Summer, Rainfall and Blowflies – How susceptible are your sheep?

Martin Preuss, Senior Land Services Officer - Livestock

Summer, rainfall, and blowflies seems to be three words that goes together at this time of year regarding blowfly strike in sheep and with the above average rainfalls caused by the La-Nina across Southern NSW, the Riverina is facing one of the worst blowfly strike seasons it has seen in years.

As we know the blowfly is potentially the greatest hazard to Australian sheep. The blowfly is not a problem until it lays eggs on the sheep, which develop into maggots on the skin, a condition called myaisis or better known as flystrike.

Flystrike causes skin damage where the sheep suffers shock and fever from tissue damage and fleece shedding with loss of productivity and infertility in rams, and if flystrike is large enough, at worst the ewe, lamb or ram may die.

Country more common with fly-struck sheep is around watercourses, either on plains or mountain areas, rather then open plains with low grass or paddocks that are bare or cultivated. When the climate favours hatching and survival of adult flies, such as now, susceptible sheep should be kept away from low lying watercourse country.

Due to this, the preferred control strategy is to reduce susceptibility of sheep to blowfly strike by reducing the attraction of female blowflies to living sheep.

Although myiasis is the same, a variety of flystrike types can be described:

  • Covert strikes – are rarely seen unless all sheep are carefully examined. Small fly strikes will be found around the body orifices or on the body, which are seldom seen at paddock inspection. These strikes usually do not spread and the larvae usually leave the sheep.
  • Overt Strikes – are those usually seen in sheep on inspection. They may be divided into breech, body strike and other areas. If left untreated, they spread and become more serious as more flies lay eggs on the infected area.
  • Breech Strikes – are the common form, which is more prevalent in sheep soiled by faeces and urine where blow flies lay eggs and larvae hatch and feed.
  • Body Strikes – are the major hazard when rain and hot weather favour emergence of large numbers of adult flies from pupae ion the soil and concurrently, the development of fleece-rot in younger sheep.

Long term control aims at breeding sheep that do not have breech wrinkles. Also, lifetime protection can be aided by leaving the tail long enough to cover bare area under the tail. Temporary measures include treatment of sheep against internal parasite to reduce diarrhoea and breech spoiling, plus removal of wool by crutching or shearing, or application of insecticides to kill larvae (maggots) hatching by flies laying eggs.

For further information on how to manage blowfly strike: please visit the following websites:

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