Internal parasites are ready to infect ewes and lambs despite limited soil moisture at the start of winter. Mild conditions in April will ensure that worm larvae deposited in dung during late summer / autumn will emerge and infect animals grazing short green and dead feed. Worm larval emergence escalates around three weeks after significant autumn/winter rain or a month earlier on irrigated pasture.
Late pregnant ewes have lowered immunity and so are unusually susceptible to worm uptake and development. A pre-lambing drench is often given to protect against this development unless worm monitoring (faecal worm egg count or WEC) in the lead-up to lambing indicate a low worm risk.
In contrast, adult dry sheep grazing worm contaminated pastures acquire immunity through long term exposure to worm larvae and are able to expel 90+% of the larvae consumed before they develop into adult egg laying worms. This usually has two outcomes: low to moderate worm burdens through much of the year; and winter scours in some of the mob due to gut irritation caused by the immune rejection of larvae.
Lambs are born with variable immunity to worms. Some are able to suppress worm development – a genetic trait that can be selected for especially in ram breeding programs. Others have little or no ability to resist worm development and become heavily infected on ""wormy"" pastures. A worm drench at weaning is always advised unless WEC pre-weaning indicates the absence of worms.
Paddock preparation for weaning should begin in autumn to ensure there are low worm risk pastures for grazing the lambs after weaning. Low worm risk means these paddocks are only grazing by cattle or adult dry sheep for 2 months (autumn lambing) to 4 months (winter lambing) before weaning to ensure negligible worm contamination.
Worm monitoring is the best tool for assessing the need to drench rather than the often costly and wasteful strategy of drenching whenever sheep are ‘in the yards’. A WEC as low as 100 worm eggs per gram of faeces may indicate a need to drench on dry feed, while a count over 200 may be the trigger for drenching on green feed. Consult your animal health advisor to discuss local recommendations.
Apart from the usual winter scour worms (""Trich/Ost"") Barbers Pole worm (Haemonchus spp) can also cause deaths and production loss in some areas that commonly receive more than 450 mm annual rainfall. Flocks in these at risk areas need to monitor WEC monthly especially during summer/autumn. In other worm risk areas WEC monitoring every 4-6 weeks in young sheep and 8-12 weeks in adult sheep is recommended.
Note: there are several other infectious and nutritional diseases of sheep that can cause deaths and scours especially during winter and so always monitor or seek advice before drenching.
For further details contact Dr Colin Trengove on 0418 808 045 or refer to WormBoss
Written by Colin Trengrove
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