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Blowfly strike in sheep can lead to these pests practically eating the animals alive. Fortunately, blowfly strike is preventable and everything possible should be done to keep open sores from forming on animals. Ewes and lambs are often the victims.
The biggest concern on wool sheep farms is the green bottle fly. The metallic coloured sheep blowfly belongs to the genera Lucilia and Chrysomyia, and are prevalent throughout South Africa. The green blowfly L. cuprina is responsible for around 90% of strikes on live animals; C. chloropyga most likely accounts for the remaining 10% of strikes. In contrast to other blowflies which complete their life cycle on or in areas close to a carcass, the green bottle fly completes most of its life cycle on live sheep.
The perfect environment
Blowfly strike is most prevalent during wet, hot times of the year, and the damage can be extreme. The adult blowfly female requires protein in order to lay eggs. Manure, carcasses, and the nectar of certain plants all serve as sources of protein. The adult female can live for up to a month, laying eggs two to three times during this period (up to 200 eggs can be laid at a time).
In almost all cases, moisture trapped in the sheep’s wool (fleece rot) is the lure preceding a blowfly strike. Blowflies typically target the breech area, the area around the anus and genitals of ewes and, to a lesser extent, other parts of the body.
In rams, the horn base is prone to blowfly strike. Shearing and other wounds, footrot, and watery eyes can also attract blowflies. These areas are perfect for maggots to hatch and start feeding. The maggots produce ammonia which is toxic to the animal. Affected sheep are lethargic, they stop feeding, develop a fever as well as secondary bacterial infections and, if animals are not treated, they usually die.
A vicious cycle
Wool that is wet to the skin is the main cause of fleece rot. This causes the skin to become inflamed and leak serum which attracts blowflies. This marks the start of the vicious cycle of females laying eggs on sheep and the process repeating itself.
Matted wool, a condition caused by a bacterium in the skin, also creates a favourable environment for females to lay their eggs. The maggots hatch in the matted wool, where they proceed to feed and burrow deeper into the skin. Wool that is contaminated with urine or runny manure also lures blowflies.
Hands-on management
Effective management practices go a long way in curbing blowfly strikes. A management plan must take into account the lambing season, time of shearing and the season (rainfall and temperature). Management tips include:
- Select for sheep with fewer folds and without sloped hindquarters, devil’s grip (depression behind the shoulder blades) or high, loose shoulders.
- Shear and crutch especially ewes so that their breech wool is short in spring, summer and autumn, as well as during the lambing season.
- Crutching ewes and shearing the areas around rams’ horns, and the testicles of rams and wethers, can be done simultaneously if these areas are prone to strikes.
It is possible to control sheep blowflies to a great extent on South African farms using blowfly traps and pesticides. The application of chemical agents can limit the number of strikes in a sheep flock. However, refrain from using pesticides in the three months leading up to the shearing period, except for emergency treatment of individual sheep.
Treating affected sheep takes time and requires additional labour. Implementing preventive measures is therefore necessary. An infected sheep is also a welfare problem. Sheep can be dipped with registered chemicals that can kill maggots and control ticks. There are regions in which blowflies have already started building resistance to some of the active ingredients in dips.
There is a single injectable endectocide registered for the treatment of blowfly strike. The agent works via the blood and kills all larvae that feed on blood components. It may also be used for the short-term prevention and control of roundworms in order to prevent diarrhoea. Ask a veterinarian to assist you in formulating a treatment programme. – Andries Gouws, Stockfarm Magazine
For more information, contact Dr Danie Odendaal at vnet1@absamail.co.za.
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