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Covert strikes - the hidden danger

by Maxine Murphy

November 2016

Sheep advisors are warning of increasing incidence of flystrike in flocks over the coming months and have advised producers to be ever watchful for signs of flystrike infestation. 

Graziers often see strikes during mustering or yarding sheep because these strikes, ‘Overt Strikes’, are conspicuous enough to be seen. Covert strikes on the other hand, are not conspicuous and are detected only by detailed inspection of individual sheep

Covert strikes are common. Wardhaugh and Dallwitz (1984) during a 2-year monitoring program on properties in southern New South Wales found the most common sites of covert strikes were the pizzle and crutch. Covert strikes occurred on 18 of the 25 properties they surveyed and were 5 and 14 times more frequent than overt strikes during the 2 year period of monitoring. 

The concern from their findings was that covert strikes were far less likely to be treated by graziers than overt strikes, and that covert strikes may be a source of further generations of flies because many strikes remained active for 1–2 months.

What sheep are at risk of strike?

  • Young sheep need to be monitored more closely than adults because sheep become less susceptible to flystrike with age.
  • Ewes and wethers differ in susceptibility depending on the circumstances.
    • If scouring occurs, then wethers may be at greater risk as they have a smaller bare area.
    • If scouring is not present, then ewes may have a greater risk of breech strike because of urine stain, wethers of pizzle strike, and rams of poll strike.

Further references:

Wardhaugh, K.G. (2001) The biology and ecology of the Australian sheep blowfly, Lucilia cuprina (Wiedemann) Proceedings of the FLICS Conference, Launceston, June 2001.

Horton B and L. Hogan. (2010). FlyBoss: a web-based flystrike information and decision support system. Animal Production Science, 50, 1069–1076.

Wardhaugh KG and R Dallwitz. 1984.  Wool technology and Sheep Breeding. March/April, 15-19.