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Editorial Information

Phil Scott DVM&S, DipECBHM, CertCHP, DSHP, FRCVS

Published 2011

Reviewed byPhil Scott DVM&S, DipECBHM, CertCHP, DSHP, FRCVS 2016

Ectoparasitic Infestations (excluding Sheep Scab) in Sheep


While sheep scab is the most important skin infestation, farmers must also be aware of the other ectoparasite infestations that can affect their sheep causing financial loss and welfare concerns.  Control of all ectoparasites infestations can be achieved by strategic treatments and strict biosecurity although these principles of disease control appear to have been largely forgotten in the 20+ years since the cessation of compulsory plunge dipping. Skin infestations in sheep are a major animal welfare concern.

Headfly

In the UK, headfly can present a major problem during the summer months. Grazing patterns are disturbed and affected sheep often isolate themselves and remain in shade where available.  They may stand with the head held lowered with frequent head shaking and ear movements.  Alternatively, sheep adopt a submissive posture in sternal recumbency with the neck extended and the head held on the ground.  Kicking at the head often greatly exacerbates damage caused by headflies and such action may also traumatise the skin of the neck and ears. The impact of headfly worry on sheep welfare must not be underestimated.  Fleece quality is adversely affected and the loss of body condition sheep experience will reduce ovulation rate in the autumn and subsequent litter size costing the farmer money.

EctoInfF1

Severe headfly lesion causing disrupted grazing and rapid condition loss in this Texel cross lamb.
EctoInfF2
Abnormal behaviour shown by this Blackface ewe is caused by headflies. Condition loss during the summer can cause poor ovulation and reduced litter size at lambing time.

Topical emollients and antibiotic preparations are not usually necessary and skin wounds heal well provided the flies are denied access to these areas. Housing is essential for sheep with large skin lesions to allow time for complete healing.  Ear tagging and notching must be undertaken carefully and not during the fly season.

EctoInfF3

Housing is essential for sheep with large skin lesions to allow time for complete healing. Sheep must not be banished to hill ground on the expectation that there will be no flies there.

Pour-on fly control preparations, such as high cis cypermethrin or deltamethrin, must be applied before the anticipated headfly season and especially to horned sheep with re-application as directed by the manufacturer's instructions.

Cutaneous myiasis (Blowly strike, maggots)

Blowfly lesions may range from centimetre diameter areas of skin hyperaemia with a small number of maggots to extensive areas of traumatised/devitalised skin causing death of the sheep in neglected cases.  It is legal requirement to inspect all lowground and upland sheep daily. Death of a sheep due to blowfly strike could lead to prosecution under animal welfare legislation.

3 ectoparasitic infestation faecal staining

Adult flies are attracted to areas adjacent to faecal staining surrounding the perineum.

Adults flies are attracted to areas adjacent to faecal staining surrounding the perineum; and less commonly virulent footrot lesions with exposed corium/exuberant granulation tissue, dermatophilosis lesions on the skin, and urine scalding around the prepuce.  In severe infestations the sheep are depressed and isolated from the flock.  Large numbers of adult flies are seen on the fleece with maggots on the blackened skin once the surrounding fleece has been lifted clear.  There is an associated putrid smell.

 

6 ectoparasitic infestation virulent footrot lesions

Adult flies are attracted to virulent footrot lesions.

 

7 ectoparasitic infestation blowfly strike

Death caused by extensive blowfly strike.

Affected sheep can be treated by plunge dipping using an organophosphate preparation but it is more usual to treat individual infested sheep with dip wash applied directly to the struck area after first clipping away overlying wool.

Before preventive measures using various chemicals are considered, much can be done to reduce the attraction of blowflies for example a grazing programme to prevent the massive build up of infective helminth larvae on permanent pasture during July and August (mid-summer rise) reduces diarrhoea caused by high parasite burdens.  Where faecal staining of the perineum occurs this wool must be removed ("dagging" or "crutching"). In adult sheep removal of the fleece and any faecal contamination by shearing during late May/June in the UK removes this attraction well before the peak of the blowfly season.

 

8 ectoparasitic infestation faecal straining

Where faecal staining of the perineum occurs this wool must be removed. What is your opinion of the care of this sheep?

Dimpylate (diazinon) and propetamphos are effective against blowfly strike for up to six weeks.  These compounds are strongly lipophilic and replenishment of dips is important to maintain effective concentrations within the bath.  It is essential to follow all instructions on the data sheet.

While topical application of high cis cypermethrin pour-on preparations provides protection against fly strike, these preparations persist for only 6 to 8 weeks at the site of application and require re-application in most situations.  The insect growth regulator, cyromazine, applied before the risk period is effective against blowfly strike for up to 10 weeks after topical application and dicyclanil affords 16 weeks' full body protection.

 

9 ectoparasitic infestation topical application

Topical application to the breech area.

10 ectoparasitic infestation blowfiles

"Recreational" rabbit shooting on this farm provides blowflies with the perfect breeding ground to rapidly multiply and attack sheep.

Lice

Louse populations are highest during late winter in sheep in poor body condition kept under unhygienic conditions rather than the reverse situation where lice cause debility.  The chewing louse Bovicola ovis is the most common infestation and may cause disrupted feeding patterns, fleece damage/loss, and self-inflicted trauma.  Spread occurs by close contact. The slow reproductive capacity of Bovicola ovis results in a gradual build-up of louse numbers over several months.

EctoInfF10

Louse populations are highest during late winter and may cause disrupted feeding patterns, fleece damage/loss, and self-inflicted trauma. Note also the faecal staining on the tail of the ewe lamb on the left

EctoInfF11

Heavy louse infestation revealed in an emaciated ewe.

The important differential diagnosis for flock problems of pruritus and fleece loss is psoroptic mange (sheep scab).   Reliance on systemic endectocides to control sheep scab has resulted in an upsurge of louse infestations in sheep flocks in the UK.  Maintenance of a closed flock and effective biosecurity measures would prevent introduction of louse infestation.

13 ectoparasitic infestation lice or sheep scab

Lice or sheep scab?

Louse infestation can be readily eliminated by plunge dipping using an organophosphate preparation.  Use of plunge dipping for other reasons, such as control of sheep scab, cutaneous myiasis and headfly problems, also effectively controls louse infestations.  Louse infestations can also be controlled with topical application of high cis cypermethrin or deltamethrin.  The presence of lice on sheep reflects poorly upon biosecurity measures and the overall flock health plan and such infestation must prompt a review of these processes before more serious and costly infections are introduced.

14 ectoparasitic infestation lice
The presence of lice on sheep reflects poorly upon biosecurity measures and the overall flock health plan
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