Controlling lice in sheep

Sheep lice remain a major problem in the New Zealand sheep and wool industry. Infestations can cause significant production losses in two ways: 

1. Fleece damage: Lice cause intense irritation. Infested sheep bite themselves or rub against trees and fences, leading to fleece derangement, cotting or broken fibres. Lousy wool often has a yellow colour and a distinctive smell caused by skin secretions, causing further downgrading. 

2. Hide damage: Lice can trigger an immune response known as cockle, which causes thickening of the skin and pelt damage. This allergic reaction usually occurs early in the infestation process 

The body louse spends its entire lifecycle on the host animal, meaning that infestations are mostly spread via direct contact. Adult females lay their eggs on wool fibres within 12 mm of the skin. Eggs require a temperature of 35–39°C and a relative humidity of 50–70 per cent for optimal development. Eggs take 8–10 days to hatch before progressing through three nymphal stages over 21 days. Mature females lay about two eggs every three days and lay an average of 30 eggs during their lifetime.

Off-shears control options

Shearing significantly reduces the size of the lice population by physical removal and subsequent exposure to the elements. This makes immediately after shearing an ideal time for sheep and wool producers to apply a pour-on ‘backline’ treatment to control lice infestations.   

Short wool control options

Dipping short wool sheep (i.e sheep with less than 6 weeks wool growth) is a cost-effective alternative to pour-on treatments. Saturation dipping with either a plunge or shower dip is acknowledged as the most effective method of lice control. Complete saturation of the sheep with diluted chemical ensures contact between the target pest with the active ingredient.  

Automatic jetting races may not deliver sufficient wash to achieve complete control of lice, even if highly effective chemicals are used. 

Long wool control options

Despite the best care and attention, lice infestations in long wool can occur and can have a severe economic impact. A long wool lice treatment may be necessary to reduce lice numbers and minimise fleece derangement until the next shearing.

Management considerations for managing lice (1)

Since the AgResearch Entomology Unit was dis-established in 2000, here has been no independent research in sheep louse control in New Zealand. However, the Australian sheep industry has continued to invest in this sector, therefore for current research results and management guidelines Australian resources are referred to. Best practice louse management includes: 

• Treat all sheep on the property at the same time. Avoid split shearing. 
• Muster each paddock thoroughly to ensure all sheep are treated. Conduct a second muster if necessary.    
• Treat all sheep on the property at the same time. Avoid split shearing. 
• Do not mix treated sheep with lousy sheep or those of unknown lice status.   
• Check boundary fences to prevent untreated or lice-infested sheep from joining treated mobs.   
• Quarantine all purchased (e.g. rams, replacement ewes) or returning sheep (e.g. coming back from agistment) until you are sure they are lice-free. If lice are suspected, shear and treat with an effective off-shears pour on or short wool saturation dip.  
• Sheep that require treatment with a long wool lousicide should be re-treated with an off-shears lousicide at the next shearing.  
• Follow biosecurity guidelines when entering sheep properties to ensure no inadvertent spread of lice.   

 Importance of Chemical Rotation

Parasite resistance may develop to any chemical if it is used continuously.

The following guidelines will help to ensure sheep and wool producers continue to have access to a range of effective lice control options:   

• All lice treatments should be applied as part of an integrated pest management program (visit www.liceboss.com.au).   
• Avoid the consecutive use of lousicides with the same mode of action. 
• If consecutive use of the same chemical group is required, careful attention should be paid to achieving eradication after treatment to help prevent the establishment of a resistant lice population.   
• Rotation between products with different modes of action may prolong the effective life of all available treatments.   
• Do not use the same mode of action for lice control and flystrike prevention during the same season. 

Trusted solutions and advice from Elanco

Elanco is an industry leader in sheep lice solutions with a product range including ExPO Extinosad^™ Pour-On, Extinosad^™, Cyrex^™ Liquid and Seraphos coupled with benchmark technical support and customer service. Always read and follow label directions. For any information or technical advice on managing sheep lice, contact Elanco Customer Service  0800446121