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Don’t Play the 'Blowfly Lottery'

21 July 2015

UK - Dealing with blowfly should be planned and calculated, not a gamble, according to a leading sheep expert.

Being ready each year requires farmers to be vigilant and rests on two factors.

As an industry we now understand more about the factors which contribute to blowfly strike, writes Dr Fiona Lovatt
  1. Number of susceptible sheep
  2. Number of flies in environment 

This is according to Independent Sheep Veterinary Consultant, Dr Fiona Lovatt, Senior Vice President of the Sheep Veterinary Society, who warns against playing the 'blowfly lottery'.

Familiar and Unpleasant

Blowfly strike is one of the most familiar and unpleasant issues affecting sheep in the UK and Ireland, writes Dr Lovatt. Research has shown that every year there will be cases on more than 75 per cent of sheep farms with every case causing pain and distress to the sheep, as well as a drain on the time and finances of farmers nationwide.

As an industry we now understand more about the factors which contribute to blowfly strike and the strategies that can help prevent its occurrence. However, each year, many shepherds fail to benefit from that knowledge and suffer the emotional and economic consequences of failing to act until after the first cases of strike have occurred.

Two major factors determine the number of cases of blowfly strike on any one farm: the number of susceptible sheep and the number of flies in the environment. Control strategies involve decreasing the susceptibility of the sheep and reducing the number of flies by applying timely preventative measures.


We are all well aware of the unpredictability of the UK weather, seen last year in a very warm spring compared to this year’s cooler, but still very unsettled weather. In terms of the weather, the occurrence of blowfly strike depends on both soil temperature and air humidity, as well as the presence of long, wet or dirty fleece. The first strike cases will not have occurred as early this year as in 2014, but as the season progresses around half a million cases will still occur nationally.

"Waiting for the first case of blowfly strike before thinking about treatment is a dangerous gamble to take..."

Due to the unpredictability of the UK weather, getting the timing right for treatment of ewes and lambs against blowfly strike can be difficult. However, evidence consistently suggests that early application is the most cost effective way in reducing the numbers of both flies and susceptible sheep.

The Gamble

Waiting for the first case of blowfly strike before thinking about treatment is a dangerous gamble to take, and commonly farmers act too late. A struck sheep can be hard to spot, having separated itself from the main flock, and, at a high risk time of year, apparently clean sheep can be heavily infested with maggots within a day or so.

The results can be devastating. Each case of strike increases the risk to the rest of the flock by increasing the blowfly population in the area. And once struck, an animal can die quickly or suffer for a week or so before succumbing.

Blowfly strike will hit half a million sheep this year

Obviously, the death of even one sheep has financial consequences, but, even when an animal does not actually die, there is a dramatic effect on growth rate as well as damage to both hide and fleece causing further loss. Of course there are additional costs associated with the time and labour required to catch and treat all affected sheep as well as the medicines needed for treatment and nursing.

The impact can be felt on an emotional level too. A severely struck sheep will be in significant distress, with foul and tender open wounds caused by the blowfly larvae quite literally eating their way through both skin and flesh.

Stop Playing the Blowfly Lottery

The gamble of the blowfly lottery can be significantly reduced with a simple but effective fly control strategy discussed in partnership with a vet.

Every case of fly strike on a farm has a significant cost to both finances and welfare, but cases of fly strike do not have to be inevitable. With the right strategy and the right products in place at the right time of year, the risk of fly strike is dramatically reduced.

Blowfly strike is a disease which should always be controlled by taking appropriate preventative action and best practice can be based on a three-tier strategy:


Arm yourself with the facts on blowfly strike and put in place a fly control strategy before it is too late this blowfly season. As soon as possible, you should consult your vet or animal health advisor as to the most appropriate strategy for your farm.

The most cost-effective strategies involve treating both ewes and lambs early in the season. Once you see a case of strike you have already incurred significant costs and it is arguably too late. However, at least you can use this unfortunate case, to prompt immediate action to protect the rest of the flock for this year and to remind you to treat earlier next year.


Don’t be fooled by a slower start to the fly season due to lower temperatures earlier this spring. Breech strike in lambs occurs irrespective of weather conditions, and the risk increases as their wool grows and the number of dirty backsides increase. Unsettled, wet weather in the early summer can give high humidity and warmth that will inevitably lead to strike.

And don’t take your eyes off the ball by the end of the summer. A warm wet autumn combined with longer fleece lengths can mean high strike risk in both ewes and lambs that were not given a long-lasting product earlier in the season.


We know that the timely use of a preventative product will limit the build-up of flies as well as protect the sheep. In accordance with SCOPS guidelines, I would always recommend that wherever possible, a narrow-spectrum preventative product is used, and for the prevention of blowfly strike this means an insect growth regulator (IGR) is ideal. The active ingredient in these products halts maggot development in its early stages, preventing damage to the sheep and subsequent flystrike.

Although the synthetic pyrethroids are essential for use in the treatment of established cases of strike, they are less suitable for prevention due to their broad-spectrum nature as well as potential residue and efficacy issues if used in anything other than a recently shorn sheep.

There are enough factors affecting the success of sheep farming that are out of our control - such as the British weather and the price of lamb – don’t introduce another risk by gambling with the blowflies that we know we can control.

Pictures courtesy of the National Animal Disease Information Service (NADIS), and

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