It’s those blasted blow flies again!

Vet's vision
with Phil Rennie
Tauranga Vets

Two quite different fly species produce most of the blowfly strike in New Zealand. Originally, it was the European Green Blowfly – more recently the Australian Green Blowfly has been the culprit.

The EGB is important because it is a parasite of temperate regions and can survive for most of the year in New Zealand. It attacks susceptible parts of the body, such as wounds, dags and urine-stained skin. It can complete its lifecycle in other food sources, such as road kill. Although the eggs are readily killed by heat and the soil-dwelling pupae cannot hatch below 15 degrees Celsius, these flies can survive in the wide variety of climatic conditions found in New Zealand.

The AGB in contrast does not need an ‘excuse’ to attack. It will lay eggs on a completely clean-fleeced sheep, with no wounds or dags – for instance, ‘shoulder strike’ in a clean sheep. However, the AGB is a tropical fly in origin and cannot survive and complete its lifecycle in such varied climatic conditions as the EGB. Typically, ‘tropical’ moist and warm conditions are required.

Damp wool

These flies normally start breeding in spring and reach epidemic proportions during warm humid weather. This is generally between November and March, although fly season can extend from October-June. The blowflies lay their eggs in the damp wool and maggots can start hatching within a day and then start to eat the sheep’s flesh. Flystrike can occur rapidly, and sheep should be checked at least once-a-day in hot weather.

Best practice management is to have all sheep shorn and with clean bottoms in warmer months. This reduces the attraction for flies, which seek damp smelly wool in which to lay their eggs. Dirty bottoms are usually due to high worm burdens and/or changes in feed quality. Well-nourished animals will cope well with parasites, as their natural immunity is strong. However, malnourished animals will succumb quickly and respond slowly to treatment. Bear in mind shearing alone is effective prevention for a few weeks, although shearing cuts and other wounds will attract flies.

Affected animals can be seen nibbling their wool, stamping their feet and twitching their tails before becoming depressed. Dark patches may also be seen on white fleece. Colonies of maggots may be difficult to find, so the wool should be systematically parted and checked.

Areas most commonly affected by flystrike are around the tail end, on the backline, on the belly where the feet touch when the sheep lies down, around the pizzle (prepuce/penis) in males, and between the ears of rams after fighting and head bashing.

Chemical prevention

Chemical prevention six weeks post-shearing is timely, as the wool length will hold the insecticide. However, it can be applied earlier as required. Earlier treatment approaches involved organophosphate, or OP, dips and showers. But these rapidly became ineffective due to fly resistance. Current treatments involve using high-powered jetting races and Insect Growth Regulators. High-powered jetting races work very well, but if the sheep move through too quickly then the duration of activity may be reduced. IGRs work by interrupting the lifecycle of the maggot stages, preventing them from forming or casting their external skeleton.

In the early-2000s, resistance to some IGR products was identified in parts of the country. Resistance is thought to have developed due to continued use of these sole products for all fly and lice prevention and treatment. While these products are still effective for lice treatment, their use for prevention of fly strike is limited. It is now recommended that to control future resistance we should consider blowflies and lice separately and use different products for prevention and treatment of clinical cases. And please note: Long-acting products tend to have longer meat withhold periods, so for lambs approaching their finishing weights careful attention needs to be paid to the product used.


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