Monitor for flystrike

30 Aug 2021

By Dr Sue Street, Livestock Officer

With increased chance of above average rainfall predicted over spring, it is best to assume a bad fly season. Therefore, the need for producers to develop an integrated pest management plan for flies will be important. The aim is to detect flystrike as early as possible, especially before systemic signs start to appear, as prevention is the best strategy. Monitor all mobs of sheep, especially weaners during high-risk periods. Early detection and prevention are the best strategy to reduce flystrike in your flock.

Strike can be covert, can be detected early or can be advanced. These are the signs:

Covert flystrikes

Not easily detected, unless handling animal
Small area of flystrike
Can last weeks before advancing
Can resolve without need for treatment

Unfortunately, covert or hidden strikes are an important ongoing source of maggots which build up the fly numbers through a season.

Early detectable strikes

Only detectable on close inspection
Strike wounds will be small
Sheep will show signs of irritation and discomfort such as:
- Biting
- Scratching
- Leg stamping and
- Ducking/hanging of the head
Animals will still be a part of the mob
Wool may appear lighter in colour from chewing or rubbing and will progressively get darker
Depression of wool and body growth

Identifying why the strike occurred will help you work out whether it is just one animal, in which case dramatic intervention may not be warranted, or if it is an indicator that an outbreak is imminent.

If you think an outbreak or fly wave might happen (a rapid increase in fly numbers bought on by an abundance of food and warm, wet weather), increase monitoring and take action. This includes crutching or chemical preventions as soon as possible. If shearers cannot be arranged, consider alternative options, such as additional jetting which may provide interim protection until shearing or crutching can be arranged. But be aware of any withholding periods, especially if planning to shear/crutch soon.

Advanced strikes

Look for signs of systemic illness, which will typically progress over a few days. Signs include:
- Depressed
- Stop eating and drinking
- Weight loss
- Laying down
- Reluctant to rise
- Can’t keep up with the mob, be left behind and often found on their own
Strike wounds will generally be large, wet and dark
Maggots will be large and will be seen to be migrating out to consume healthy tissue
There will be secondary strike from other flies, such as Chrysomya and Calliphora spp., they will be smaller but more serious as the maggots ‘underrun’ the skin and cause extensive damage and illness
Skin will be swollen and inflamed
Remaining fleece will become tender

Without treatment, sheep with advanced strike will generally die quickly, anywhere from hours to three or so days. Early detection can help reduce flystrike, but if advance strike has been seen, you will need to monitor your flock more often as more strike is likely to occur in the following days. Once strike has been detected, the animal will need to be treated.

Treatment

Shear struck wool with a 5cm barrier of clean wool around the strike zone. Make sure it is close to the skin and remove all maggots
Collect all maggots and infected wool. Place into a plastic bag, preferably black and leave the bag in the sun for a couple of days to kill all the maggots.
- This helps to break the lifecycle of the fly as not all chemical dressings kill larger maggots, and many may escape treatment.
- Unless maggot infested wool is collected and bagged, most maggots will survive and pupate and come back as adult flies.
Apply a registered flystrike dressing to the shorn area, preferably using a low-pressure applicator, to prevent re-strike. You will need to use a product that will prevent strike, while the animal is healing as some products do not last long enough to prevent restrike.
Remove struck sheep from the mob and run separately to reduce the attractiveness of the mob to blowflies
Ultimately, cull struck animals from your breeding program.

Although flies might seem a bit daunting, especially after last summer, the good news is, there are several great resources out there to help get you through.

If you have any questions or would like more advice on monitoring your flock for flystrike, please contact your local LLS Ag team member or district vet.