Subscribe to the Boss Bulletin
Subscribe the the Boss Bulletin for monthly updates and articles about all things parasite management
Subscribe hereSubscribe now to receive email updates from one or more of our ParaBoss suite of websites, ParaBoss, FlyBoss, LiceBoss and WormBoss.
Subscribe“Targeted Treatment” involves drenching only a portion of specific mobs at particular times, to slow down the development of drench resistance across a property by having more of the worm population “In refugia“.
More details on the background of this strategy are included below the guidelines.
It is not uncommon to find ewe mobs with mean counts of less than 50 epg in summer; no drench can be justified for these. For a count of say 100 epg, 50% could be left undrenched.
Table 1. Proportion of sheep to drench and not drench based on mob average WEC
(choose the WEC nearest to yours)
Mob average WEC (epg) | Proportion of mob (%) | |
Drench | Do NOT drench | |
50 | 0 | 100 |
100 | 50 | 50 |
150 | 70 | 30 |
200 | 75 | 25 |
250 | 80 | 20 |
300–350 | 85 | 15 |
400–500 | 90 | 10 |
Note: research on Australian properties showed that the selection of sheep to not drench can be done very quickly. Even when the proportion of “not drenched sheep” were selected at random, there were no significant ill-effects. You do not need to agonise over getting the scores absolutely correct.
There is little to be gained by using Targeted Treatment for drenches when rapid re-infection is inevitable (winter and spring in winter rainfall regions) as there are larger numbers of worms already in refugia and so it is far less selective for drench resistance.
“Targeted Treatment” is a strategy to reduce the development of drench resistance in worms of ruminants, by leaving some animals untreated when a mob is drenched. Treatments are “targeted” to animals judged most likely to benefit, and the more parasite-resistant or resilient individuals can be left undrenched.
Ensuring that some non-resistant worms survive in a mob or on the property dilutes resistant worms that remain after a drench is given. This is the “refugia” strategy: sufficient parasites that are susceptible to the drench are deliberately allowed to survive as either adult worms or as larval stages on the pasture, “in refuge” from drenches. Any resistant worms surviving drenches given to the current mob being drenched are only a small proportion of the total worm population on the property. With successive generations of parasites over time and movement of animals in different mobs over the property, the dilution of resistant worms will minimise the increase in the overall level of resistance.
Targeted Treatment (or more usually “Targeted Selective Treatment”) has been extensively trialled around the world in sheep, goats and cattle (UK, EU, New Zealand, US, South America, South Africa), and shown to reduce resistance development with no appreciable loss of animal production (growth rates or milk production). The basis of the decisions about which individuals to leave untreated varied in the trials, and has mostly involved animal weights at frequent intervals, but also body condition score, milk production (dairy cattle and sheep), and indicators of parasite numbers (worm egg counts and milk ELISA readings). But practical decisions in the drenching race are much more easily done.
Of special note is the use of the FAMACHA system, based on frequent individual-animal anaemia assessments using eye membrane colour to indicate the Haemonchus burden, and hence an animal-by-animal decision on the need for treatment. The low percentage of animals justifying frequent treatment on this basis emphasises that routine whole-mob treatments are unnecessary, and Targeted Treatment is an obvious approach to reducing increases in drench resistance in Haemonchus zones. However, this is currently only practical on small farms with relatively few animals to check, or on large farms where the labour cost of repeated mustering and checking of the individuals is quite low (the latter being unfeasible on commercial Australian farms).
In Australia, there is now considerable literature on the use of Targeted Treatment based on body condition scores in adult sheep to indicate which animals can be safely left undrenched when a mob treatment is planned. The research is specific to winter rainfall regions, where the major worms are Teladorsagia, Trichostrongylus and Nematodirus. In these regions, Haemonchus is absent or rarely of clinical significance. Annual control programs for these scour worm-dominated regions have been based on drenches in summer to remove harmful worm burdens and minimise the contamination of pastures with worm eggs in autumn (to prevent winter parasitism). Trials include computer simulation modelling, controlled field investigations, detailed observations on flocks on commercial farms, and reports from farms where Targeted Treatment has been practiced for some years.
The use of body condition score as the index for selecting which animals may be safely left untreated recognises that the key factor should be individual animal resilience, not resistance, to worms, i.e. the ability to tolerate a worm burden, rather than the number of worms (some can tolerate a heavy worm burden with no ill-effects, whereas some will show signs when carrying only a moderate worm burden).
Assessments on sheep in regions where scour worms predominate (not coastal, summer rainfall or summer irrigation areas) are typically done at specific times when a critical drench is to be given, usually summer or autumn, when the selection pressure for drench resistance is at its seasonal highest. In early summer, especially, sheep in southern Australia are generally in their best body condition scores, as they have come from (or are still on) the best quality pasture for the year, and the lambs have been weaned from ewes.
Research in WA has shown that worm egg counts of adult ewes are typically low (below 200 eggs per gram) in early summer. This suggests that little production benefit will be obtained by treating all sheep in these adult mobs, and that the level of pasture contamination in autumn will be low (where there is a hot dry summer preventing egg hatching to larvae) even if a small proportion are not given the usual summer drench. However, counts in weaner and yearlings were significant at that time, justifying the retention of the summer drench program in these groups.
Trial results from WA, SA and Victoria show that a proportion of adult sheep mobs can be left untreated with no reduction in the annual wool production or lamb birth or survival rates, no increase in the occurrence of winter parasitism, but a significant reduction in the rate of development of anthelmintic resistance.
In some trials in WA, over 50% of ewe mobs were left undrenched in summer with no subsequent ill-effects, with similar results in SA, where, based on their worm egg count index, anywhere between 0 and 100% of sheep were left undrenched in each mob.
Computer modelling has helped develop the guidelines (further down the page) indicating the appropriate percentage of sheep that may safely be left undrenched under different circumstances (environment, initial worm burden, time of year, drench efficacy).
It appears that until recently we have not fully appreciated the strength of resistance and resilience to worms of adult sheep in good body condition, and that this can be safely used as the basis of refugia strategies where some animals are not treated when annual pre-emptive treatments would normally be given.
Targeted Treatment is not or less applicable:
However, it should be noted that a planned drench resistance management program should be in place for all flocks, and that targeted selective treatment strategies have been developed for environments that favour worm survival, such as in the UK, Europe and New Zealand.
In most situations, modelling shows that provided refugia strategies are applied to a number of mobs on a property, sheep movements between paddocks over the year ensure that the whole-flock and whole-worm population dilution effect can effectively minimise the further development of anthelmintic resistance.
Local recommendations can be developed that optimise the application of the refugia concept to ensure a balance between the sustainability of drench efficacy and the minimisation of the impact of worm burdens on sheep productivity and health.
Subscribe the the Boss Bulletin for monthly updates and articles about all things parasite management
Subscribe herewww.wecqa.com.au is a secondary ParaBoss website hosted by the University of New England (UNE). Whilst this is still an official ParaBoss website, UNE is solely responsible for the website’s branding, content, offerings, and level of security. Please refer to the website’s posted Privacy Policy and Terms of Use.