Sheep parasites
Management of body lice

Ken Wilson and Bob Armstrong, formerly of DPI&F
Reviewed by Geoff Knights, DPI&F

Key points

• Lice breed only on sheep and complete their entire life cycle on the animal. They usually spread by direct contact between sheep.
• Sheep lice populations are primarily influenced by the time of shearing, but can also follow a seasonal pattern.
• Lice-infested sheep will bite at themselves and rub against anything that is available. This damages the fleece by matting the fibres and tearing the tip of the staple.
• Not all rubbing and fleece damage is due to lice. Grass seeds also cause irritation and it is important to arrive at an accurate diagnosis before any remedial action is undertaken.
• Up to 90 per cent of lice are removed with the fleece at shearing. Shearing is by far the most effective time to achieve control through chemical application.
• The economic effects of lice will depend on the severity of infestation, the class of sheep affected and the proportion of each mob affected.
• Effective lice control is dependant on an appropriate mix of animal management and chemical usage. The degree to which chemicals are needed will vary with the level of control that can be achieved.
• Every effort must be made to reduce the over-reliance on chemical control methods and reduce chemical residues on wool. This is particularly important in relation to chemicals applied later than six weeks after shearing.

Introduction

The sheep body louse (Bovicola ovis) is arguably the most economically important parasite of sheep in western Queensland. Unlike blowflies and worms, which are essentially seasonal parasites, lice are able to survive and breed under almost any conditions.

Adult lice are about 2 mm long, with a brownish head and creamy coloured abdomen. They can be seen by parting the fleece, as lice do not like direct sunlight and become active when exposed. The areas of heaviest infestation are on the neck in short wool animals and along the sides of long wool animals.

Life cycle

Lice only breed on sheep and complete their entire life cycle on the animal. They are found only on the wool growing parts of the body (ie not on the face, udder, scrotum).

The female louse lives for about 30 days and lays 10-20 eggs during this time. Eggs are laid close to the skin surface and are attached to a wool fibre. These hatch in 9-10 days (approximately equal numbers of males and females) and then pass through three nymphal stages during the next 21 days. The adults mature 3-4 days later. The life cycle from egg to egg is completed in 34-36 days. Figure 1 depicts the sheep body louse life cycle.

The eggs develop and hatch in a temperature range of 33-39°C. Hatching is not affected by low humidity but exposure to humidity levels above 92 per cent during the last 24 hours of development prevents hatching.

 

Figure 1.  Life cycle of the sheep body louse (Bovicola ovis)

Spread of lice

Lice spread rapidly between sheep by direct contact.

Lice generally do not live for more than four or five days, in ideal conditions, once separated from sheep. However laboratory studies at various temperatures have shown that some lice can live (and be reproductively viable) up to four weeks after separation, although most died within two to three weeks.

If lice are unprotected and exposed to sunlight they are unlikely to survive for more than a couple of hours (eg on wool in trucks, yards, catching pens, fences or in the paddock). Lice can be transmitted on shearers' moccasins (surviving for up to 10 days), but microwaving boots for five minutes in a plastic bag kills these lice.

Sheep lice can transfer to goats and survive the remainder of their normal life span on the goat. Sheep and goats need to be running very closely together (eg yarded or shedded) for this to occur. Sheep lice will not breed on goats and are very unlikely to be the cause of re-infestation. Sheep lice will not transfer to other animals.

Lice do not move far from the site of hatching. Most movement is up and down the wool fibre and is governed by temperature and light. In hot, bright conditions lice stay close to the skin surface. In moderate (cool to warm) and shaded conditions, they move towards the tip of the fibre.

This is important, as lice are most likely to spread when near the tip of the fibre. Conditions that favour transfer will therefore occur between late afternoon and early morning (ie at sheep camps, while watering), on cloudy days and during the cooler months of the year.

Seasonal variation

Lice populations are primarily influenced by the time of shearing, but can also follow a seasonal pattern. Numbers often decline during spring, remain low through summer and increase in autumn and winter. Apart from shearing, higher temperatures and an increase in rainfall are the main factors responsible for depressing lice numbers over the summer months. Lice control is much easier with a summer shearing.

However, lice numbers can increase and spread within a flock very quickly. In one experiment where 1.25 per cent of sheep in an otherwise clean mob were infested with lice, 65 per cent of the mob had lice within one month. At five months the entire mob was infested, with 60 per cent showing wool damage due to lice.

Effects on sheep

Sheep lice do not suck blood. They are biting lice that feed on the outer layer of skin where the wool fibres emerge. The feeding action of body lice on the skin surface causes intense irritation to the sheep. In an attempt to gain relief, animals will bite at themselves and rub against anything that is available. This damages the fleece by matting the fibres together and tearing the tip of the wool fibres. Associated stress may also cause a break in the fibre.

Other causes of irritation and rubbing

Not all rubbing and fleece damage is due to lice. Grass seeds also cause irritation and it is important to arrive at an accurate diagnosis before any remedial action is undertaken. A close examination of 8-10 sheep that are showing signs of fleece damage will give a good indication of the cause. However, grass seed and lice can both be present at the same time and can both be contributing to the problem. Don't put it all down to seed just because seed happens to be present.

Itch mite is normally not a problem in western Queensland. There is a remote possibility that sheep introduced from southern areas may bring some with them, but conditions do not favour the establishment of this parasite.

Economic importance

The effect that lice have on wool prices and animal productivity will depend on the severity of infestation, the class of sheep affected and the proportion of each mob affected. A light infestation in a small percentage of animals, which has caused little or no rubbing, will have only a minimal effect on overall returns. Medium to heavy infestations will result in some or all of the following:

• discolouration (yellowing) of the fleece
• decrease in yield of clean wool
• cotting of the most valuable fleece lines
• worn, wispy fleece tip
• lower tensile strength
• wool lost in the paddock
• increased susceptibility to flystrike (fleece rot, biting)
• increased cost of chemical control
• higher chemical residues in wool due to the need for extra control measures
• lower price and/or difficulty in selling lice-infested sheep or sheep showing signs of lice infestation.

Management and control

Effective lice control is dependant on an appropriate mix of animal management and chemical usage. The degree to which chemicals are needed will vary with the level of animal control that can be achieved. However, in most cases, a control program will include a chemical application at shearing.

Shearing

Up to 90 per cent of lice are removed with the fleece at shearing. The lice remaining on the sheep are exposed to a hostile environment and are very vulnerable. Shearing is by far the most effective time to achieve control through chemical application.

It is preferable to shear all sheep at the one shearing. If this is either not desirable or possible, animals with different shearings must be kept isolated from each other.

A clean muster at shearing is mandatory. As is stock proof boundary fences to prevent re-infestation from outside sources.

Chemical control

Shearing is by far the most effective time to achieve control through chemical application. Stragglers must be cleaned up well within the protection period offered by the chemical used for treatment of the main mob at shearing. Please check the product label for the protection period of various chemicals.

Sheep must be cleanly shorn. Backliners are much less effective on roughly shorn animals. Also the more wool that is left on, the more difficult it is to wet sheep in either shower or plunge dips.

Backliners must be applied within 24 hours of shearing and preferably on the same day that animals are shorn.

Where chemicals are used off the board, please check the product label for restrictions on time of lambing after treatment.

Where young lambs are not shorn at the same time as their mothers, they should be examined for lice. If necessary treat with a chemical whose label states that it is suitable for lambs.

Although lice on some properties in Queensland have developed resistance to the synthetic pyrethroid (SP) chemical group they are easy to kill with the correct application of an effective product.

Chemical treatment facilities need to be adequate and fully functional. This includes dips (nozzles, pump pressure, spray pattern) and working races for the application of backline treatments. (Automatic jetting races are not recommended for lice control treatments.)

Chemical application must be accurate. This includes:

• proper dose rate and site of application of backliners
• maintaining dip sumps at full strength
• allowing sufficient time in the dip to properly wet animals to skin level
• being carried out by a competent person
• not adding other chemicals/substances to the dip unless specifically recommended.

Chemical residues on wool

Pollution and environmental controls in many wool-processing countries are becoming increasingly stringent. To avoid the possibility of restrictions being imposed on the marketing and processing of our wool, the Australian industry needs to voluntarily reduce the level of chemical residues in greasy wool. Being able to provide a product with minimal residues would reinforce the marketing advantage that Australian wool currently enjoys.

This is not to say that chemicals should not be used at all. In fact, all registered products can still be applied to sheep provided they are used in accordance with label directions. However, there is an urgent need for producers to take maximum advantage of non-chemical control measures and to limit the use of chemicals to situations where no other alternative exists.

Chemicals applied to sheep with greater than six weeks wool growth will leave an unacceptable level of residue in the greasy wool at the next shearing. Where chemical usage is unavoidable, the type of product and method of application will affect the residue level.

Where a long wool lice control treatment is necessary, hand jetting along the backline (only) is the preferred method of application. Automatic jetting races apply chemical indiscriminately and only achieve very limited control. Long wool backliners apply high concentrations of chemical and result in some of the highest residue levels.

Introduced sheep

All introduced sheep (including rams) should be closely inspected for lice. If lice are found or suspected, sheep can either be treated in the wool (bearing in mind residue implications), shorn and treated offshears, or quarantined from other sheep on the property.

Because of the effect that shearing has on lice numbers, it can be difficult to find lice on short wool sheep. Bear this in mind when introducing sheep carrying less than three to four months wool.

Rainfall

Rainfall that is heavy enough to saturate the fleece can kill 50-100 per cent of adults, nymphs and eggs that are on the verge of hatching. Other eggs are not affected.

Further information

DPI&F Note Sheep parasites: integrated pest management to control blowflies and lice.

The book Blowflies and lice information manual: A practical approach to producing low residue wool (DPI Shop) takes you step-by-step through best practice management of blowflies and lice following an integrated pest management approach to reducing pesticide use and residues on wool.

The Quick guide to commonly used treatments for blowflies and lice lists commonly used pesticides.

For further information contact the DPI&F Call Centre on 13 25 23 (Queensland residents) or (07) 3404 6999 (non-Queensland residents) between 8 am and 6 pm weekdays, or e-mail callweb@dpi.qld.gov.au.

This DPI&F Note is also published on the DPI&F's PrimeNotes CD-ROM.

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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File No: SW0102 . Date created: June 2002  . Reviewed: February 2005