Lice in Sheep

Introduction

Chewing or biting lice such as Bovicola ovis are small, parasitic insects which live and feed on the skin surface and fleece of sheep. In small numbers they are clinically insignificant, and are principally of concern due to the similarities of clinical signs with the early stages of sheep scab, although occasional heavy louse infestations of lice may be of concern in their own right and often indicate an underlying health or management problem.

Cause

Bovicola ovis lice live exclusively on sheep and are mainly spread by close contact between animals for example crowded housing, at feed troughs and during gathering and handling. Their lifecycle takes 4-5 weeks to complete, meaning infestations develop over several months, usually peaking in mid/late winter.

Figure 1: Lice spread readily between housed sheep in winter

Welfare and Economics

At low levels, lice are considered a coincidental finding. Light infestations are generally well tolerated, with little to no impact on welfare, productivity and economics. Occasionally, heavy louse infestations cause profound irritation and itching which compromises welfare, damages the fleece and can reduce feed intakes and growth rates. Severe irritation and rubbing behaviour may result in skin damage and secondary infections leading to marked production costs. Heavy louse infestations are usually associated with poor underlying nutrition, husbandry and/ or flock health and should therefore prompt further investigation to identify the underlying cause(s).

Figure 2: Lice commonly affect sheep in poor condition or with underlying health problems

Clinical signs and Diagnosis

Louse infestations tend to intensify during the winter period when sheep are housed or in closer contact, and are generally more of a problem in fully fleeced animals.

Affected animals may be mildly to intensely itchy, rubbing and scratching themselves on fenceposts and other such objects in the environment. This can lead to damaged fleece and even skin damage where irritation is particularly intense. Heavy infestations lead to disrupted feeding patterns, reduced weight gain and/or poor body condition. These signs are indistinguishable from those caused by sheep scab, particularly in its early stages, making it of vital importance that a veterinary diagnosis is sought. Adult Bovicola ovis are red/brown in colour and measure 2-3mm in length. These are easily visible in wool samples taken from affected areas of fleece. However, it is not uncommon for sheep to have both louse and scab infestations, meaning sheep scab should not be excluded as a possible cause of itching based on the observation of lice in wool samples alone.

 The severity of louse infestations appears to depend on a range of factors including breed, fleece length, overall flock health, temperature and humidity. Lice do not like very wet or hot weather and populations tend to reduce after periods of heavy rain and in summer.

Figure 3: It is impossible to determine if itchy sheep have lice, sheep scab or both without seeking a veterinary diagnosis

Prevention

Clinical effects of lice infestation are less frequent in well fed, healthy sheep in low stocked housing with good ventilation. Lice can indicate other underlying problems which should be addressed. Thick fleece protects lice from unfavourable conditions. Simply shearing animals can help reduce burdens by 30-50% and also effectively reduces stocking density. Insecticides should not be required routinely to treat lice in housed sheep over winter, but may be indicated in heavy infestations. Treatments for other ectoparasites such as diazinon dips to treat scab and topical synthetic pyrethroid products to control flystrike over the summer will also effectively treat for lice.

Treatment

Diazinon dips and a number of topical synthetic pyrethroid products carry a licence of efficacy against lice, although it should be noted that duration of protection and efficacy varies between products, meaning it is essential to check the datasheet and administer following the manufacturer’s recommendations and observe stated withdrawal periods.

Plunge dipping using an organophosphate preparation for other reasons, such as control of sheep scab, blowfly strike and headfly problems also effectively controls louse infestations. Louse infestations can also be controlled with topical application of synthetic pyrethroids, which should ideally be applied following shearing. Application to animals in full fleece will be less effective and may result in resistance developing. Injectable group 3-ML products used to treat sheep scab and roundworms are not considered effective treatments for chewing lice and do not carry a licence for this purpose in sheep.

Figure 4: Veterinary advice should always be sought to investigate the causes of itching or wool loss in sheep.