Blowfly strike in sheep

16th November 2016

Blowfly strike is the most common ectoparasitic infestation affecting sheep in the UK: every year up to 80% of farms will report one or more cases of strike, and this equates to at least 500,000 animals affected in the national UK flock.

Female adult blowflies are attracted to odours of sweat and faeces on the fleece of sheep, and their potential for multiplication of larvae (maggots) is accelerated in the warm and humid microclimate provided by fleece. Devastating welfare consequences and economic implications can arise as soon as twelve hours after the first flies land on the affected animals.

Damage caused affects wool clip, hide quality and daily liveweight gains, and can rapidly progress to fatalities within 36 hours. EBLEX estimates a cost to the English sheep industry of £2.2 million annually.

It's not over yet...

Traditionally, the peak season for strike would be between May and September, but as weather patterns change, the risk period may now extend later into the year: lowland flocks could be at risk any time between March and December. It is important to remain highly vigilant, even at this time of year.

Usually the first signs of infestation are discoloured wool and agitation, which will progress to depression. Common predilection sites are the breech, withers, shoulders and neck. However, it can also happen secondary to severe footrot. There is a substantial risk that affected sheep may become toxaemic and die; the prognosis is hopeless if greater than one third of the skin surface is affected. Treatment must be initiated as soon as possible to avoid such complications.

Recommended treatment

Affected sheep can be treated by plunge dipping in an organophosphate preparation, however it is usually more practical to treat individual animals with topical treatments.

The wool must be clipped away from the wounds, to beyond the margin between normal skin and infested discoloured skin: this will ensure that any unhatched eggs which lie around the margin are also removed. Clipping also disturbs maggots, to make them easier to manually remove.

Spotinor Spot-on Solution for Cattle & Sheep contains 1% deltamethrin, and is licensed for the treatment of established blowfly strike. It is recommended to be applied directly to the maggot infested area as soon as signs appear, and one application will ensure blowfly larvae are killed in a short time.

Your vet may also prescribe broad spectrum antibiotics and anti-inflammatories to help control pain, inflammation and secondary bacterial infection, which could if left unchecked result in toxaemia. Affected animals may require a course for as long as five days. If there are signs of dehydration, it would be prudent to consider administering oral rehydration fluids to improve demeanour and prognosis. Unfortunately, cases which are recumbent and toxic on discovery will require euthanasia on welfare grounds.

Veterinary Treatment of Sheep & Goats. Graham R. Duncanson. 2012. Accessed online 11/11/16.

 

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