Lice: Chewing: Werneckiella (Damalinia) equi; Sucking: Haematopinus asini

Horses around the world, including in Canada, are infested with sucking (Haematopinus asini) and/or chewing (Wernekiella (Damalinia) equi) lice. The life cycle occurs entirely on the host.  Transmission is by direct horse to horse contact, and less commonly via the environment or inanimate objects (fomites), for example grooming equipment.  The major problem with lice is irritation and the attempts by the horse to relieve this.  As with other hosts, lice burdens on horses tend to be larger during the winter than at other times of the year.

Horse lice are not known to be zoonotic.

Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda
Subclass: Pterygota
Order: Mallophaga (Chewing Lice)
Order: Anoplura (Sucking Lice)

Other arthropods of veterinary importance within the Class Hexapoda are bugs (Order Hemiptera), fleas (Order Siphonaptera) and flies (Order Diptera). The chewing louse of horses is Werneckiella (Damalinia) equi, and the sucking louse is Haematopinus asini. Lice are very host specific, and although they may sometimes be found – but only rarely established - on other than their normal hosts.

Larval and adult lice are dorso-ventrally flattened and have six legs arising from the thorax. The legs terminate in claws. Adult W. equi and H. asini are up to approximately 5 mm in length.

In common with other chewing lice, the head of adult W. equi is wider than the thorax, is blunt anteriorly, and has two prominent, segmented antennae, one on each side.

In common with other sucking lice, the head of H. asini is narrower than the thorax, is pointed anteriorly, and has two prominent, segmented antennae, one on each side.

The eggs (nits) of W. equi and H. asini are whitish in colour, measure up to approximately 750 µm by 300 µm, and are fixed to the hairs. Louse eggs are readily visible to the naked eye, particularly if present in large numbers.

Chewing and sucking lice are found on horses and other equids around the world. In Canada, lice infestations are more prevalent and more intense in the winter than at other times of the year.

Apparently, some species of chewing lice of chickens can infest horses if the two hosts are housed together.

For W. equi and H. asini, like other lice, the entire life cycle occurs on the host. Adult lice probably cannot survive off the host for more than a few days. Adult females lay eggs (nits) attached to hairs. A larval stage develops in each egg, which hatches. There are three larval stages before the adult. All the larval stages and the adults feed, chewing lice primarily on skin debris, sucking lice on blood. The life cycle is completed in a few weeks.

Life Cycle: Werneckiella equi and Haematopinus asini

Infestation with lice is more common in horses than is clinical disease, meaning that there are asymptomatic carriers. Transmission of lice is usually direct horse-to-horse, although grooming equipment and tack may also transmit the parasites. An infested horse in a group usually means that all the animals in the group are infested.

Werneckiella equi tends to be found on the dorso-lateral trunk, while H. asini prefers the mane, tail and fetlocks. Both chewing and sucking lice cause pruritus. Horses that are otherwise stressed may show greater effects from lice than horses that are otherwise healthy. Problems with lice in horses are more common in the winter than at other times of the year.

Clinically, attempts by the horses to relieve the irritation by rubbing and biting leads to hair loss and damage to the skin. Eventually, the horse may look "moth-eaten" and the constant efforts at relief may interfere with feeding and result in ill-thrift. The loss of haircoat may also reduce the infested animal’s insulation, potentially a major problem in Canadian winters. The blood loss associated with sucking lice may, in heavy infestations, cause a clinically apparent anemia.

Recovery of adult lice (which can easily be identified to species) or eggs is the best method. Nits can be differentiated from Gasterophilus (bot) eggs on the basis of their location on the hair (lice eggs tend to be towards the base of the hair, bot eggs towards the tip), and the absence of a dipteran larva inside the egg.

There are several topical products containing carbaryl, pyrethrins, permethrin, or combinations of pyrethrins and permethrins that are approved in Canada for louse treatment in horses. The use of these products in Canada is governed by the Pest Control Products Act.

Treatments need to be repeated, ideally three times at 10-day intervals, because the lice eggs, but not the hatched immature stages, are resistant to the treatments. There are no good data on the efficacy of ivermectin or moxidectin for lice on horses.

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of lice in horses depends on rapid detection and treatment of infested animals, with particular attention to new arrivals in a group, and on appropriate cleaning of any potential fomites, for example tack.

Lice of horses are host-specific and are not commonly zoonotic, although horse lice may occasionally be found on people.