by Bas Van Luijk,
Bas is the Head Veterinarian at East Coast Farm Vets
Flystrike is a serious disease that affects many sheep in New
Zealand every summer. Anyone who has had animals with flystrike will know how awful this disease is.
It is, therefore, a concern both from an economic and animal welfare perspective. Flystrike is particularly common in the warm and wet months of Summer and Autumn between November and April. In NZ, four different blowfly species contribute to this disease.
Sheep that have long unshorn fleece, especially prior to summer, are most at risk. Blowflies are attracted to the foul odour that is emitted from urine stained and dirty areas of the fleece, laying eggs directly on these damp warm areas. Once the eggs hatch, maggots are released, which bury into the sheep’s tissues so that they can eat flesh as a food source. During the feeding process, maggots cause extensive tissue damage and also release toxic ammonia secretions. Once flystrike has been initiated, further flies are attracted to the damaged area, and the sheep can die from ammonia poisoning as quickly as 2-6 days.
Early signs of flystrike can be very difficult to observe. Sheep may appear restless, nibble at areas of the body, exhibit shade-seeking behaviour or stamp their feet. Sometimes affected sheep will not show any signs until a significant number of maggots are present and have caused the animal to feel uncomfortable. Only at this stage will the wool fall out, the fleece become blackened or sores become visible.
Immediate treatment is required for any sheep with active flystrike. Remove the wool from around the area to be treated using clippers or shears. Apply an effective chemical which will kill maggots and prevent further flystrike.
The extensive tissue damage involved can also cause substantial pain, distress and infection, leading to toxaemia. A vet should be consulted so that the animal can be adequately treated with pain relief medication, anti-inflammatories and antibiotics. Any affected animals will need to be checked daily to ensure they do not become fly-struck again. Any flystrike sores need to be treated as early as possible. Wounds that are extensive or deep can be difficult to treat and in severe cases, euthanasia is often the only humane option remaining.
Shearing sheep and lambs is key in preventing this disease. Shearing should be done twice a year, in May/June, before the weather gets wet and again in November/December before the heat of summer. Shearing sheep regularly helps to remove dags and prevents the wool from getting wet and dirty.
Unfortunately, despite shearing, there is one type of blowfly (Australian green blowfly) that can strike even relatively clean and well-shorn areas on sheep. Checking areas of the skin along the back or around the poll and ears regularly is recommended in summer.