Fly strike

Fly strike

The thought of flystrike can send a chill through any shepherd; though no illness or disease in sheep is pleasant, this is one of the worst.  After a mild, very wet winter, the numbers of fly larvae around will be greater than usual, so it will be doubly important to watch out for strike and deal with it without risking  compromise to the health of the sheep or the potential to use the wool.

The economic cost of flies is estimated to reduce milk yields by 0.5 litres per cow per day and growth rates by 0.3kg per head per day in cattle, which can easily be extrapolated to sheep – it’s not just the damage to an animal which has been struck, but the irritation which distracts the whole flock from getting on with eating and growing.

What causes strike?
Blowflies are the cause of strike; attracted by the scent of faeces, dirt and sweat, they lay eggs in fleeces.  Once these eggs hatch, the maggots feed off the flesh of sheep, creating wounds which in turn attract more flies in a constant cycle.  By the time you notice flies, the population explosion is already happening.

Phases of flystrike and an example strike-riskmap from Strike Wise.

The various types of fly succeed each other across the season, and it is worth noting that the “green bottle” is by far the worst for sheep, cashing in on early and lesser damage by the “blue bottle”.


What are the signs?
Most sheep that have been struck will show signs of discomfort; increased stamping or scratching and quite often a tendency to move away from the flock.  There are also signs on the fleece itself: patches that look damp are a sign that there is something happening that isn’t outwardly visible.  There is also a sweet smell.  There may be a loss of fleece and in advanced cases, an animal that is reluctant to move due pain and weakness.  Untreated flystrike will result in death.

How can it be prevented?
Constant vigilance!  If you are unable to select pasture less prone to flies, you will need to take more action …

Flies will take advantage of any dirty animal with dags, so the first priority is to keep the flock as clean as possible.  This may mean dagging or crutching sheep once they are wormed, or if they have be put on fresh grass.  Also, open dung heaps, slurry puddles and old hay and straw stacks are ideal locations, along with water-courses sheltered by trees – a nice open draughty downland or moorland is much less comfortable for flies, so the location of your sheep is crucial in reducing the risks.

Fly larvae need a temperature of a minimum of 9C to develop, so if the temperature rises earlier in the year then flies will be out before they are usually expected: ‘Fly Season’ can start at any point! 

Treatments such as Clik and Clikzin can also be applied to the fleece to act as a deterrent, as can Stopmyasis or Neem (organic versions) or home-made preparations using aromatherapy oils which deter flies.  With these chemicals is it vitally important to get the timing right to ensure that all withdrawal periods are met for meat and wool production.  We have a useful Flies Information Sheet on our website.  See also Sue’s item on sheep and drugs!

It is also worth considering fly traps, which are cheap and effective.  Here is a simple version.  These are effective in catching and killling flies for sheep, cattle and also horses.

Make your own fly trap using a plastic bottle, instructions are here. Images courtesy of Healthy Home Economist.

How can it be treated?

If possible, please avoid using pour-ons as a deterrent: hair clipped from the feet of cattle several weeks after treatment still contains sufficient deltamethrin to kill midges, so the effluent from scouring fleeces will kill fish and algae.  Once shorn, the sheep can be treated and the treatment will be more more effective on the short fleece, and will have worn off by next year.  Apart from fly traps (as above), you can consider aromatherapy oils such as neem, geranium, tea-tree and eucalyptus, all of which also deter flies.

If the worst comes to the worst, regular observation should mean that you can treat the flies and remove eggs and larvae before they have managed to do more than create a slight graze on the skin, but if left untended the wounds may become extensive and deep.

Generally the following steps should be taken:

–          remove the fleece from the affected and surrounding area by carefully clipping it away (make sure you go further than necessary to ensure no lurking predators remain)

–          clean the wound with antiseptic, salt water or a pesticide treatment

–          apply antiseptic as required

–          apply summer fly cream (or equivalent) around the area, but not in the wound

–          if the wound is big or the sheep is underweight or seems in poor condition, also give an anti-biotic injection and vitamins, to help fight any infection remaining.

Removing fleece from the surrounding area will make the extent of the attack or wound clear.  Cleaning the wound is probably the most unpleasant part as maggots will surface once water and any cleaner has been applied.  Cleaning should be repeated until no more maggots appear, though it is imperative to keep a close eye on the wound just in case any have been missed, or any more fly activity is apparent.

Though it is unpleasant for the shepherd, it is important to remember that it is much, much more unpleasant for the sheep and if left untreated can result in a very slow and painful death.