Lice

Description

• Lice are dorsoventrally flattened wingless insects (i.e., with three leg pairs and a distinct head) and are small (about the same size as fleas). Lice are highly host-specific, so differentiations of head morphology, host species, and sometimes location on the host are usually sufficient to identify lice for diagnostic purposes.
• There are two major groups of lice with distinct head morphologies reflective of differences in feeding habits and vector competence.
  • Anoplura (sucking lice) have narrow heads because their mouthparts are adapted for sucking blood or fluids; sucking lice also have more developed claws for grasping fibers (such as hair).
    • Head is narrower than the thorax
  • Mallophaga (chewing lice) grasp host hair (or feathers) with their mouthparts; these “chewing” lice have large mandibles that result in wide heads.
    • Head is wider than the thorax

Species

Linognathus setosus (canine sucking louse)

Trichodectes canis (canine chewing louse)


Felicola subrostratus (feline chewing louse)


• Linognathus setosus is a sucking louse occasionally found on dogs (particularly on long-haired breeds) and wild canids throughout the world. It causes little irritation except in heavy infestations, in which the lice can cause anemia due to blood loss. It does not occur on kennel-reared dogs except under conditions that likely are unsatisfactory in many other ways.
• Trichodectes canis is a chewing louse found on dogs and wild canids throughout the world and is a vector of the dog tapeworm, Dipylidium caninum. Trichodectes canis causes little irritation except in heavy infestations, where it induces a moderate skin inflammatory reaction. It does not occur on kennel-reared dogs except under conditions that likely are unsatisfactory in many other ways.
• Felicola subrostratus, a chewing louse of cats, is usually found in large numbers only on aged or diseased cats and on some wild felids. It is probably cosmopolitan in location. Infestations often go unnoticed.

Linognathus setosus 50X

Trichodectes canis 50X

Felicola subrostratus 50X

Overview of Life Cycle

• A female lays several eggs daily throughout her life, which is about  30 – 45 days.
• Eggs (also known as “nits”) are cemented near the base of the hair and hatch in 1 to 2 weeks.
• Lice undergo simple metamorphosis. Once the egg has hatched, the nymphal louse, which resembles a small adult louse, experiences two additional molts until becoming an adult. This process takes 2 to 3 weeks.
• The chewing lice, T. canis and F. subrostratus, feed on tissue debris; the sucking louse, L. setosus, feeds on blood.
• Lice can survive only 3 to 7 days if separated from the host.

Felicola subrostratus nit 100X

Stages

• Adults (1 to 2.5 mm in length) are yellowish to tan with dark markings.
• The head of a chewing louse is wider than the thoraxes due to mouthparts adapted for grasping hair shafts of the host.
• The head of T. canis has thick, short antennae consisting of 3 segments.
• The head of F. subrostratus also has a characteristic triangular anterior end.
• In the canine sucking louse (L. setosus), the head is narrower than the thorax due to needle-like mouthparts adapted for sucking blood and tissue fluids. The antennae are short, stout, three-segmented, and fully exposed. Each leg has only one claw. The abdomen is oval and has six pairs of spiracles on the lateral edges of each abdominal segment. The abdomen may also appear a variety of shades of gray to red depending on the amount of blood that has been consumed.
• Eggs (nits) are operculate and glued to the hair shafts.

Disease

• Usual signs of louse infestation are irritation and damage caused by the host such as rubbing, scratching, and biting of infested areas. Heavy infestations may cause severe pruritus, restlessness, intense scratching, a ruffled or rough matted coat and sometimes alopecia.
• Chewing lice are attached by their claws or mandibles to the base of a hair, usually on the head, neck, and tail. These lice may be found in larger numbers near body openings or skin abrasions where they are seeking moisture.
• Trichodectes canis is a competent vector for the tapeworm, Dipylidium caninum.
• All stages of sucking lice feed on blood, therefore infestations can lead to anemia, especially in immunosuppressed or young dogs.

Prevalence

• Infestations are more prevalent in very young, old, or debilitated animals or animals maintained in unsanitary environments.
• Chewing lice are fairly common on dogs and cats around the world. Another chewing louse, Heterodoxus spiniger, is found on dogs in tropical areas such as the Philippines.
• Infestations with the sucking louse are most common in the colder climates to which this louse is mainly restricted.
• Infestations are also more common in areas where flea and tick burdens are low, and owners are not consistently using products to control those ectoparasites.

Host Associations and Transmission Between Hosts

• Lice of dogs and cats are fairly restricted parasites. They are not shared between dogs and cats and are not transmitted from pets to people.
• Transmission between hosts is usually through direct contact, but transmission can also occur via eggs on fomites such as brushes, combs, or similar grooming equipment.

Prepatent Period and Environmental Factors

• Adults become mature about three weeks after hatching. Infestations generally are most common when animals are held under unsanitary conditions and during the cooler months.

Site of Infestation and Pathogenesis

• Effects of louse infestation on the hair or skin are usually minimal to nonexistent. The eggs and nits can be seen on the hair and may give it a scruffy appearance.
• Cats can develop heavy infestations resulting in restlessness, pruritus, scratching, a ruffled coat, and sometimes alopecia.

Diagnosis

• Diagnosis is based on clinical signs and on demonstration of the louse or its eggs.

Treatment

• Louse infestations are readily treated with many flea control products. Fipronil, imidacloprid and selamectin are all effective for dogs and cats. Topical permethrin or flumethrin/imidacloprid can be used on dogs with good effect.
• In the isoxazoline class, afoxolaner has been used to treat T. canis, and fluralaner was effective against L. setosus, suggesting this drug class is also a potential treatment option for lice infestations of dogs and cats.
• Treated animals should be placed in a clean, disinfected cage or living space. Treatment may be repeated in a week to ensure that any nymphs hatching from eggs are also killed.
• The eggs and other stages on fomites will die gradually over time due to desiccation, which can be accelerated by several hours under hot dry conditions such as in a clothes drier.

Control and Prevention

• Newly acquired dogs should be examined and treated if infested. Infested pets should be quarantined and treated before coming into contact with other pets. Nits are not susceptible to treatment, so treatment may be repeated in a week to kill any recently hatched nymphs before they mature into egg-producing adults.
• Monthly treatment with fipronil, imidacloprid, flumethrin/imidacloprid, or selamectin for control of fleas will also control lice. The use of monthly isoxazolines may also reduce the risk of infestations.

Public Health Considerations

• These parasites pose virtually no public health risk, because canine and feline lice do not infest people and human lice do not infest pets.
• Rarely, T. canis has been reported as a source of cysticercoids of D. caninum that could infect humans or dogs and cats that happen to ingest infected lice.

Selected References

• Akucewich LH, Philman K, Clark A, Gillespie J, Kunkle G, Nicklin CF, Greiner EC. 2002. Prevalence of ectoparasites in a population of feral cats from north central Florida during the summer. Veterinary Parasitology. 109: 129-139.
• Arther RG. 2009. Mites and Lice: biology and control. Vet Clin Small Anim. 39: 1159-1171.
• Endris RG, Reuter VE, Nelson J, Nelson JA. 2001. Efficacy of a Topical Spot-on Containing 65% Permethrin Against the Dog Louse, Trichodectes canis (Mallophaga:Trichodectidae). Veterinary Therapeutics. 2: 135-139.
• Kohler-Aanesen H, Saari S, Armstrong R, Péré K, Taenzler J, Zschiesche E. 2017. Efficacy of fluralaner (Bravecto™ chewable tablets) for the treatment of naturally acquired Linognathus setosus infestations on dogs.
• Mihalca AD, Deak G, Panait LC, Ravi S, Beugnet F. 2022. Efficacy of afoxolaner (NexGard^®) against natural infestations with Trichodectes canis in dogs under field conditions. Parasites & Vectors 15:317.