Online learning: Sheep Lice

There are three species of lice that occur on sheep in Australia.

The sheep body louse (Bovicola ovis, formerly called Damalinia ovis) is a pale-yellow insect 1.5 to 2 mm long with brown transverse stripes on the abdomen and a broad, red-brown head (Figure 1). It is a chewing louse and feeds on skin scurf, lipid and sweat gland secretions, superficial skin cells and skin bacteria (Sinclair et al. 1989). Males are smaller than females and have more pointed abdomens.

Sheep are also host to two other species of lice that are both sucking lice, which feed on blood, have long thin heads and appear bluish in colour:

• the face louse, Linognathus ovillus, occurs mainly on or close to the face
•  the foot louse, Linognathus pedalis, is found on the legs and on the scrotum in rams.

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Sheep lice do not breed on animals other than sheep (with the possible exception of goats, in very rare instances).  Birds do not carry sheep lice and they do not remain in wool rubbed onto fences, trees or other structures, so these are not sources of infestation. Read More

Infestation with sheep lice can reduce clean wool cut by up to 1 kg per head. Lice also reduce yield because the sheep becomes itchy from the irritation and they will scratch or bite at the fleece causing fleeces to become cotted and yellow, resulting in increased losses during processing.

In New Zealand, sheep lice have been shown to cause a defect in sheep leather known as cockle. This manifests as multiple, sometimes discoloured, lumps visible in sheep leather after processing. Infestation with B. ovis does not affect fibre diameter and, contrary to popular belief, does not cause reduction in body weight. Read More

Temperatures (in the wool where the lice are situated) outside very specific ranges will prevent various stages of louse reproduction from occurring.

• Egg development only occurs between 37°C and 42.5°C.
• Egg laying only occurs between 35°C and 40°C.
• Development of eggs to nymphs occurs only at temperatures between 30°C and 39°C and the incubation period is generally between 9 and 11 days. Temperatures of greater than 45°C will rapidly kill eggs.
• Nymph develop through 3 stages over about 21 days only occurs at temperatures between 35°C and 39.5°C.

A female louse will cement individual eggs to wool fibres, most within 12 mm of the skin; she will lay 1-2 eggs every 3 days and these will hatch 9 or 11 days later. The entire life cycle from egg to egg takes about 34 to 36 days, which includes three nymph (instar) stages.  Read More

Lice move to the surface of the fleece when it is shaded and warm. Transfer between animals occurs when sheep are in close contact, such as when they are shedded, held together in yards, or perhaps when feeding or drinking from a trough. Transfer is fastest:

• When sheep have short wool.
• With management practices, such as hand-feeding and working sheep through yards, which increase the amount of close contact between sheep.
• When sheep are held tightly together in sheds or yards that are shaded.
• When ambient temperatures are moderate, neither cool nor very hot.

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Lice can be found on most woolled areas of sheep, although they are rare on the belly and don’t appear to breed there. They are not evenly spread, but have a clumped or aggregated distribution. At most times of the year densities of lice are highest along the sides and sometimes on the back of sheep. At times, significant numbers of lice can also be found on the head, underlining the importance of thorough coverage when dipping sheep or applying backliners.

After shearing, a greater proportion of the population are found at sites on lower body regions such as under the neck, lower flanks and upper legs and in areas where the wool has not been closely shorn. Read More

• Time from infestation: Usually, a sheep becomes infested by transfer of one or a few lice during contact with another infested sheep. Increase in lice numbers occurs very slowly in the early stages of an infestation and it can take many months for numbers to build up to levels where lice are easily found.
• Shearing: Shearing directly removes 30–50% of lice and many more die subsequently because of exposure to environmental influences. Louse numbers are lowest 30­–60 days after shearing.
• Temperature: Optimum temperature for B. ovis is between 37 and 39°C. Environmental conditions that subject lice to temperatures outside of this range reduce louse reproduction. Exposure to 48°C for 60 minutes, 50°C for 30 minutes or 55°C for 5 minutes kills all nymphal and adult stages of lice and most eggs (Murray 1968). However, under most conditions, lice can thermoregulate by moving up and down the wool fibre to avoid unfavourable temperatures.
• Solar radiation: High solar radiation can cause temperature gradients in the fleece from 70°C at the fleece tip to 45°C near the skin within 5–10 minutes of exposure. This has severe effects on louse populations, especially if the wool is short.
• Rainfall: If the fleece remains saturated for more than 6 hours, many nymphs and adults can drown and hatching of eggs is inhibited.
• Sheep susceptibility to lice
  • Breed: Merinos appear to be more susceptible than many other breeds, but all breeds of sheep, including shedding breeds, such as Dorpers and Damaras, can carry lice.
  • Differences among sheep within breed
  • Age: Lambs are more susceptible to lice than older sheep.
  • Sheep health and nutrition: Heaviest infestations of lice are found on lambs with low growth rates and sheep under stress from poor nutrition or disease.

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