The reproductive ewe or doe and the weaned lamb or kid are the classes of sheep and goats most susceptible to worm infection. For this reason, many of the WormBoss regional programs have instructions on how to prepare low worm-risk lambing/kidding and weaning paddocks, and in some regions, a drench is mandatory for ewes and does just prior to lambing or kidding and for lambs and kids at weaning.
Low worm-risk paddocks expose these susceptible classes of stock to lower worm challenge and provide production benefits. An effective drench on entry to the prepared paddocks prolongs their lower risk status and also simply removes worms from the stock when they are quite susceptible.
The following describes the process in ewes, but will be similar for does.
Ewes start to lose immunity to worms during late pregnancy with the effect being earlier and more severe with poorer feed quantity and quality. The establishment rate of worms, that is, the percentage of infective worm larvae consumed with pasture that establish to become adult worms in the sheep, increases at these times; this is described as the peri-parturient rise in worm infection. To put this into perspective, only 5–10% of worm larvae will establish in a dry ewe in good condition and on good feed. During early lactation that may rise to a level where 40–60% of the infective larvae are able to establish to adult worms.
Before worm egg counts (WEC) start to rise, changes occur in the immune response of the pregnant ewe that signal the ewe is losing immunity. These changes can occur as early as 3 weeks prior to lambing. These changes lead to an increase in WEC (Figure 1) and worm burden (Figure 2). The rise in WEC is worse in ewes with multiple lambs and on low nutrition.
Figure 1: Worm egg counts of dry (dashed bottom line), pregnant/lactating (black top solid line) and early-weaned (grey middle solid line) Merino single bearing and rearing ewes.
Figure 2: The worm burden (black scour worms) of dry, pregnant/lactating and early-weaned Merino single bearing and rearing ewes on either a low or high quality diet. Note that for the dry ewes no bars are evident as the results were close to zero.
If there were any doubt that lactating ewes were susceptible to worm infection, examining the response to weaning of lambs at two days of age in Figures 1 and 2 would be informative. Weaning led to an increase in worm resistance such that the differences in WEC and worm burden from dry ewes had disappeared within 5–6 weeks.
The main cause of the loss of immunity is the negative nutritional balance of the late pregnant and lactating ewe. Put simply, ewes typically lose weight during this time and can typically lose 30–50% of their fat and 10–20% of their protein reserves from 2 weeks prior to 5 weeks after lambing. While not fully accounting for all of the changes that increase susceptibility of reproductive ewes to worms, these losses are the main cause. Increasing the quality of the diet results in lactating ewes eating more and being in better condition and this improves worm resistance and reduces worm burdens (Figure 3).
Figure 3: The worm burdens (black scour worms) of single-rearing Merino ewes at six weeks after lambing when fed a low or a high quality diet.
There are two key approaches to manage the increased susceptibility of the lambing ewe to worm infection.
Finally, also remember that increasing genetic resistance to worms by purchasing rams with negative Australian Sheep Breeding Values for WEC will produce ewes that are less susceptible to worm infection during lambing.