Blowfly strike or maggots is a seasonal disease, which in average years begins in mid-May and continues until first frosts in the autumn.

It involves the infestation of sheep tissue with the larve of the greenbottle, black blowfly and bluebottle. Various products exist to aid in the prevention, and the key determinants of choice remain the longevity of cover and how rain-fast the product is; this is particularly important in a wet year.

Cause

The female blowfly lays clusters of eggs on wounds, soiled fleece (tails) or other areas where an odour of decomposition attracts the fly. Under warm conditions of early summer, the eggs hatch into larvae in about 12 hours. These then feed, grow rapidly and moult twice to become mature maggots in three to 10 days. They then drop to the ground and undergo further change to emerge in three to seven days as adult flies. The female can lay 2,000 to 3,000 eggs in batches of hundreds and they live for approximately 30 days. This allows about four generations between May and September and the final generation overwinters in the soil.

Blowflies fall into two broad categories – primary flies who are capable of starting a strike and secondary flies, which cannot initiate a strike, but act on areas damaged by an existing strike worsening the injury. Factors which increase the prevalence of blowfly strike include:

  • High climatic temperatures – which speed up the life cycle of the blowfly.
  • Humidity and rainfall – can create fleece rot which attracts flies.
  • Host susceptibility – breeds of sheep with long fine wool are particularly susceptible though soiling of hindquarters due to urine or diarrhoea.
  • Injuries – Wounds due to shearing, fighting or wire scratches all attract flies to lay eggs on affected areas.

    • Symptoms

    Affected sheep are anorexic and restless. In the early stages they may run along in short gallops with their head held low accompanied by switching of the tail. The affected area of the fleece appears damp and has a foul odour. Nibbling at the flanks may also be evident.

    This progresses to dullness, separation from the flock and eventually recumbency due to septicaemia which results from secondary infection of the damaged area. On parting the fleece, the maggots become visible and extensive areas of raw skin become obvious on closer examination. Deep lesions may be exuding copious amounts of pus.

    Treatment

    The only effective treatment of an affected case is to fully immerse the affected sheep in fly dip, though some pour-on options also claim efficacy. The animal should also be treated with an effective broad-spectrum antibiotic coupled with an anti-inflammatory for pain relief.

    There is a good argument for clipping the wool from around the infected area. However, such areas, particularly if they are on the back, are subject to sunburn that can retard the healing process. After treating one or two animals in the flock, the best management practice that can be advocated is to immediately take preventative action on the rest of the flock.

    Prevention and control

    In order to control blowfly strike it is critically important that farmers understand the difference between a product that is licensed for treatment and prevention as opposed to a product that is only licensed for prevention of blowfly strike. This difference determines the optimum time for product usage.

    There are numerous ways to aid in the prevention of blowfly such as:

  • • organophosphate dips which are very effective at treating and preventing blowfly strike. Plunge dipping ensures whole body protection. In practice, in a dry year, one dipping in June may carry the flock through the season but with wet summers redipping has been necessary in mid-August and some are finding three sessions necessary.
  • • Synthetic pyrethroid dips are products that have not been used much, however they are claiming similar efficacy to the organophosphate dips version without the safety risks.

    • Before dipping it is wise to at very least dag the sheep to ensure improved efficacy. Where sheep are shorn it is advisable to allow at least three weeks of wool re-growth between shearing and dipping. During dipping, care should be taken with very young lambs and stress should be minimised. Care should also be taken from an operator viewpoint to ensure totally waterproof clothing and face shields are used and exposed skin should be washed immediately.

    Sprays are also an option for farmers and, just as with dips, it may be advisable to leave a few weeks between shearing and spraying.

    Other control measures include prevention of diarrhoea by regular dosing and the prompt removal of carcases (which act as a breeding ground for the fly) is also helpful.