Levels of drench resistance of sheep worms in WA are some of the highest in the world and without resistance management action at individual farm level, it will continue to worsen and spread. Although effective drench options are generally available, many sheep owners are unaware that resistance has reduced the effectiveness of worm control programs and this can affect profitable sheep production.
There are two main groups of gastro-intestinal worms that affect sheep in WA.
Firstly, scour worms — including black scour worm (Trichostrongylus), brown stomach worm (Teladorsagia (Ostertagia)) and to a lesser extent, the thin-necked intestinal worm (Nematodirus) and large mouthed bowel worm (Chabertia ovina). These occur throughout the agricultural areas of WA and larvae are abundant on pastures in winter and spring. Signs of scour worm infection include ill thrift, diarrhoea and in severe cases, death. However, significant production losses (decreased liveweight gains and wool growth rates) usually occur before clinical signs become obvious.
Secondly, barbers pole worm (Haemonchus contortus) occurs mainly in areas that have significant rain during warm weather and pastures with areas that remain green over summer. Larvae are most abundant on pasture in autumn and spring and after significant summer rainfall. This worm sucks blood from the sheep and can cause anaemia (visible as pale mucous membranes of the gums and around the eyes), subcutaneous oedema (bottle jaw) and sheep deaths with little warning, in certain environmental conditions. Barbers pole worm infections do not cause sheep to scour.
Tapeworms (Moniezia) are also common in young sheep but there is little scientific evidence that tapeworm infection significantly affects sheep production and control measures are not considered necessary.
The worm life cycle
Adult worms of each of the species are found in a specific location within the sheep — black scour worm in the small intestine and barbers pole and brown stomach worms in the abomasum, or fourth stomach. Male and female adult worms at this location mate and the females lay eggs, which pass out of the sheep’s gut and onto the paddock in the sheep’s faeces.
To complete the life cycle, eggs in the faeces hatch to release first-stage larvae. These develop through two stages over several days to become infective third-stage larvae (L3s). Infective larvae move from faecal pellets to pasture plants, with most L3s located up to about 25 millimetres (mm) above the ground. If grazing sheep ingest them, L3s develop through further stages inside the sheep to become adult worms within about three weeks.
Development of parasites on the paddock depends on environmental conditions and the type of worm. Barbers pole worm thrives in warm, wet conditions, for example, in areas which experience regular summer rainfall. Black scour worms and brown stomach worms are generally present in largest numbers in the autumn, winter and spring in a typical Mediterranean climate and therefore occur throughout WA.
Larvae survive on pasture for long periods when temperature and moisture conditions are favourable. In winter, some larvae may survive upwards of four months, while in summer, with hot and dry conditions, most larvae are destroyed within a few weeks. In areas with relatively mild summers, small numbers of larvae will survive over summer, protected in dung pellets and can emerge later to infect grazing sheep when conditions improve, such as after autumn rains.