by Lewis Kahn, ParaBoss Executive Officer
ParaBoss News March 2015
The rise of drench resistance
Many sheep producers have heard of partial flock treatment (targeted treatment), which is the concept of leaving a proportion of a mob of sheep untreated as a means to slow the development of drench resistance. Why this is of interest requires some thought about how drench resistance develops.
Where drenches are not fully effective, the eggs passed in dung are all from worms that survived treatment. That is, they are the eggs of the drench-resistant worms that remain in the sheep. Even with new treatment actives, drench resistance might already exist, just by chance, in a tiny proportion of the worm population. So even then, the very few eggs—often way too low in number to detect in laboratories—that are passed in dung after treatment are likely to be those from drench-resistant worms.
So, drenching is a process of selection that sees a gradual increase in the proportion of drench-resistant worms, and subsequently, drench efficacy falls. Of concern is that by the time a DrenchTest indicates 95% drench efficacy (that is, when worm egg count is reduced by 95%), the genes in that worm population that code for susceptibility to a particular drench active have already declined to 75%; with 25% now being for drench resistance.
Good drenching practices
An important risk factor for development of drench resistance is the number of worm larvae on pasture when the sheep are drenched. These larvae are often referred to as being in refugia because they are in refuge from (not exposed to) the drench. Where partial flock treatment (also called targeted treatment) is used, that is, leaving a proportion of a mob undrenched, the worms in undrenched sheep are also in refugia, as they also have not been exposed to the drench. As the worms in these sheep have not been selected for resistance—the level of both susceptible and resistant worms they had before their flock-mates were drenched are still present—the eggs that pass in the dung from these undrenched sheep can help to offset the predominance of eggs from drench-resistant worms in the drenched sheep.
When a pasture has a relatively low level of worm larvae and the drenched sheep are grazed there, the pasture will become increasingly contaminated with eggs from resistant worms, as they are the only ones that survived the drench. This is likely to increase the proportion of drench-resistant larvae that subsequently develop on the pasture and go on to infect sheep, resulting in a more rapid increase in drench resistance.
In contrast, when the number of worm larvae on the pasture is high, eggs shed in dung from resistant worms will make up a small proportion of the population, with a slower increase in drench resistance.
So, strategies to maintain worms in refugia inside the sheep are likely most important where the environment and management practices result in periods where the number of worm larvae in refugia on pasture fall to very low levels. This might typically include the Mediterranean region during its hot, dry summers, but also, to some extent, the summer rainfall tablelands region of NSW after its long, cold winter. In the WormBoss regional program for Western Australia, adult ewes are drenched in autumn instead of summer to avoid drenching when there are few or no larvae in refugia on the pasture.
Partial flock treatment in summer rainfall: some early thoughts
The concern for using partial flock treatment in any region has been loss of production. This has been shown to be negligible for Merino ewes in Western Australia and for cross-bred ewes in winter rainfall regions of Australia. However, where barber’s pole worm is common, the concern is sheep deaths, rather than body weight or fleece weight loss.
In summer rainfall tableland and slopes regions, the most effective time to leave a proportion of adult ewes undrenched so as to maintain worms in refugia is during spring, after the long and cold winter. During winter, barber’s pole worm is unable to develop from egg to infective larvae, so the number of larvae on pasture at the beginning of spring is low. However, lambing is often in spring, and lambing ewes suffer a decline of their natural immunity to worm infection. This alone could be a deterrent regarding leaving some sheep undrenched.
Nevertheless, the concept of partial flock treatment needs to be seriously considered because of the prevalence and severity of drench resistance in the region.
Meat and Livestock Australia are currently funding the Lifting the Limits project that is determining the production cost of worms in cross-bred flocks and identifying best practice worm control. This involves 17 farms from the northern tablelands of NSW through to Victoria, with similar flock experiments in Western Australia. In the northern tablelands of NSW, this project provided the opportunity to see if ewe body condition score can be used to select the most suitable cross-bred ewes to leave undrenched as part of a partial flock treatment. Body condition score was used as a possible trait because it is a subjective measure of whole body muscle mass that is known to benefit the sheep’s resistance to worms.
The large dataset* collected from twin-bearing Border Leicester x Merino ewes during the three years of the Lifting the Limits experiment showed that worm egg count (WEC) was favourably associated with body condition score (BCS). In other words, at any point in time from pre-lambing to weaning, fatter ewes (higher BCS) were more likely to have a lower worm egg count. On this basis, the fatter ewes seemed to be the best ewes not to drench, so as to maintain a proportion of the worm population in refugia to slow development of drench resistance.
But the real question remained, would those ewes with higher pre-lambing body condition scores continue to have lower WEC up until weaning? The results in the figure above indicated that higher BCS ewes had a lower WEC at any point in time, but they did not indicate what the future held for these ewes.
Unfortunately, pre-lambing BCS was not predictive of subsequent WEC at marking or at weaning and this means it is unlikely to be a useful tool for identifying the pre-lambing ewes that can safely be left undrenched.
The reason why body condition score (BCS) did not indicate future WEC was because those ewes in the highest BCS just prior to lambing lost the most condition over the lactation period. These ewes likely transferred body condition to milk and more successfully reared twin lambs. The downside for these ewes was that loss of BCS at this time was associated with an increase in WEC.
In summary, the results indicate that ewes in better body condition score prior to lambing are likely to have lower worm egg counts initially, but do not maintain a lower WEC because they lose more condition.
At this stage, the safe use of partial flock treatment in summer rainfall tableland and slopes regions doesn’t seem possible. This is because of the risk of deaths from barber’s pole worm and the absence of a trait that can readily be measured or assessed to allow real-time and within-yard decision-making on which animals could remain untreated.
These are only early thoughts and WormBoss will continue to keep a close eye on this topic.
*The data collected were from 120 Border Leicester x Merino ewes on each of 3, 5 and 5 properties during the years 2012, 2013, 2104 respectively in the Armidale region, New South Wales. Body condition score (BCS; 1–5 scale) was estimated and faeces collected to determine worm egg counts (WEC). Adult ewes were identified as twin-bearing. Lambing occurred in September of each year. Measurements were done at pregnancy scanning (July), prelambing (August), lamb marking (November), weaning (January) and mating (April). Prelambing, lamb marking and weaning data were used for this analysis. There were 3,989 observations with both BCS and WEC recorded.