Flystrike in sheep

Flystrike is a condition that is caused by flies laying eggs in the flesh of sheep. It can, if untreated, lead to the death of the sheep. The eggs hatch, producing hundreds of small maggots. These insects feed on the flesh of the sheep, causing slow and probably painful blood poisoning.

The most common fly species that causes flystrike (in Australia, at least) is Lucilia cuprina, the Australian sheep blowfly. Flystrike comes about due to the nesting characteristics of flies. Their eggs are given a much greater chance of survival if they are layed somewhere warm and moist. Sheep provide these nesting spots in a few different ways.

The most common is through urine and fecal matter being caught in long wool around the sheep's back end. This creates the moistness, while the sheep's body heat contributes the warmth.

Another common nesting spot on a sheep is in weak or broken parts of the sheep's skin. This can be caused from skin conditions such as dermatitis, or from simple open wounds, caused by factors such as predatory attacks (e.g. foxes, dogs) or cuts inflicted while the sheep was shorn.

The "deadly double" for flystrike attack is simply moistness and warmth. Any factor or combination of factors that provide this environment contribute to flystrike on a sheep.

Flystrike can be prevented using chemical protection in the form of insecticides. Products such as Vetrazin are examples of this. Application is most often in the form of a liquid spray to susceptible areas, namely the rear end, plus any other patches the farmer identifies as areas of risk. Chemical protection can be undesirable, however, around shearing (wool harvesting) time, as it contaminates the wool.

Another common preventative measure is crutching, which involves removing long wool from around the anus area and down the back legs of the sheep. This stops urine and faeces from getting caught up on the animal.

Flystruck sheep are mainly identified by flock inspection. The farmer drives around his flock, and spots any weak or slow sheep. Flystrike usually looks like a darker, maybe even black, spot on the sheep. The sheep is then run down and caught, and treatment applied.

The most popular treatment of flystrike on Australian farms is to remove the fleece from the affected area and treat the sheep with a chemical product. The fleece is removed using shears; either hand shears or their mechanical equivalent. This removes many of the maggots as well. Insecticides are then applied, to kill the eggs and maggots, and to prevent further spread of the flystrike.

This treatment can be undertaken in the paddock if the flystrike is in its early stages or is not particularly nasty. If it is well developed, though, the sheep is often brought back to the shed where it is treated, and observed closely.

Flystrike is a common and annoying problem in Australian sheep flocks. The smell of it is nearly as bad as the problem itself. It smells like dead sheep. I think it's the rotting flesh that gives it that charming characteristic.

References:
1. Agriculture W.A. Farmnote(s) 28/97, 72/94 and 3/95
2. Personal experience.