Scab & Lice in Sheep

Introduction

Sheep scab and lice are the two most significant external parasites affecting sheep flocks in the UK and Ireland. Both cause intense irritation and fleece damage and spread rapidly between animals, but they are distinct conditions requiring different approaches to diagnosis and treatment.

Sheep scab, caused by the mite Psoroptes ovis, is the more serious condition. It is a notifiable disease in parts of the UK and Ireland and can severely affect both flock health and productivity if not identified and managed promptly.

Understanding how these parasites spread, recognising them early and implementing effective management is essential for every sheep farmer.

The Impact of Sheep Scab and Lice

External parasites are a major economic and welfare concern for sheep farmers across the UK and Ireland.

Sheep scab alone is estimated to cost the UK sheep industry £80-200 million every year, with approximately 10,000 outbreaks recorded annually, the majority occurring between September and April.

In Ireland, sheep scab is a notifiable disease, with 42 confirmed cases reported in 2023.

Beyond the financial impact, the welfare consequences are severe. Infested sheep experience intense irritation, rapidly lose body condition and may develop secondary infections. In severe cases, sheep can die.

Economic Impact

• Sheep scab costs the UK sheep industry an estimated £80-200 million per year.
• Approximately 10,000 outbreaks occur each year in the UK.
• 42 cases of sheep scab were confirmed in Ireland during 2023.
• Most outbreaks occur between September and April.

What Is Sheep Scab?

Sheep scab is an allergic dermatitis caused by an intense immune reaction to the faeces of the sheep scab mite (Psoroptes ovis).

The mites live on the surface of the skin and are just visible to the naked eye. Their lifecycle takes approximately 14 days, while mite populations can double every six days under favourable conditions, allowing outbreaks to escalate rapidly.

Importantly, mites can survive away from sheep for 16 to 17 days.

Potential sources of infection include:

• Wool tags
• Rubbing posts
• Gates and fences
• Shared handling equipment
• Contaminated trailers

This means infection can spread even without direct sheep-to-sheep contact.

How Does Scab Spread?

The primary route of infection is direct contact between sheep. However, because mites survive in the environment for prolonged periods, infection can also spread through:

• Purchase of infested sheep, including animals showing no obvious signs
• Shared handling, shearing or scanning equipment
• Common or shared grazing
• Contaminated lorries and trailers
• Wool tags caught on bushes, brambles or fencing
• Contaminated clothing worn by visitors or contractors

Clinical Signs

One of the greatest challenges with sheep scab is that up to 60% of a flock may already be infected before obvious clinical signs appear, and this can take several months.

As a result, disease may spread extensively before it is recognised.

Early-stage Signs

• Restlessness and irritation
• Rubbing against fences, gates and walls
• Head tossing, nibbling or biting at the fleece
• Stained or soiled wool
• Deranged or tagged fleece

Advanced Infestation

As mite numbers increase, signs become more severe:

• Excessive rubbing and head tossing
• Expanding areas of wool loss
• Open wounds and skin lesions
• Rapid loss of body condition
• In severe cases, neurological signs including fitting

The early signs of sheep scab and lice infestation are extremely similar. Diagnosis should always be confirmed by your vet before treatment, as each condition requires a different treatment approach.

It is also possible for sheep to be simultaneously infested with both scab mites and lice.

Diagnosis

Accurate diagnosis is essential before treatment is selected.

Skin Scrapes

The traditional method involves scraping material from the edge of a skin lesion for microscopic examination.

Although useful, this technique requires experience and may miss infestations where mite numbers are low.

ELISA Blood Test

The serum ELISA detects antibodies against Psoroptes ovis from approximately two weeks after exposure, before clinical signs develop.

This test is particularly valuable for:

• Screening purchased sheep
• Monitoring quarantined animals
• Confirming suspected treatment failures
• Flock-level surveillance (groups of up to 2,000 sheep using 12 samples)

A thorough flock history remains an important part of diagnosis alongside laboratory testing.

Treatment

Treatment should always follow an accurate diagnosis. Your vet or SQP/RAMA can advise on the most appropriate treatment plan for your flock.

There are two principal treatment options for sheep scab:

• Plunge dipping
• Injectable macrocyclic lactone (ML) wormers

Each differs in:

• Speed of action
• Duration of protection
• Potential impact on gut worm resistance
• Practical and safety considerations

Because mites survive in the environment for 16 to 17 days, returning treated sheep to contaminated areas may lead to reinfestation.

Not all products provide the same level of persistent protection, so treated sheep should either be moved onto clean grazing or treated with an appropriately persistent product.

Your vet can advise which option best suits your flock.

Resistance: An Emerging Concern

Resistance of sheep scab mites to moxidectin has now been confirmed in the UK.

SCOPS and Animal Health Ireland recommend that suspected treatment failures should always be investigated thoroughly before resistance is assumed.

Potential causes include:

• Incorrect dosing
• Poor administration technique
• Reinfestation
• True resistance

Any suspected treatment failure should be reported to your vet.

Injectable wormers used to treat sheep scab also treat gut worms, which may increase the selection pressure for gut worm resistance. This should be considered when choosing a treatment strategy.

Managing Sheep Scab

Before Treatment

• Quarantine all purchased sheep.
• Assume incoming sheep may be carrying scab even if no signs are visible.
• Confirm the diagnosis using skin scrapes and/or ELISA testing.
• Work with your vet or SQP/RAMA to select the most appropriate treatment.
• Consider previous treatment failures, withdrawal periods and your flock management system.
• Treat all in-contact sheep.
• Inform neighbouring farms where adjacent grazing exists.

During Treatment

• Dose according to the heaviest sheep in the group.
• Check and calibrate all dosing equipment.
• Ensure correct administration throughout treatment.

After Treatment

• Move sheep onto clean pasture where sheep have not grazed during the previous 16 to 17 days.
• Prevent contact between treated and untreated sheep for at least 14 days following treatment.
• Report any suspected treatment failures to your vet.

Notifiable Disease

• Republic of Ireland: Report sheep scab to DAFM through your local Regional Veterinary Office.
• Northern Ireland: Report suspected cases to DAERA through the local DAERA Direct Regional Office.
• Scotland: Sheep scab is notifiable. Report cases to your local APHA Field Service.

Lice in Sheep

What Are Sheep Lice?

The sheep body louse (Bovicola ovis) is a chewing louse that feeds on skin debris and hair rather than blood.

Lice populations peak during winter when fleece length is greatest and sheep are housed more closely together.

Shearing removes most lice and can reduce populations by 30 to 50%.

Clinical Signs and Welfare Impact

Signs include:

• Irritation, itching, rubbing and scratching
• Fleece damage and wool tags
• Cockle, a nodular skin condition caused by the immune response to lice

In healthy flocks, low to moderate infestations rarely result in significant production losses.

Heavy infestations are usually associated with animals already in poor health and may indicate an underlying management or health issue.

Diagnosis and Differentiation from Scab

Because early lice infestation closely resembles sheep scab, veterinary diagnosis is recommended before treatment.

Diagnosis may involve:

• Parting the fleece to identify lice visually
• Coat brushing or Sellotape testing followed by microscopic examination
• Skin scrapes to rule out sheep scab

Treatment

Treatment should be targeted and ideally timed around shearing.

Different treatment options are available, and your vet or SQP/RAMA can advise on the most suitable choice.

Remember: Injectable macrocyclic lactones used to treat sheep scab will not treat concurrent lice infestations.

Where both sheep scab and lice are present, plunge dipping provides the most comprehensive treatment.

Prevention

• Quarantine and treat all purchased sheep before introducing them to the flock.
• Treat sheep at, or shortly after, shearing when products penetrate the fleece most effectively.
• Maintain good overall flock health, as heavy lice burdens often indicate wider management or health problems.