosis is manifested by pruritus and dermal irritation, resulting in 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 lead to anemia; in rare cases, extreme infestation with chewing lice on dogs also can result in anemia. The constant crawling and piercing or biting of the skin may lead to restless behavior in hosts, and the wounds resulting from feeding by lice and rubbing by hosts can become infected.
Infestations can be more common in the winter in production animals and more common in stressed animals and in animals in a poor system of management. Crowding (eg, in the winter housing of production animals, or among caged and indoor free-roaming birds) also can increase lice infestations.
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. 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 nits are more readily visible. Occasionally, when the coat is matted, lice become evident when the mass is broken apart. Diagnosis may be aided by use of a magnification device. An otoscope, without the otoscopy cone, may be useful for this purpose.
Treatment of Lice in Animals
Extermination of adult and nymphal life stages
Prevention of transmission within a herd, flock, or household
Washing of animal equipment and bedding to prevent transmission via fomites
Modification of animal management to avoid overcrowding, poor nutrition, and stress
Successful louse control requires addressing multiple factors, including treatment of the affected animal(s), treatment of contact animals, environmental control, and elimination of stressors that either permitted initial infestation or exacerbated infestation. Effective treatment results in prompt improvement of clinical signs.
It is the veterinarian's duty to recommend a safe and effective treatment regimen. In the US, ectoparasiticides are regulated by the 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 veterinarians 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 retreatment interval. Because products regulated by the FDA are approved animal drugs, extra-label use may be allowed under the Animal Medicinal Drug Use and Clarification Act, AMDUCA (see Ectoparasiticides Ectoparasiticides and Anthelmintics Anthelmintics ). The Food Animal Residue Avoidance Databank (FARAD) can be consulted for extra-label drug use recommendations and calculated meat and milk withdrawal times.
Many systemic and nonsystemic products are registered for the treatment of lice infestations, including oral, topical and injectable formulations. Compounds that are effective include pyrethrins, pyrethroids, organophosphates, macrocyclic lactones, fipronil, and imidacloprid. Other compounds, such as spinosad and fluralaner, also are effective; however, they are not registered for this use. Compounds with persistent efficacy can kill nymphs as they develop; other treatments might require administration at 7- to 10-day intervals until the infestation is controlled.
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 decrease the parasite burden on an affected animal, allow topical products to be distributed evenly, and enable treatment of secondary infections, if present.
Contact animals should also be treated to prevent transmission of the infestation within a herd, flock, or household. Animal equipment or bedding should be washed frequently with hot, soapy water until the infestation is controlled. Finally, addressing animal overcrowding, improving feed quality, and treating underlying health problems are the final steps to manage pediculosis and prevent recurrence. New animals should be quarantined and inspected for infestation before herd or flock integration.
Lice are host-specific ectoparasites.
A poor coat, pruritus, dermal irritation, and, in heavy infestations, anemia, are common clinical signs.
Many effective treatments are available; however, they can differ for chewing and sucking lice.
Control and prevention must also address underlying conditions and management methods that lead to overcrowding and poor nutrition.
For More Information
Companion Animal Parasite Council: Lice
American Association of Equine Practitioners: Lice
Holdsworth P, Rehbein S, Jonsson NN, Peter R, Vercruysse J, Fourie J. World Association for the Advancement of Veterinary Parasitology (WAAVP) second edition: guideline for evaluating the efficacy of parasiticides against ectoparasites of ruminants. Vet Parasitol. 2022;302:109613. doi:10.1016/j.vetpar.2021.109613
Durden LA. Lice (Phthiraptera). In: Mullen GR, Durden LA, eds. Medical and Veterinary Entomology. 3rd ed. Academic Press; 2019:79-108.