Flystrike is when a blowfly lays eggs on the skin of the sheep and the emerging larvae create an open wound as they feed on the underlying skin tissue. Depending on conditions for the larvae, flystrike can quickly become fatal or result in a slow and painful demise (AWI, 2019).
The Australian sheep blowfly (Lucilia cuprina) is responsible for initiating over 90% of all flystrike in Australia. The sheep blowfly is metallic green/bronze in colour and as an adult can reach 9mm in body length, and produces a smooth skinned white maggot. Adult flies usually live for 2 – 3 weeks (NSW DPI, 2016).
The blowfly larvae pupate emerges after winter when temperatures are optimal, usually when soil temperatures increases (>15°C). The male is sexually mature at emergence, while the females require a protein meal for her reproductive organs to mature. This protein meal is generally manure, carcasses, weeping skin or existing flystrike (DPIRD WA). Once mated, the females then search for susceptible sheep, particularly those with odour and/or fleece rot damage in damp fleece to lay their eggs. Lucilia can mature up to 200 - 300 eggs in each ovarian cycle and lay a new batch of eggs every 4–8 days.
The eggs hatch after 8–24 hours and the maggots immediately start feeding on the sheep, moulting twice during the 3–5 days before they drop off and burrow into the ground to pupate, usually at night or in the early morning. This means that a large proportion of blowflies will emerge around sheep camps. The normal blowfly life cycle in warm weather takes 2.5–3 weeks (FlyBoss).
So when are sheep most at risk?
Flystrike in sheep is mainly dependant on environmental conditions and the susceptibility of the flock.
The perfect environmental conditions for flystrike tends to be:
· The presence of primary fly species (Australian blowfly)
· Optimal temperatures between 15 – 38°C
· Warm, humid weather
· Recent rain - enough to keep suitable sites on the sheep moist for around 3 days
· Suitable sites for fly strike
· Wind speeds below 9 km/h as this gives flies the best opportunity to disperse
Blowflies are particular about where they lay their eggs, and will only lay on susceptible sheep. Newly hatched maggots require a moist protein source to survive, so if it is not present the sheep will not be struck. Flies are attracted to:
· Urine or faecal stained wool around the crutch (breech/crutch strike)
· Moist fleece infected with fleece rot or lumpy wool (body strike)
· Urine stained wool causing weeping around the pizzle (pizzle rot)
· Wounds from fighting injuries or sweat around the horns (poll strike)
· Any wound regardless of type or size that provides suitable protein when moist (wound strike)
Due to the struck areas tending to have high amounts of a moist protein source, they become increasingly attractive to blowflies. This leads to further egg laying or secondary strike by the hairy maggot blowfly. This secondary strike causes rapid extension of the wound and is commonly associated with illness and death in struck sheep (NSW DPI, 2008).
Preventing flystrike
There are many ways in which flystrike can be prevented and managed, this is dependent on the producer. Prevention can be broken down into two categories, short term and longer term prevention.
Short term prevention includes:
· Strategic chemical application
· Crutching
· Shearing
· Controlling worms and dags
· Correct tail length
While long term prevention includes:
· Breeding to reduce flystrike susceptibility
o Select rams with low wrinkle, low dag and low breech cover ASBV's
o Assess ewes for fleece rot, wrinkle, dags and breech cover
o Develop a breeding strategy
o Assess your lambs and make a decision about mulesing and hogget classing
Flystrike prevention is not just about applying chemical. We urge producers to consider using the Flyboss 'flystrike decision support tools' when deciding how to approach flystrike on their farm.
References
AWI: Managing breech fly strike
NSW DPI (2008) Primefacts: Triggers leading to blowfly strike
NSW DPI (2016) Primefacts: Sheep Blowflies
