What is Blowfly Strike in Sheep?

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

What is Blowfly Strike?

Blowfly strike is caused by the larvae of dipteran flies which invade the tissue on living sheep causing lesions that can lead to tissue and muscle damage. The primary flies (green bottles) strike first laying their eggs on soiled fleeces or wounds. After approximately 12 hours the eggs will hatch into larvae which feed on the skin of the animal and 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 therefore meaning attacks often happen in waves. In optimum conditions the whole cycle from egg to mature fly can take less than a week, and with an adult fly being able to lay over 1000 eggs, means cases can get out of hand very quickly.

How do I know if any of my flock is suffering from blowfly strike?

Regular observation of a flock is the best way of noticing blowfly strike in individuals. Sheep affected by blowfly are often observed to be 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 may not be partaking in normal grazing behaviours. The animal will have discolouration of the fleece and on closer inspection you will notice many flies around the area and maggots on the skin. A strong unpleasant smell will most likely be present.

What are the impacts of blowfly strike on my flock?

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

How can I protect my sheep from blowfly strike?

The most common area that strikes happen is the area surrounding the tail as this can become damp and soiled with urine and faeces. Therefore it is important for a stockperson to prevent diarrhoea from occurring through good parasite control measures. Management techniques such as dagging, crutching and shearing will also help limit the creation of the optimum strike sites on your animals, for example shorter wool around the tail area is less likely to become as quickly soiled as longer wool is. Strikes can also happen on the feet, therefore good foot management and monitoring to prevent incidences of foot rot is also important. There are some chemical formulas, particularly larval growth inhibitors that prevent any maggots from developing, that can be utilised 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 and therefore the summer months are when flocks are most at risk so particular care should be taken at these times to monitor the health of a flock. By observing sheep grazing you are more likely to spot abnormal behaviour than if you are to gather the sheep into a group as this can obscure abnormal behaviour.