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Be on the look-out for blowfly strike; VET'S VIEW.

BLOWFLY are the main ectoparasite affecting sheep in the UK; they cause Strike (Cutaneous Myiasis) which is a serious condition of animals all over the world.

In Britain the disease is primarily caused by Lucilia sp, mainly Lucilia sericata (the greenbottle fly) which are attracted by the odours released by moist skin associated with open wounds, excessive wetting, urine splashing and faecal soiling.

Strike is normally seen in the warmer months of the year, especially when the weather is humid.

In average conditions of humidity and a temperature of 20degC, the life cycle of the greenbottle is completed in about 20 days, faster with higher ambient temperature and humidity.

Initially the maggots may only be present within the wool but as the disease progresses they produce enzymes that break down the skin, enabling them to invade the tissue. This gives rise to wool loss and large open sores with active maggots around the edges of the lesion.

Struck animals are restless, spend less time feeding and move from place to place with their heads held close to the ground. They may continually waggle their tails and bite or kick at 'struck' areas of the body.

The affected areas are moist with a distinct smell, have a brown discolouration and the wool is usually raised from the surface of the skin.

At this stage if the disease goes unrecognised, the temperature, pulse and respiration are increased and the animal may die from septicaemia and toxaemia.

It is a legal requirement to check all lowland and upland sheep daily and if a sheep dies due to blowfly strike, this could lead to prosecution under animal welfare legislation.

Control is usually managed by reducing the main risk factors. Shearing before the main risk period where possible, dagging around the back end at other times - especially if there is faecal staining and adhesion - careful management of wounds to avoid infection and foot trimming to help control footrot/ovine digital dermatitis which can also attract the blowfly.

Confirmation is also a factor and breeding from sheep that are habitually struck should be avoided. The use of Organophosphate dips will aid in the control of strike but with worries over these products, many people now use pour-on preparations; these are either repellents or insect growth regulators which interfere with the development of the maggots.

The pour-on products are formulated to give long-lasting activity and are divided into two effect groups, those designed to prevent strike and those that prevent and treat.

It is important before using these products to check which type they are and the withdrawal times and duration of activity to ensure the product meets your requirements.

Treatment of strike is aimed at killing the maggots, preventing re-infection and assisting wound healing.

The wool should be clipped away and the wound cleaned and maggots removed; drying of the area can also be helpful.

Pour-ons containing synthetic pyrethroids or injections of macrocytic lactones such as ivermectin can help in killing the maggots, and control of bacterial infections can speed healing.

It is important to remember that in hot muggy weather, strike can become a problem much faster than you might think!

Iain R Carrington BVM&S MRCVS
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Publication:The Journal (Newcastle, England)
Date:Jun 10, 2013
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