Lice on Horses

Lice on horses in Ontario are not common but questions arise about treatment. Both sucking and biting lice can affect horses. The chewing louse, Bovicola equi, feeds off tissue and cells sloughed from the surface of the skin. The sucking louse of horses, Haematopinus asini, feeds off blood. The species of lice is important since it is easier to kill sucking lice with a systemic insecticide than it is to kill biting lice. Lice cause intense itching (pruritus) around the base of the tail, head and mane. Horses will rub themselves raw.

Lice are normally species-specific. However, the chewing lice of poultry can also affect horses when housed together. The horses should be removed from the building. If the poultry is removed, the lice will continue to harass the horses unless a good cleanup and premise insecticide treatment is used.

Lice and their Life Cycle:

Lice undergo a simple life cycle. They transform from egg to nymph to adult with the entire cycle being completed on the host. The chewing louse is about 1/10 of an inch long, chestnut brown in color, with a yellow abdomen with dark cross bands. They are flat with a broad, rounded head and slender legs. The eggs are attached to the hair and hatch in 5 to 10 days. Nymphs immediately start feeding and mature in 3 to 4 weeks.

The sucking louse is about 1/8 of an inch long and a dirty grey color. They have a broad abdomen which contrasts with their long narrow head. Sucking lice are more common and more irritating than chewing lice. They have piercing mouth parts. When present in large numbers, they can cause anemia. The eggs are attached to the hair and hatch 11 to 20 days later. Nymphs begin sucking blood immediately. They complete their life cycle in 2 to 4 weeks.

Control and Treatment:

Transmission of lice is by direct contact between horses and through contact with infected brushes, blankets and tack. Successful control requires that equipment is cleaned thoroughly with the same insecticide as used on the horses. The cleaning should be repeated in two weeks. Sterilization of equipment by boiling will also kill the lice, nymphs and eggs as will placing blankets and coolers in the drier at the highest heat setting.

The only approved products for the treatment of horses with lice are the powder products, Dusting Powder and Sevin. All other products are used in an off-label, or extra-label, manner and the veterinarian is required to re-label the product, providing directions for use, e.g. the route of administration and frequency as well as human and animal precautions associated with the product.

• Powders should be used by dusting the entire animal and ensuring that the dust reaches the skin. It is difficult to get the powder down to the skin when animals have a long winter coat. Wetable powders are intended to be mixed with water and applied to improve contact with the skin. Powders may contain any of several chemicals including: rotenone, coumaphos, carbaryl, and fenthion.

• Systemic treatments such as the avermectins are not approved for treatment of lice on horses. Systemic products usually come as a pour-on and an injectable. Care must be taken before contemplating the use of a pour-on form, since the carrier base (e.g. alcohol) may cause a local skin reaction. The injectable form may possibly be used orally for sucking lice treatment but no research has been reported (1).

• Pour-on Products

Permethrins can be used as non-systemic, pour-on, insecticides to control lice. A 4% solution is recommended when treating donkeys with lice (2). A 1% solution was very effective in controlling lice in cattle and a 0.1% solution of permethrin was effective against chorioptic mange in cattle. It may be possible to use a 0.1% solution of permethrin for biting lice on horses and reduce the potential for skin reactions which is sometimes seen with a 1% permethrin solution (3). The permethrin solution should be applied along the back and down the face. Two treatments, 14 days apart, are recommended for the optimum control of lice.

The following are products containing permethrin with their concentration of permethrin: Coopers Delice Pour-on - 1%, Disvap Equine 0.1%, Nix (a human product) - 1%, Siecon - 0.5%, and Vetolice - 1%. These can be diluted, as necessary, to get a 0.1% solution (3). Some of these products have an oil-based carrier and will create a mess if the horse rolls in a sandy arena. Siecon, and Disvap-Equine are labeled for use for flies on horses.

• Shampoos

Seleen shampoo, Sanofi Animal Health, has been reported as being successful in treating lice on horses. Seleen is approved for the treatment of seborrhea in small animals. The 1% selenium sulphide has antiparasitic action. Patterson reported that whole-body bathing of horses with Seleen three times at 10 day intervals was successful in treating lice. Treatment consisted of using the following amounts of Seleen: for ponies - 150 ml; horses up to 500 kg. body weight - 300 ml; and horses greater than 500 kg. body weight - 450 ml (4).

Choice of treatment will vary depending on time of year, ambient temperature and the number of horses being treated. Your veterinarian will provide you with the best treatment option.

References:

1. Knight P., personal communication, Pfizer Animal Health
2. Svendsen E. D., The Professional Handbook of the Donkey, 3rd Edition, Whittet Books
3. Heal J., personal communication, Environmental Biology, University of Guelph
4. Paterson S., Orrel S., Treatment of Biting Lice in Horses Using Selenium Sulphide, Eq. Vet. Educ. 1995; 11:11-28.

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