Monitoring for flystrike

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