Fly Strike

Fly strike, or Blow-fly strike, (Myiasis) is a condition caused by maggots living on the skin and in the fur of animals. Sheep are most commonly attacked by fly strike around the rear end where the wool becomes soiled by faeces and urine. This may result from diarrhoea caused by sudden changes in diet, or illness. Fly strike can be fatal. Fly strike can occur on any animal, and also in humans.

What is Fly Strike Symptoms Treatment and Prevention References and Further Reading

Fly strike is predominantly caused by the greenbottle (Lucilia sericata) and related fly species which lay eggs on the living sheep - or any other mammal. Fly strike may occur on any part of the body where the wool or fur has become soiled or infected with bacteria. Fly strike is a common problem in sheep. In goats it is less common than in sheep, largely because goats tend to have less fibre around their tails and close in to the anus. Fly strike can also be a common problem in rabbits, especially where they are kept in closely confined conditions or small cages in warm and humid environments.

Castration and tail wounds, clipping wounds, head wounds on fighting rams and any open wound will attract flies (wound strike), particularly if infected by bacteria. The smell of foot rot is particularly attractive to flies, and especially blowflies. When an animal with foot rot lies down, the skin and wool or fur over the chest wall becomes contaminated from the infected foot, and this area can then serve as a attractant to the flies.

Any animal lying down and urinating without standing, as a result of some other disease (e.g. pregnancy toxaemia) or physical injury, is particularly vulnerable.

Fly eggs, once laid, hatch within a few hours or days. The emerging larvae or maggots are especially active. They secrete enzymes which liquefy the skin and flesh of the animal on which they are feeding. Larvae feed for a variable time period depending on the availability of food. The mature maggots crawl off the fleece and pupate in the soil. In suitable weather conditions it can take less than a week for the adult fly to emerge. The adult flies are capable of laying thousands of eggs, and a population of blowflies can increase rapidly suitable weather conditions.

Infestation of an animal by maggots in areas of thick hair or wool as well as on nearby skin, usually when the area is constantly wet or fouled by faeces and/or urine.

Animals suffering from fly strike show obvious signs of distress. They spend less time grazing and more time rubbing the affected areas and biting those areas of the wool or fur that they can reach.

If signs go unrecognised and secondary strike occurs, the wounds can become very extensive. Bacterial infection of these wounds may lead to death from septicaemia and toxaemia.

There is often a foul smell from the wound and visible signs of maggots. In severe and neglected cases lumps of flesh may come away in your hands when the animal is handled.

In smaller animals such as rabbits, symptoms also include lethargy, seizures and shock from maggot infestation.

The maggots are usually larvae of flies belonging to the genera Phaenicia, Phormia, Chrysomyia, or Calliphora (greenbottles, blackbottles, bluebottles).

Treatment of fly strike should aim to kill any maggots present, and prevent the likelihood of further fly strike and assist the wound to heal. The wool should be carefully clipped away from around the wound and surrounding area. A suitable insecticidal cream should be applied to the infected areas. Mild cases should heal quickly with correct treatment.

A method of treatment that can be used with smaller livestock, such as rabbits, is to submerge the infected part of the body in water and wash the maggots off. Moisture attracts more flies so make sure that the area is dried well after washing off all maggots and any faeces adhering to the fur. Individual maggots can also be picked off by hand. Keeping rabbits in a clean living area is the best method of prevention.

It is also important to catch the problem early and to prevent any fly maggots leaving the animal and pupating in the soil.

Routine dagging should be carried out. This is the removal of excess and soiled fur or wool - usually in the region of the tail - and in wool sheep is usually carried out with the aid of dagging shears.

Insecticidal sprays or pour-ons can be used as a preventative measure during high risk periods.