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Home News Feature articles Managing Lice in Sheep: From Inspection to Treatment

Managing Lice in Sheep: From Inspection to Treatment

22 Aug 2023

Sheep with lice are like a dog with fleas- they scratch, rub, bite, and itch but they can’t get rid of them!

Lice are fascinating insects as they are very easy to kill, but on farm they can be very tricky to control or eradicate. There are a few things we should consider about lice that will help us combat them in our flock. Lice are host-specific meaning they will only breed on sheep. They may pass to other species but will not reproduce. This is because sheep lice prefer to eat cells of the sheep’s skin and have a claw size ideal for grasping the wool fibres of sheep.

Lice are very sensitive to the environment, especially sunlight, temperature, and humidity. This make them easy to kill, but hard to eradicate as they breed in very large numbers and as they live at the base of the fibre it can be difficult to reach them with some chemical applications. Although lice prefer to live at the base of the fibre, they will migrate up in order to regulate temperature. If sheep are yarded, they may become hot, encouraging the lice to migrate up the tip and transferring onto other animals. Lice can also persist for short periods (up to 2 weeks) off their host when the environment is conducive i.e., shorn fleeces and inside shearers moccasins.

The life cycle of lice has some important features that should be kept in mind when planning control programs. Interestingly, lice have a long life cycle of between 34- 36 days and have proven to be quite efficient breeders. The implication of a long life cycle is that once we see lice in our flock it means that they were introduced anywhere from 2 -6 months previously. So, the neighbour’s sheep that broke through the fence last week are probably not to blame! As a result, it is important to consider what happened on farm 2-6 months prior to when you first see lice.

Sheep lice cause irritation to the flock as the biting leads to scratching and rubbing. This also predisposes some animals to fly strike. Other production losses, such as reduction in fleece quality through fleece derangement, cotting and discolouration can also be experienced. These lead to reductions in fleece quantity and quality.

Identifying lice on sheep

If your sheep are rubbing, how can you be sure lice is the problem?

Inspect inspect inspect! Lice are small, about, 2-4 mm in size, but are visible. When inspecting sheep for lice, it is important that you are able to see them clearly. If you use reading glasses normally then use them when inspecting your mob. Also, adequate light is essential. Outside on a sunny day is ideal. This has an added bonus, as lice don’t favour light so they will wriggle around making them easier to detect. It is also important to inspect a number of sheep (preferably those that are rubbing) and undertake 20 partings per side of each sheep. By counting the number of lice and their maturity per parting you will get more information about your infection. Large numbers of adult lice will indicate an older infection, while a number of juveniles would indicate a new infection.

Treating sheep for lice

As mentioned previously, lice are easy to kill but hard to control. It is estimated 70% of properties will fail to eradicate lice with a single treatment. Failure can result from a number of sources: missed muster; failure to treat new acquisitions; split shearings and lambing ewes making effective control difficult.

Regardless of the method you use, the best time to control lice is off-shears (in short wool) This is because shearing removes most of the lice and exposes the remaining lice to sunlight which will kill them. Short wool also enables maximum chemical penetration. Follow label directions for each product and bear in mind that dips can only be used at 2-6 weeks off shears due to the risk of wound infection of shearing cuts if used earlier.

It is vital to treat every sheep at the same time. This includes the kids pets and rams. Remember not to mix treated and untreated mobs. Ensure all sheep are treated correctly. If you miss just one all sheep can potentially be re-infected.

The gold standard for eradication is plunge or cage dip. Backliners are known to be easy and efficient, but in reality, they are difficult to apply correctly. This frequently results in failure to eradicate. Regardless of the application method, it is important to ROTATE chemical groups- not just product names but the chemical group within a product. As a final note, it is important when using an abamectin product for lice control (e.g., Maverick, Fennec) that you use a different chemical compound for controlling worms at the same time.

Lice control resources via LiceBoss:

The rubbing assessment allows users to determine the cause (or causes) of rubbing in their sheep: https://bit.ly/41rIHv8

The ewe/lamb treatments tool suggests the type of treatment for the ewes and considers whether their lambs may also require treatment: https://bit.ly/3owBITl

The lice treatment decision guide helps you determine the source of your infection on farm and give recommendations on treating your flock: https://bit.ly/3AhCOoI

The short wool lice tool helps to assess whether sheep should be treated after shearing: https://bit.ly/3AhwmOg

Author – Dr Fiona Macarthur

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