When monitoring for flystrike, the aim is to detect strike as early as possible, and certainly before the toxins produced by the maggots affect the entire sheep (systemic effects).
Small strikes that cannot be detected easily on general inspection, called ‘covert’ strikes, are quite common and can persist for some weeks before either advancing to an ‘overt’ strike—one that can be detected by a close-paddock inspection—or resolving without the need for treatment. These covert or hidden strikes are an important ongoing source of maggots that build up the fly numbers through a season.
Aim to detect strikes before systemic signs appear
Early detectable strikes:
- Signs of strike become noticeable on a close paddock-inspection of a mob.
- The strike wound is not particularly large.
- The sheep will typically be biting or scratching at the affected area—especially with breech strike, but less so with body strike—and may stamp its feet and duck its head, but will be staying with the mob.
- Maggots have commenced feeding on the skin of the sheep and the affected area will appear different—the wool may appear lighter coloured from the chewing or rubbing, but will progressively become darker with more exudate.
- A limited amount of toxins from this feeding will have entered the sheep’s system.
- The sheep will be suffering local discomfort and be irritated, but there will be no obvious systemic effects (as described in advanced strike, below).
- A cost will occur from the loss of fleece removed from the strike site and the chemical treatment costs, but a larger cost is associated with the time to detect and treat these sheep. There may also be some depression of wool and body growth.
Monitoring should detect strikes before they are advanced
Advanced strikes:
- Once signs of systemic illness commence, they typically progress over a few days.
- Sheep can’t keep up with the mob while grazing, will be left behind, and are often found on their own. They will become increasingly depressed and will stop eating and drinking and will lay down and be reluctant to rise. Without treatment, they generally will die in a few days.
- The strike wound will generally be large, wet and dark, and the maggots will be migrating out to consume healthy tissue.
- Strike from secondary blowflies (flies other than Lucilia cuprina, such as Chrysomya and Calliphora spp.) can be smaller, but more serious, as the maggots ‘underrun’ the skin and cause extensive damage and illness.
- Maggots will be large and feeding extensively on the skin, which will be swollen and inflamed.
- Significant amounts of toxins released by the maggots will have entered the sheep’s system and be affecting the entire sheep (systemic effects).
- The sheep will be suffering from local pain, as well as a fever and other systemic (whole body) effects.
- A significant cost will occur at this stage. Either the sheep dies or is destroyed, or if it is treated, there is the loss of fleece removed from a large strike site, as well as the remaining fleece becoming tender, the chemical treatment costs, and significant labour. On top of that is a loss in weight from the systemic illness.
If sheep with advanced flystrike are found, increase your frequency of monitoring
Whether or not the whole mob should be treated when struck sheep are found will mostly depend on your continuing ability to detect and treat struck sheep before strikes are advanced and the associated cost, compared to treating the whole mob or specifically identified high-risk sheep.
The three recommended “times” for applying preventative treatment
Early season flystrike preventative treatment
In areas where flies overwinter (are dormant) for an 8-week period or more during winter, apply a preventative treatment to the entire flock during this period of dormancy.
It is essential that no strikes (including the hidden ‘covert’ strikes) occur, as maggots from these will allow the fly population to build to levels high enough for strike to occur later in the season.
Fly-trapping or previous experience can be used to plan when treatment should be applied. The FlyBoss Optimise Treatment tool can be used, but treatment must be applied several weeks before the tool suggests that any flystrike is expected. Your treatment time should also allow you to comply with chemical withholding periods.
The strategy involves applying a long-acting flystrike preventative to the whole flock before any fly activity in spring, followed where possible, with shearing or crutching when the chemical protection period is ending.
This prevents a fly population building up after a winter dormancy, so that strike is ultimately prevented later in the season by a lack of flies, rather than by making sheep less susceptible or applying another chemical treatment.
Set treatment time at the start of the flystrike season
On properties where preventative treatments are required in more years than not, treatment can be applied at a set time every year.
This can be at a time just before significant amounts of flystrike is likely to occur, and can just be applied to at-risk mobs.
Treatment ideally occurs just before the number of strikes start to build up, but not before any strike occurs. The aim is to gain the maximum amount of protection for the chemical applied across the flystrike season.
If the chemical is applied too early, only a small amount of high-risk sheep are gaining the benefit in the first few weeks or so. But at the end of the season, when the fly numbers have built up and more strikes are liable to occur, the protection period of the chemical may have finished earlier than needed. As a result, more sheep may be struck at the end of the fly season, or more chemical will need to be applied than was necessary. This can end up with a higher cost and more sheep struck than if initial treatment was done a few weeks or so later (with the small number of sheep struck early being treated individually).
To decide on the optimum set time to apply the preventative treatment on your property, use the FlyBoss Optimise Treatment Tool. The Tool uses long-term weather data for your area. However, occasionally the weather will result in a fly season starting earlier, so you must be prepared to monitor and treat earlier in these years (see the threshold method below).
This tool allows you to easily select your situation: location, shearing time etc, to calculate and optimize the best time to apply treatment.
Of course, you may need to apply treatment earlier if labour will not be available at the ideal time, such as being away from the property or when labour is diverted to other jobs such as harvesting of crops.
Ensure treatment times allow you to comply with chemical withholding periods.
Threshold method
On properties where preventative treatments are not required in the majority of years, a threshold method, where treatment occurs once a set amount of strike is occurring, can be used to decide when to treat.
Also, on properties that use the method listed above—treatments applied at a set time each year before significant amounts of strike occur—monitoring prior to this time should still occur, and the threshold method should be applied in the occasional years when a wetter, warmer spring causes earlier strikes.
Threshold level
Once about 0.5% of sheep (1 sheep in 200) are struck in any one week it is almost always most economical to treat the whole mob with a preventative treatment, as the costs of labour for monitoring and treating individuals are very high, as are the costs from deaths and severely struck sheep.
However, you should treat earlier, at a lower strike threshold, if you or your staff cannot check frequently enough to detect and treat struck sheep before the strike is advanced.
In some cases the threshold may be reached, but has been brought about by an isolated rain event. If weather forecasts indicate that wet, warm and humid weather that presents a high flystrike risk is about to cease for a lengthy period, it is likely to be more economic to continue treating affected sheep individually over the next few days, rather than apply a mob treatment.
The other situation where individual treatment should continue is if you are close to shearing or the sale of animals and the various withholding periods for mob treatments will exceed the time until shearing/sale. In this case, if the flystrike risk is severe, consider bringing shearing forward or applying a product with a very short withholding period.
Monitoring frequency and process
With susceptible sheep, monitoring should be carried out every second day during periods of heavy challenge, provided the sheep can be properly observed. Note that body strike is often more difficult to detect because sheep respond less in the early stages (compared with breech strike), so strike can become extensive before detection.
Where the country is steep or timbered or there is otherwise poor access to all sheep, or where lack of labour prevents frequent checking during these heavy-challenge periods, then preventative treatments will be required in advance of expected flystrike.
Flytraps are also a useful tool for determining the timing of preventative treatments.
Monitoring, done properly so that strikes are detected before they are advanced, is time-consuming, but essential on wellbeing and productivity grounds if sheep are at risk.
To identify affected sheep the mob must be held, such as against a fence, while you walk slowly among them or have the sheep drifted by you so that you can see both sides of all sheep—particularly the shoulder area) and the breech.
Advanced strikes are much easier to detect, but come at a major production and animal welfare cost.
If sheep with advanced flystrike are found, increasing your frequency of monitoring will deliver benefits for the wellbeing and productivity of the sheep
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