Fly Strike is also known as Myiasis.
There are two types of Fly strike.
1. One is caused by opportunistic fly larvae i.e. those which are usually free living but when the opportunity arises can adapt themselves to a parasitic dependence on a host. These include house flies, blow flies and flesh flies.
2. The other is caused by obligatory fly larvae, which are completely dependent on a host in order to complete their life cycle and without which they would die. These include the New World Screwworm Fly and the Old World Screwworm Fly, which is found in Africa. Screwworms are so called because the larvae have a "wood screw" shape. Screwworms tend to target cattle.
All animals get Fly Strike but sheep get it worst and most often, usually due to attack by opportunistic fly larvae. Humans are sometimes affected by fly strike.
Mode of spread
Adult flies are attracted to moist wounds, skin lesions or a soiled hair coat. Young animals with softer skin and sheep with skin folding or wrinkling around the thighs, back or tail are susceptible. Castration wounds, docking wounds and wounds around the head are favoured sites for fly strike.
Eggs, usually laid below the tips of the fleece, hatch within 24 hours if conditions are moist. Warm, humid weather favours fly strike and by far the most common site affected is the breech, because of soiling and irritation of the skin by urine in this area. Moisture, and nutrients from serum, faeces etc are necessary for survival of first stage larvae.
Opportunistic larvae move over the skin surface, ingesting dead cells, exudate secretions and debris, but not live tissue. The larvae irritate, injure and kill successive layers of skin and produce exudates. The maggots can then tunnel through the thinned top layer of the skin and go deeper, producing tissue cavities in the skin which may be several centimeters in diameter.
Advanced wounds may contain several thousand maggots and the animal may die of shock, toxaemia and infection.
Second stage larvae can scrape the skin with their mouth hooks to obtain food. Once established, Fly Strikes can spread rapidly and attract more flies. Mild Strikes can cause rapid loss of condition. Bad Strikes can cause death from shock, toxaemia or infection.
Signs of Fly strike
- Affected animals become depressed, stand with their heads down, do not feed, and attempt to bite the infested areas.
- Sheep are restless, wriggle their tails continuously and move about from place to place.
- There may be an obvious smell.
- The wool may be lifted slightly above the surrounding wool.
- The affected wool is moist and usually brown in colour.
- In the early stages larvae may be found in pockets in the wool before they have reached the skin.
Prevention and control
Prevention is by clipping the wool around the crutch, docking the tails of lambs to lessen faecal contamination, controlling scouring and reduction in fly numbers by burying carcasses and destruction of fly breeding areas, and the application of insecticides and larvaecides to the skin or fleece by dipping or jetting. All skin wounds should be promptly treated and the wool clipped away from the affected area.
Screwworms have been controlled in a number of countries by the release of irradiated sterile male flies. The female mates only once, and when mated with a sterile male lays eggs that do not hatch. The sterile males are able to mate and the release of sufficient sterile males in an area will lead to eradication.
Treatment involves killing the larvae in the skin lesions, clipping the hair or wool, removing as many larvae as possible, applying an insecticide to kill larvae and to prevent secondary re-infestation, treating wounds promptly and effectively and providing an environment hostile to flies.
Ivermectin at dosages of 50, 100 and 300 micrograms per kg to infested cattle resulted in 100% larval mortality for at least 6, 12 and 14 days respectively.