Red lice in sheep: A cause of poor wool production and quality


Article supplied by Afrivet

Red lice (Bovicola ovis) are a parasitic problem in sheep that can cause damage to wool quality and quantity if left undiagnosed in a herd.

Red lice are approximately 2mm in size, reddish brown and are mostly found at a height of 6mm in the wool of the back and flanks. The louse is of the biting type and feeds on skin cells and secretions. Transfer occurs through direct contact between animals.

The parasites cannot survive long without the host. They prefer temperatures of between 33 and 39°C and can regulate the temperature themselves by moving higher or lower on the wool. The louse is unable to survive and reproduce amid high temperatures. Hence, larger numbers occur during the cooler winter months.

Signs of infestation

The louse causes an immune reaction on the skin of the sheep, causes itching – the sheep bites and scratches itself and rubs against objects. This results in lower wool quality because the wool tends to break, gets pulled out, becomes caked and sometimes discolours. Lumpy lesions form on the skin.

Close contact between sheep is needed for the lice to transfer from one sheep to the next – the louse cannot spread by itself over distances. Aspects to bear in mind can include new sheep that are added to the herd or a sheep that wanders off and comes into contact with other herds. The louse is more common in lambs and can therefore re-infest treated ewes.

The louse moves to the surface of the wool in ideal environmental conditions – a warm, moist, shady environment is optimal, for example sheep kept in a barn. In warm, sunny weather, extremely cold weather and rainy conditions, the louse will hide deep inside the wool, close to the body.

Influence of warmer weather

Shearing of sheep in warmer weather conditions can help to control red lice, as the higher seasonal temperature together with greater exposure to radiation, inhibit their reproduction and survival.

In the past, red lice were not such a common problem, because compulsory controlling the sheep scab mite with dip automatically resulted in control of these lice. However, the problem is on the increase and can persist unnoticed for some time, while production is negatively affected.

Table 1: The effect of red lice on production.

Quality Effect
Fleece quality Decreases by around R49 to R310
Clean fleece weight Decreases by 0,2 to 0,9kg
Yield Decreases by 2,6 to 6%
Wool colour More yellow and dull
Fibre length Moderate decrease
Tensile strength Possible decrease
Skin quality Sometimes deteriorates

As few as 5 000 lice per sheep can result in a decrease of 5% in wool value and 25 000 lice up to 20%.

Good biosecurity

A basic aspect to focus on is good biosecurity through fence maintenance, good shearer and shearing equipment hygiene, as well as keeping new sheep under quarantine.

Annual dipping after shearing and/or dipping to prevent sheep scab will prevent infestations, provided that biosecurity on the farm is applied properly.

Because the louse is not of the sucking type (such as Linognathus sp.), injectable macrocyclic lactones do not control red lice. Immersion, spray or pour-on remedies must be used. Active ingredients that target lice are organophosphates, pyrethroids, macrocyclic lactones and insect growth regulators.

Dip treatment after shearing

Treatment by dipping sheep and goats after shearing will control the condition and prevent animals from being treated when their wool or fleece is long. Dip treatment is more effective post shearing because shearing helps to remove the nits (eggs). Treatment with organophosphate and pyrethroid dips is very effective if performed correctly and repeated within ten to 14 days.

Pour-ons can be used on sheep that cannot be dipped. Because wool penetration and spread of the pour-on is less effective on a long, thick fleece, the use of a high-flow applicator is recommended. Application of pour-ons at least six weeks prior to lambing is recommended to ensure that no lice transfer occurs between the ewe and lamb.

Each sheep must be treated, regardless of whether the dipping or pour-on method is used. If one sheep is skipped, this animal will be the source of infestation of the treated sheep.

Sheep scab mites and any conditions that cause itching in sheep are communicable diseases according to the Animal Diseases Act, 1984 (Act 35 of 1984). Hence, it is important that a veterinarian confirms the diagnosis before animals are treated.

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