Livestock News

  • Written by: Farmers Guide
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Dont play the blowfly ‘lottery’

By Dr Fiona Lovatt, Independent Sheep Veterinary Consultant and SeniorVice President of the Sheep Veterinary Society.

Blowfly strikeis one of the most familiar and unpleasant issues affecting sheep in the UK andIreland. Research has shown that every year there will be cases on more than 75per 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 industrywe now understand more about the factors which contribute to blowfly strike andthe strategies that can help prevent its occurrence. However, each year, manyshepherds fail to benefit from that knowledge and suffer the emotional andeconomic consequences of failing to act until after the first cases of strikehave occurred.

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


We are allwell aware of the unpredictability of the UK weather, seen last year in a verywarm spring compared to this years cooler, but still very unsettled weather.In terms of the weather, the occurrence of blowfly strike depends on both soiltemperature and air humidity, as well as the presence of long, wet or dirtyfleece. The first strike cases will not have occurred as early this year as in2014, but as the season progresses around half a million cases will still occurnationally.

Due to theunpredictability of the UK weather, getting the timing right for treatment ofewes and lambs against blowfly strike can be difficult. However, evidenceconsistently suggests that early application is the most cost effective way inreducing the numbers of both flies and susceptible sheep.

The gamble

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

The resultscan be devastating. Each case of strike increases the risk to the rest of theflock by increasing the blowfly population in the area. And once struck, ananimal can die quickly or suffer for a week or so before succumbing. Obviously,the death of even one sheep has financial consequences, but, even when ananimal does not actually die, there is a dramatic effect on growth rate as wellas damage to both hide and fleece causing further loss. Of course there areadditional costs associated with the time and labour required to catch andtreat all affected sheep as well as the medicines needed for treatment andnursing.

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

Stop playing the blowfly lottery

The gamble ofthe blowfly lottery can be significantly reduced with a simple but effectivefly control strategy discussed in partnership with a vet.

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

Blowfly strikeis a disease which should always be controlled by taking appropriate preventativeaction and best practice can be based on a three-tier strategy:


Arm yourselfwith the facts on blowfly strike and put in place a fly control strategy beforeit is too late this blowfly season. Assoon as possible, you should consult your vet or animal health advisoras to the most appropriate strategy for your farm.

The mostcost-effective strategies involve treating both ewes and lambs early in theseason. Once you see a case of strike you have already incurred significantcosts and it is arguably too late. However, at least you can use thisunfortunate case, to prompt immediateaction to protect the rest of the flock for this year and to remind youto treat earlier next year.


Dont befooled by a slower start to the fly season due to lower temperatures earlierthis spring. Breech strike in lambs occurs irrespective of weather conditions,and the risk increases as their wool grows and the number of dirty backsidesincrease. Unsettled, wet weather in the early summer can give high humidity andwarmth that will inevitably lead to strike.

And dont takeyour eyes off the ball by the end of the summer. A warm wet autumn combinedwith longer fleece lengths can mean high strike risk in both ewes and lambsthat were not given a long-lasting product earlier in the season.


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

Although thesynthetic pyrethroids are essential for use in the treatment of establishedcases of strike, they are less suitable for prevention due to theirbroad-spectrum nature as well as potential residue and efficacy issues if usedin anything other than a recently shorn sheep.

There areenough factors affecting the success of sheep farming that are out of ourcontrol – such as the British weather and the price of lamb dont introduceanother risk by gambling with the blowflies that we know we can control.


  • Written by: Farmers Guide
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