Sheep Body Lice - Itchy Sheep
Signs, Control and Eradication
Sheep body lice (Bovicola ovis) are biting insects that cause affected animals to become itchy. Signs of lice infestation include rubbing, biting and scratching. Lice are a common problem, but most importantly are controllable, eradicable and preventable. Significant economic losses result from decreased wool quality and quantity, high treatment costs, and limited sale opportunities.
Sheep lice usually spread by direct body contact between sheep. Humans may spread lice on their clothing and footwear, but this is a remote possibility compared to the risk of stray, or purchased, infected sheep.
Biology of the sheep body louse
The sheep body louse (Bovicola ovis) is a cream-coloured insect 1-2 mm long with a red-brown head. It feeds on dead skin, lanolin, sweat gland secretions and skin bacteria.
When mature, the female louse lays approximately 1 egg every day for a month. The cycle from egg to adult-layer only takes 34-36 days, and takes place entirely on the sheep.
Short cool days in the cooler winter months are the preferred conditions for sheep lice, although buildup can occur at other times but at a reduced rate.
Shearing removes 30 to 50% of lice population on a sheep and solar radiation kills most of the remainder on late Spring/ Summer shorn sheep. Hence, untreated, Summer shorn sheep, will go into Autumn with very low lice populations.
The relatively slow rate of reproduction during the warmer time of the year means that it will take months for lice populations to build to a heavy infestation. Lice may be present for some time before rubbing and wool on fences, draws attention to their presence in a sheep flock.
Signs of lice infestation
Lice are only one cause of irritation to sheep, so a detailed inspection of the mob should be undertaken to establish the true cause. As lice are small insects they can be quite difficult to see, however most people can see them with the naked eye in good light, particularly if large numbers are present.
The best way to check for lice is by parting the wool a number of times where sheep are rubbed or matted. Otherwise make at least 10 partings on each side of the sheep, moving down the neck and along the side to the thigh. With the sunlight shining on the skin, lice can be seen moving away from the parted area back into the wool, as they do not like to be exposed to sunlight.
Protected areas, such as the folds around the neck, are preferred areas for the insects on bare shorn sheep. In woolly sheep, the mid-sides generally have the greatest lice numbers.
Any mobs that you suspect are infected should be thoroughly checked, as should all mobs before shearing.
Prevention is the best way to minimise the impact of lice on farm. Lice spread primarily by direct contact between sheep: they are host specific and cannot live off sheep for very long. What this means is the most likely source of new infestation is from sheep. Lice are unlikely to be transmitted by wool on posts or fences or picked up from long grass and trees. With this in mind controlling the movement of sheep is the most critical aspect of lice infestations, and there are several strategies to employ to prevent the introduction of lousy sheep into your flock.
Fencing needs to be sheep-proof at all times. Pay particular attention to boundary fencing, roadways, creek crossings, drains and timber plantations. With the advent of Landcare; increased environmental work on farm, and the increase of timber plantations, the risk of stray sheep has increased because stray sheep can hide undetected in fenced off creeks, gullies and plantations. Further, some timber companies lease out the grazing rights to their plantations as a means of grass control and fire mitigation. Often new plantations have been established on old farm land and retain several watering points making it difficult to detect and remove sheep. These stray sheep can breach boundary fencing and mingle undetected with your sheep.
Introducing new sheep
Most farms introduce sheep at some time. These introductions represent another possible source of infection. By having stock-proof internal fencing, these risks can be managed by quarantining the introductions from other sheep until you can be sure they are not carrying a lice infestation (at their next shearing). Inspections should be carried out prior to introducing them on to the farm, but light infestations or short wool will make detecting lice very difficult. Good record keeping of introductions and mob movements around the farm will help in managing these risks. While not mandatory, it is good policy to issue and request a Sheep Health Statement when moving sheep on and off farm.
Be a good neighbour
Good biosecurity should also exist between neighbours. Talk to neighbours and request that should they find one of your sheep on their farm they do not put it back over the fence. Get them to ring you for your collection and management of the straying sheep: you don't know if the neighbour has lice or how long your sheep has been with theirs. If in doubt, destroy the sheep as it is better to sacrifice one sheep than to risk your whole flock. If a number of sheep have strayed treat them as you would for any other new sheep introduced to your property.
Make assessments of the farm and use natural barriers or buffer zones. The creation of buffer zones further enhances biosecurity providing you with an area free from livestock to help prevent disease spread, including lice. Such buffer zones can be created using unused roads, old railway lines, creeks and rivers etc. Arrange an agreement with the neighbours that these zones do not carry stock. Have a policy about what happens to sheep caught in these buffer zones. Do you really want them back?
Non sheep vectors
Consideration should be given to shearing teams. There is potential for adult lice to live in shearing footwear such as shearers moccasins for up to ten days. While the risk might be small, all risks should be analysed given the cost of a lice infestation on farm. If shearers have previously worked at a shed with a heavy infestation, adult lice could be transferred to your sheep inadvertently. Microwaving moccasins for 5 min is a quick and convenient way of minimising the risk of transfer.
All good stock mangers are on the look out for disease; lice should be no different. When performing normal animal husbandry routines on the farm make random inspections of suspicious looking animals and always look for the signs of lice. Rubbing on posts and trees, biting at wool and wool on the fences are all signs that sheep are itchy. Itching and rubbing sheep does not necessarily indicate lice, but it should raise suspicions. Always inspect new introductions very carefully.
Lice can be difficult to detect and many partings of the wool might be necessary to find them. It is critical to actually see lice before an eradication program is initiated and it is important to actually identify the sheep body louse.
There are several other causes of sheep rubbing that are not lice. The treatment of sheep for lice is expensive, time consuming and with every extra chemical treatment hastens the risk of the development of resistance. If sheep body lice are detected then eradication and future prevention is necessary.
Inspections prior to shearing are critical. This is crucial to decision making if lice are detected because shearing and post shearing treatment is a key component in eradicating lice. Shearing can remove up to 90% of lice on the sheep and these will consequently die in the wool bale. The secret to successful eradication is the treatment post shearing.
If lice are detected several months prior to shearing then several factors need to be considered prior to embarking on eradication.
The level of infestation will determine the best course of action. The damage to wool caused by a light infestation may not warrant the cost treatment until shearing.
Detection of lice has now been made easier with the development of a lice detection kit to be used at shearing. Low levels of lice are difficult to detect but the kit will detect infestations at very low levels.
The test works by detecting a unique protein found in sheep lice. When lousy sheep are shorn the protein is trapped on the wool grease left on the comb and cutter. The kit enables producers to collect the wool grease from suspect mobs and submit the sample to the laboratory for testing.
Shearing and treatment to eradicate lice
Chemical applications in long wool will not eradicate lice. A full eradication effort will be required post shearing, therefore, consideration needs to be given to how long before the next shearing. Most chemicals will have withholding periods for wool, meat and an export slaughter interval. This will impact on what products can be used. Shearing may be brought forward to avoid the need to treat for lice in long wool.
Pregnant ewes and ewes with lambs at foot can present a serious challenge to a lice eradication program because it is essential that ALL sheep are treated for any lice eradication program to be successful. An isolation period of 6 weeks should be observed prior to mixing treated and non treated sheep.
The use of some chemicals (particularly the Insect Growth Regulators - IGRs) will limit the ability for sheep to be mixed for a period of time post treatment. For the same reasons, pregnant ewes should not be treated within 6 weeks of lambing.
Animal welfare concerns should be considered with respect to dipping ewes and lambs. Dipping of late pregnant ewes and young lambs should not be considered due to the risk of stress and subsequent welfare consequences.
I’ve got lice - what are my options?
Disposing of lousy sheep at saleyards is not an option and is illegal. If disposal rather than treatment is a considered option there are methods of disposal available to producers that have lousy sheep which require permission from an authorised inspector of livestock.
If treatment is being considered, several methods are available to treat sheep infested with lice. Effective chemical treatment requires adherence to several key principles.
Rotate chemical groups
Several different chemical groups have been used to kill lice. Some groups have lost their effectiveness and should not be considered. Of the effective chemicals, it is important to rotate to a different chemical group every time you treat for lice.
Read the label
Always read and follow label instructions. For chemicals to effectively kill lice they need to be applied at the correct rate at the correct time, and for the required length of time. Failure to follow the instruction may result in a sub lethal kill. This will increase the need to retreat next year, increase the risk of developing resistance, and cost money.
Observe Withholding Periods (WHP)
Australia enjoys market access to many countries around the world because we can assure them the products we produce are safe. Sales of animal or animal products that are sold for human consumption detected with chemicals above the Maximum Residue Limit (MRL) threaten international trade, domestic market confidence, and can result in prosecution.
It is very important to observe the withholding periods stated on the label:
- The wool withholding period is the time the product should not be used before shearing. This ensures there are no chemical residues in the wool.
- The meat withholding (or export slaughter) interval is the period of time after treatment that animals should not be sold for human consumption.
- Where the milk from sheep may be used for human consumption the withholding period for milk should be observed.
Keep accurate records
Keeping accurate records is an excellent way of understanding and observing withholding periods. Under the Agricultural & Veterinary Chemicals Regulations any person applying veterinary chemicals for which a withholding period applies must keep accurate records.
Chemicals for controlling lice
- Organophosphates (OPs) until recently were the main chemicals used for controlling lice in sheep. The most commonly used OP contained the active ingredient diazinon. In May 2007 products containing diazinon were suspended from use for dipping and jetting sheep. Since May 2009 it is illegal to treat sheep with diazinon by jetting or dipping unless under special permit.
- Insect Growth Regulators (IGRs) inhibit the growth and development of lice to break the life cycle of the insect. As they do not kill adult lice they are not generally recommended for late pregnant ewes, or where recently-treated sheep are to be mixed with untreated, lice-free sheep. Research undertaken collaboratively by Queensland DPI and Industry & Investment NSW have identified strains of lice with reduced susceptibility to IGRs. Over the last two years there have been an increasing number of reports of suspected resistance.
- Synthetic Pyrethroids (SPs) interfere with the insect's nervous system. They are not as dangerous to humans as OPs, but can have a devastating affect on aquatic life if contamination of waterways occurs. Lice resistance to these chemicals are commonplace now. For these two reasons, the use of synthetic pyrethroids is no longer recommended.
- Macrocyclic Lactones (MLs) (ivermectin) also affect the insect's nervous system and are used for long wool treatments by means of hand jetting. Follow up treatment off shears is recommended after using any long wool product to eradicate lice.
- Spinosyn (SPINs) (Extinosad) causes nerve dysfunction in insects, with negligible human health and environmental risks.
- Other registered chemicals (i.e. not OPs, IGRs, MLs) such as magnesium fluorosilicate can be used for treating lice-infected sheep. Magnesium fluorosilicate will control body lice and is approved for organic production enterprises. Its mode of action is to severely and rapidly dehydrate lice.
Louise Wood, Katie Rutter and Richard Keys, October 2003. Revised Sept 2009 by Leon Watts.
© The State of Victoria, Department of Primary Industries, 1996-2013
By Agriculture Victoria - Last updated