Targeted Treatment for sustainable worm management: guidelines for refugia-based drenching strategies in winter rainfall regions

by Dr Brown Besier, Brown Besier Parasitology
December 2020

Summary

“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”.

This summary provides an overview, but producers contemplating this strategy are advised to carefully read this whole document, in particular, the guidelines at the end of the document, and they should seek advice from an animal health advisor should they have any questions.

• Targeted Treatment suits winter rainfall regions where haemonchosis is not a routine occurrence, and where hot and dry summer conditions limit the survival of worm larvae on the pasture.
• Use the strategy only in adult sheep mobs (>2½ years of age) with average body condition score of 3.0 or more.
• The best individuals to leave undrenched are those with a body condition score of 3.5 or more, based on a visual check when in the race, often when yarded for drenching.
• At least 10–20% of the mob should not be drenched to gain a benefit, leaving a mean of 50 epg or less after treatment.
• Monitor the mob that received a Targeted Treatment: check worm egg counts in autumn, or after 1–2 months at other times.

Introduction

“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).

Australian trials

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).

Translating Targeted Treatment into recommendations

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:

• in younger or poor-condition sheep, where worm burdens are high
• where Haemonchus is a significant risk and checking cannot be done frequently enough, as leaving even a small proportion of a mob untreated may risk disease in those animals or the mob, due to the excessive worm egg contamination of pastures
• in environments where ecological conditions result in a significant proportion of the population being in refugia for most of the year (temperate parts of southern states in Australia), so that drenching is relatively less selective for resistance and worm control requirements are more stringent.

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.

Targeted Treatment guidelines

Strategy summary:

• Winter rainfall regions where haemonchosis is not a routine occurrence
• Adult sheep mobs (>2½ years of age) with average body condition score of 3.0 or more
• Individuals to leave undrenched: body condition score 3.5 or more (visual check when in race, often when yarded for drenching)
• Percentage to leave undrenched: 10–20% of the mob, leaving a mean of 50 epg or less after treatment (if mean mob egg counts are suspected to be relatively high, check before implementing Targeted Treatment and base the percentage on this, see below)
• Post-Targeted Treatment monitoring: check worm egg counts in autumn in mobs that received summer drench Targeted Treatments, or after 1–2 months at other times

Strategy details:

Where is it applicable?

• Winter rainfall regions of Australia, where the main worm genera are Teladorsagia, Trichostrongylus and Nematodirus:
  • Mediterranean climates (Western Australia, most of South Australia): as a routine strategy (subject to guidelines below).
  • Southern SA, parts of Victoria and parts of southern NSW (non-haemonchosis endemic), with monitoring of worm egg counts prior to Targeted Treatment.
• Haemonchosis endemic zones: only in small mobs where FAMACHA is feasible and is the routine. Refugia creation in Haemonchosis endemic zones depends on worm egg counting to determine which mobs can be safely left undrenched as an alternative to a schedule of preventative treatments. ParaBoss does not recommend Targeted Treatment in these zones except under the supervision of an appropriately trained adviser.

Which mobs?

16. Adult sheep (ewes and particularly wethers, but not rams due to their high value and higher susceptibility to worms) in good body condition: entering their third summer, with a fully-developed acquired immunity to worms (a function of age and worm exposure). Mobs subject to Targeted Treatment should be at a mean body condition score of 3.0 (recommended for ewes in early summer under LifeTime Ewe Management guidelines).
16. Yearling sheep with a mob average worm egg count less than 250 epg: well-grown, early-born hogget-age sheep may have developed an adult worm immunity and have sufficient resilience to no longer justify a summer drench to all in the mob. A worm egg count is essential to confirm their suitability.

• Lambs (current year drop): not recommended. (Targeted selective treatment in the UK and Europe has concentrated on this age group, but with frequent (monthly) weight monitoring and worm egg counts, which is not usually practical in Australia.)

What proportion should be drenched/not drenched?

• Modelling studies indicate that at least 10 to 20% of sheep should be left undrenched to provide an effective degree of refugia.

• The aim is for the mob—after a proportion have received the Targeted Treatment drench—to have an average WEC of 50 epg or so after the summer drenches are given, as this indicates that there is no real production loss and, in adult sheep, pasture worm egg contamination in autumn will be below a significant level.

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

Which sheep in the mob should be left undrenched?

• Base selection on condition score: sheep to be left undrenched should have a minimum body condition score of 3.5.
• This can generally be done visually (except in full wool); it is easy to pick the sheep in high condition score when moving along a race, and many will be fat (score 4+). Low-condition score sheep (<2.5) are very evident in a well-managed ewe mob in early summer.
• For sheep with heavy wool cover, a quick hands-on body condition score check shows likely candidates—this takes less time than drenching, as it is quickly evident whether the animal is over BCS 3.0.
• When the mob mean is BCS 3.0, leaving only those in BCS 3.5 is an easy criterion to meet. However, where only a few individuals would qualify, Targeted Treatment is not appropriate.

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.

When should Targeted Treatment be used?

• When the anthelmintic resistance selection pressure is highest: summer for winter rainfall regions is the most applicable time as most worms are in the sheep (rather than on the pasture) and so most of the worm population will be exposed to any drench given at that time. This is the time when the need for refugia is most critical and the resistance-reduction potential is greatest.
• When sheep go onto prepared low-refugia (“worm-safe”) pastures (at any time of year) that have been managed so there are few infective larvae on the pasture to reinfect sheep and most worms are in the sheep, so will be exposed to a drench. If this is a consistent tactic across the property and involves most of the flock, there is heavy selection pressure for drench resistance. Leaving a percentage undrenched (especially for adult sheep mobs) will increase the level of refugia.
• Other situations and seasons where a worm egg count shows a relatively low count and there is little gain from drenching sheep in good condition and not scouring. Drenching may not be warranted at all in these situations, but where it is, and they are adult or yearlings with a mob average WEC under 250 epg, then Targeted Treatment can be used

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.

Monitoring of the strategy

• Prior to a possible Targeted Treatment drenching: where there is little information on typical worm burdens, an egg count will indicate whether these are higher than expected. Checking for a couple of years will indicate the pattern of counts in various adult sheep mobs (e.g. younger ewes may have higher counts at particular times). Experience suggests that in dry inland areas of Mediterranean climatic regions, ewe mobs of mean body condition score of 3 or more typically have worm burdens in the range of 50–250 epg.
• After a Targeted Treatment: the key requirement is for low mean mob worm counts in autumn. Adult sheep counts can rise from late summer onwards, which is suspected to be due to the resumed development of hypobiotic* worms, and which appears variably between properties and mobs. Checking worm egg counts of part-drenched mobs in early autumn will indicate whether mean counts remain low (below 100 epg is recommended for Mediterranean climatic regions).
  *Hypobiosis is a worm survival mechanism where the development of a proportion of the worm larvae is inhibited, generally during unfavourable environmental conditions, at which time these recently ingested larvae will cease development, but survive as  fourth stage larvae in or on the intestinal lining. When environmental conditions improve (as evident to the worms through better nutrition of the sheep), they are triggered to continue their development to adult worms.
• Drenches at critical times (summer and autumn) should be fully-effective. Periodic drench checks or drench resistance tests are recommended.