Lewis Kahn, Associate Professor, Animal Science, University of New England, Armidale
Adult sheep typically have reasonable levels of immunity to most types of round worm with their greatest struggle often being immunity against Barber’s Pole worm. A reasonable level of immunity will mean that no more than 5% of infective larvae are able to establish as adult worms in the sheep. For the lambing ewe, this situation starts to change as early as three weeks before lambing when ewes start to lose immunity. Loss of immunity gets continually worse over the next 8-12 weeks (till lamb marking or a little after) which makes lambing ewes highly susceptible to worm infection. Ewes that rear twin lambs suffer an even greater loss of immunity. During this time 30-40% of infective larvae can establish as adult worms causing high worm egg counts in the ewes, lower milk production and higher worm challenge for lambs.
Over the years a number of reasons have been put forward to account for the loss of immunity in the lambing ewe. Research has demonstrated that rather than hormonal changes associated with lambing and lactation it is the supply of protein and energy that is most important for maintaining immunity. The energy and protein requirement of the lambing ewes increases two and threefold respectively in the 4 week period between lambing and peak milk production. It is common for increased requirements to outstrip the supply of energy and protein from pasture and the gap contributes to a loss of immunity. The larger the gap the bigger the loss of immunity and this explains why twin-rearing ewes are much more susceptible to worm infection.
The most effective ways to manage worm infection in lambing ewes are to ensure ewes have: