Pediculosis

Infestation by lice, a condition called pediculosis, is uncommon in dogs and cats and predominantly affects debilitated and poorly nourished animals as well as those in precarious conditions of hygiene.

MORPHOLOGY AND BIOLOGICAL CYCLE OF THE PARASITE

Lice are insects without wings (Order Phthiraptera = louse without wings); they are obligate parasites with marked species specificity.  The lice that infest dogs and cats belong to two suborders: Anoplura, sucking lice, and Ischnocera, biting or chewing lice (previously known as Mallophaga). The former have mouthparts suitable for sucking blood, whereas the second have mouthparts adapted to biting and chewing and are not, therefore, haematophages, but feed on epidermal debris and hairs.

The biological life cycle is carried out entirely on the host (permanent parasitism) on which the female lays operculated eggs (nits) which, through the effect of cement-like substances, adhere firmly to the hair shaft. The egg hatches a nymph, a miniature version of the adult (incomplete metamorphosis), which moults three times before becoming an adult louse. The whole cycle usually takes 2-3 weeks and the female can lay as many as 200-300 eggs during her life, which lasts about 1 month.

Transmission occurs almost exclusively by direct contact because lice only abandon a host to move onto another animal and are not, therefore, able to survive for long in the external environment. In areas with a temperate climate seasonal fluctuations in infestations have been described, with increased infestation in the winter period, probably facilitated by characteristics of the host’s coat. Lice have a dorsoventrally flattened body and are divided into a head, thorax and abdomen with three pairs of limbs and one pair of antennae.

Canine lice

Trichodectes canis
Trichodectes canisis a biting louse (suborder Ischnocera). It is about 1-2 mm long and has a rectangular-shaped head that is wider than its body, two antennae formed of three segments and a well-developed buccal apparatus that enables the parasite to hook firmly to a hair shaft. The body is covered by bristles and has three pairs of very short limbs, which terminate with a single claw (Fig. 1). Trichodectes canis is a possible intermediate host of Dipylidium caninum.

Linognathus setosus
This is a large, sucking louse (suborder Anoplura) with mouthparts adapting for sucking blood (haematophagous); it can reach 2 mm long after a blood meal. The head is elongated, conical, narrower than the thorax and has antennae formed of five segments. The limbs are short and thin and terminate with a single, robust claw (Fig. 2). Immature stages of Dipetalonema reconditum have been found in Linognathus setosus although this insect species is not known to be vector for the nematode.

Feline lice

Felicola subrostratus
Felicola subrostratus is a biting louse (suborder Ischnocera) that is 1-2 mm long and has a pentagonal shaped head that is larger than the thorax, terminating with a pointed tip with a longitudinal sulcus in which a hair can be lodged. The antennae are formed of three segments. The buccal apparatus is well-developed and enables the parasite to remain hooked to the hair. The limbs are short and terminate with a single claw (Fig. 3). Felicola subrostratus is the only louse that infests the cat.

CLINICAL SIGNS

Lice infest all parts of the body, but tend to collect predominantly on the head, neck and dorsal lumbar region. The clinical lesions that can be found on the animal vary depending on the number of parasites and the intensity of the pruritus, which is extremely variable (from absent to moderate). Some animals are asymptomatic and the lice can be observed moving on the hair shafts (suborder Ischnocera) or on the skin of the dog (suborder Anoplura) (Figs. 4, 5 and 6). However, often only the eggs (nits) can be seen attached to the hair shafts; at macroscopic observation from a distance, these nits may be confused with scales, but are easily distinguished at close-up inspection by their oval outline and whitish colour (Figs. 7, 8 and 9). Other infested animals may have primary lesions (papules, scales) or secondary non-specific lesions due to self-inflicted trauma (alopecia, excoriations, scabs) (Fig. 10). Possible signs in cats are self-induced symmetrical alopecia and miliary dermatitis. In dogs, severe infestations by Linognathus setosus can sometimes be responsible for profound anaemia, particularly in puppies.

DIAGNOSIS

Lice and their eggs can be easily seen by direct inspection or with a magnifying glass. Microscopic examination of hairs or transparent sticking tape, to which the parasites and nits adhere, enables them to be identified by optical microscopy. Another diagnostic method is to brush the host’s hair, which gives a collection of the parasites and their eggs on the examination table (Fig. 11).

In cases in which no adult parasites are found, but only nits, these must be differentiated from the eggs of Cheyletiella spp., mites which, like lice, lay eggs on the hair shaft. The eggs of lice are much larger than those of Cheyletiella and have a covering on the dorsal part; furthermore, about two-thirds of the length of nits are firmly cemented to the hair, whereas the eggs of Cheyletiella are attached to the hair by thin, interwoven fibrils (Fig.12).

DIFFERENTIAL DIAGNOSIS

• Other ectoparasitic infestations
• Allergies
• Disorders of keratinisation

TREATMENT

Lice are sensitive to most insecticides on the market. The compounds currently registered in Italy for the treatment of canine pediculosis due to Trichodectes canis include permethrin (shampoo, spot-on and spray), fipronil (spot-on and spray), selamectin (spot-on), the combination of  imidacloprid - moxidectin (spot-on) and the combination of metaflumizone – amitraz (spot-on), while the treatments for feline pediculosis are fipronil (spot-on and spray) and selamectin (spot-on). A single administration is recommended for all the compounds but, given the resistance of nits to most insecticides, it is wise to repeat the treatment 14 days after the first administration to kill the lice that have hatched from the nits in the time following the first treatment.

Other compounds that are effective, but that are not registered for use in dogs and cats are ivermectin (0.2 mg/kg subcutaneously in a single administration) and imidacloprid, which can be used for infestations by both Trichodectes canis and Linognathus setosus. The treatment should always be extended to all animals of the same species in contact with the infested ones.

Suggested readings

1. Albanese F, Leone F. Pediculosi. In Manuale Pratico di Parassitologia Cutanea del Cane e del Gatto. Ed. Pfizer 2007; 79-97
2. Genchi M, Traldi G, Genchi C. Pidocchi. In Manuale di Parassitologia Veterinaria. Casa Editrice Ambrosiana, Milano 2010; 142-5
3. Ghubash R. Parasitic miticidal therapy. Clin Tech Small Anim Pract 2006; 21(3): 135-44.
4. Guaguére E, Beugnet F. Pediculosi. In Guaguère E, Prélaud P: Guida Pratica di Dermatologia Canina ed Merial, Milano, 2006; 207-8
5. Gunnarsson L, Christensson D, Palmér E: Clinical efficacy of selamectin in treatment of naturally acquired infection of sucking lice (Linognathus setosus) in dogs. J Am Anim Hosp Assoc 2005, 41:388-94
6. Hanssen I et al. Field study on the insecticidal efficacy of Advantage against natural infestations of dogs with lice. Parisitol Res 1999; 85(4):347-8
7. Littlewood JD. Treatment of scabies, trombiculidiasis, cheyletiellosis and pediculosis. In Proceedings 22^nd Annual Congress of the ESVD-ECVD, Mainz 2007; 79-83
8. Page SW. Antiparasitic drugs. In Maddison JE, Page SW, Churh DB. Small Animal Clinical Pharmacology. second edition, Saunders Elsevier 2008; 198-260
9. Pollmeier M et al.: Effective treatment and control of biting lice, Felicola subrostratus (Nitzsch in Burmeistrer, 1838) on cats using fipronil formulations.Vet Parasitol. 2004, 121:157-65
10. Pollmeier M et al.: Evaluation of the efficacy of fipronil formulations in the treatment and control of biting lice, Trichodectes canis (De Geer, 1778) on dogs.Vet Parasitol. 2002, 107:127-36
11. Scott DW, Miller WH, Griffin CE. Pediculosis. In: Muller and Kirk’s Small Animal Dermatology 6^th edn. Philadelphia: W.B. Saunders 2001; 487-90
12. Shanks et al.: Efficacy of selamectin against biting lice on dogs and cats. Vet Rec 2003, 152:234-7
13. Shastri UV. Efficacy of ivermectin against lice infestation in cattle, buffaloes, goats and dog. Indian Vet J 1991; 68:191
14. TaylorMA, Coop RL, Wall RL. Pidocchi. In Parassitologia e Malattie Parassitarie degli Animali. EMSI, Roma 2010; 442-4
15. Wall R, Shearer D. Lice. In Veterinary Ectoparasites: biology, pathology & control. Second edition, Blackwell Science, Oxford 2001; 162-78