Fly strike (myiasis) was not a problem in the early days of sheep farming in New Zealand. Flies were a nuisance for people and there are plenty of stories of woollen blankets and socks crawling with maggots after being blown by flies. The first reported case of fly strike in sheep is from a South Canterbury farm in 1881. A leading English expert recommended applying coarse whale oil, as the flies would be put off by the strong smell.
Flies are attracted to lay their eggs in dirty, urine- or dung-stained wool, and on wounds. The emerging maggots eat the flesh of the living sheep. If untreated, the sheep will die from secondary complications such as septicaemia or toxaemia. Fly strike can be prevented by crutching to remove dirty wool from the sheep’s tail, and by dipping.
Lamb tailing or docking
Lambs’ tails are removed, generally at an early age, to reduce faecal contamination of this area and reduce the risk of fly strike. The most common and humane method is to put a rubber ring around the tail, or it can be cauterised or cut off. Ram lambs are usually castrated at the same time.
The sheep louse (Bovicola ovis) spends its entire life cycle on the sheep. Lice numbers can build up to create a heavy infestation in autumn and winter. Infected sheep can be seen rubbing against fences and often leave behind tags of wool. A lice infestation downgrades the quality of wool and the pelt. The best way to control lice is by dipping all the sheep on the farm three to four weeks after shearing, or by using a pour-on dip immediately after shearing.
The sheep ked (Melophagus ovinus) is often called a tick, but is a wingless, bloodsucking fly. Its entire life cycle is spent on a sheep, and if dislodged it can survive only about four days. Blood loss from a heavy infestation of keds can cause anaemia in young lambs and reduced production in older sheep. Reduced capillary flow to the skin lowers the quality and quantity of wool, while ked faeces and pupae give it a dirty appearance. Ked numbers build up in cool weather in full-woolled sheep.
Keds can be controlled using a pour-on dip immediately after shearing or a saturation dip three to four weeks later. This parasite has largely been controlled by the annual dipping for lice that was compulsory until the 1980s, and is unlikely to be a problem in the future.
Facial eczema is a serious disease characterised by photosensitisation (sunburn). It is caused by the fungus Pithomyces chartarum, which develops on pasture and produces a toxin (sporidesmin) that damages a sheep’s liver. Spore numbers increase in warm, humid weather in late summer and autumn in the North Island, and occasionally in the top third of the South Island. Spores in pasture and sheep can be counted to assess the risk of disease.
A pasture fungus (Fusarium sp.) capable of producing an oestrogenic compound, zearalenone, is found throughout the country and can cause infertility in ewes.
These include scabby mouth, a virus generally spread after thistle prickles or other trauma has irritated skin and lips, and periodontal disease, which results in loose teeth, especially incisors.