What is Blowfly Strike in Sheep?

Blowfly strike is a common annual battle that sheep farmers must face, particularly during the summer months. When the weather is warm and wet, it creates optimum conditions for flies and maggots to thrive.

http://lovegodlovelife.org.uk/author/admin/ What causes blowfly strike?

Green bottle flies are responsible for the primary incidence of blowfly strike.
Green bottle flies are responsible for the primary incidence of blowfly strike.

Blowfly strike is caused by the larvae of dipteran flies. They invade the skin of living sheep, causing lesions that can lead to tissue and muscle damage. The primary flies (green bottles) strike first, laying their eggs on soiled fleece or wounds. After about 12 hours, the eggs hatch into larvae. These feed on the skin of the animal and on faeces on the wool. Mature maggots will then drop off the animal and pupate, emerging as flies after about 3 to 7 days.

After the primary flies have attacked, secondary flies (black bottle and bluebottle flies) will then attack areas that have already been targeted. These flies are attracted to the odour given off from the primary strike. As a result, attacks often happen in waves. In optimum conditions, the whole cycle from egg to mature fly can take less than a week. With an adult fly able to lay over 1000 eggs, this means cases can get out of hand very quickly.

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Regular observation of a flock is the best way to notice blowfly strike in individuals. Sheep affected by blowfly are often agitated and display behaviours such as kicking and nibbling at the affected area. These individuals are also likely to be isolated from the rest of the flock and might not show normal grazing behaviour. An infected animal will have discolouration of the fleece. On closer inspection, you will probably notice many flies around the area and maggots on the skin. A strong unpleasant smell is almost always extremely noticeable.

buy Lyrical dance costumes online What are the impacts on my flock?

Blowfly strike has a significant impact on the productivity of a flock. It interferes with grazing patterns, and when left untreated sheep can rapidly lose weight. Secondary bacterial infections can occur as a result of blowfly strike, which can then lead to death via septicaemia. Blowfly issues cause significant costs in both time and money for farmers. This is due to the potential loss of livestock and the costs of treating cases.

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Thick fleece can stay damp and dirty for longer, attracting flies.
Thick fleece can stay damp and dirty for longer, attracting flies.

The most common area for blowfly strike is the area surrounding the tail. When this becomes damp and soiled with urine and faeces, it provides a perfect habitat for the flies. Therefore, it is important to prevent diarrhoea from occurring, especially through good parasite control measures. Management techniques such as dagging, crutching and shearing will also help limit blowfly strike. Shorter wool around the tail area, for example, is less likely to become as soiled as longer wool. Strikes can also happen on the feet, so good foot management and monitoring to prevent incidences of foot rot is also important.

There are some chemical treatments, particularly larval growth inhibitors that prevent any maggots from developing. However, these management techniques are not a substitute for daily observation of the animals. Warm and wet weather are the ideal conditions for flies and maggots. Summer months, therefore, are when flocks are most at risk so particular care should be taken at these times to monitor health. By observing sheep grazing, you are more likely to spot abnormal behaviour. If you are gather the sheep into a group, it can obscure abnormal behaviour and make spotting signs of blowfly strike more difficult.


Wall, R., & Lovatt, F. (2015). In Practice37(4), 181-188.

Wall, R. et al. (2011) Veterinary Parasitology. 180(1-2):82-89.