Internal parasite drench resistance has been identified as the most costly health problem facing sheep producers in Australia, costing 369 million dollars annually in 2006 (Sackett et al. 2006). In the summer rainfall area of NSW Haemonchus contortus (Barbers Pole Worm-BPW) presents the most challenging drench resistance problems for sheep producers and their advisors (Love et al. 2003).
Included is data from drench resistance surveys undertaken over the past 15 to 20 years and recent data that demonstrates the continuing challenges to managing BPW.
The History of Drench Resistance
Resistance to anhelminthics is not a new phenomenon and starts the very first time a drench is used. In fact the only way not to select for drench resistance in sheep is not to drench them at all.
Drench resistance was first detected in the 1960s to Thibenzole (BZ group) and has systematically emerged within 5 to 10 years for all drench groups3. This is summarised in Table 1.
Closantel Resistance Survey 1997
Because of its important role and widespread use in the Wormkill program to manage BPW, Closantel (Dash, 1986) was put under high selection pressure for the development of resistance. By 1987 cases were emerging throughout the northern tablelands (Lloyd et al 1988).
Out of 97 flocks sampled 39 (40%) showed resistance with a further 21% having emerging resistance.
With the demise of Closantel as an effective narrow spectrum sustained activity drench for BPW, more frequent treatment with other effective drenches was required and the resistance juggernaut rolled on.
A review of the prevalence of resistance in sheep worms published by Steve Love in 2011, is summarised in Table 2.
WECRT RESULTS 2012
During 2012 worm egg count reduction tests were carried out by local pharmaceutical company representatives and LHPA staff. This data has been combined below.
Tests were carried out from Walcha in the south to Deepwater and west to Nullamanna.
The level of resistance from data provided found that resistance to drenches used on properties was high. This is in line with previous surveys and studies undertaken and reported in this paper (Love 2011; Dash 1986).
Figure 1 takes the individual reduction data for each active and focuses on the number of likely effective drenches available to manage worms on the property.
Over 50% of farms have 2 or less actives available and of those with 3 or more the majority are combinations containing an organophosphate (Figure 2).
To put this in context, managing worms relies on a combination of strategies based around the principle of Integrated Pest Management.
To do IPM correctly sheep producers need effective drenches. Or more importantly need to know which actives are effective, and to make it really challenging for quarantine drenching it's not what's effective on their property it's what is effective on the property or properties the introduced sheep originate from.
All the data for the New England would indicate that a general recommendation would have to include Monopantel, an OP and levamisol.
The data used for this paper do not include the new active Monopantel and the soon to be new active Derquantel. So those with no effective drenches actually have two and those with five have seven.
Individual producers find some formulations more appealing than others based on price, sustained activity, concurrent fluke activity and route of administration.
This is the first time in the living memory of the sheep industry that 2 new actives with no recorded resistance have hit the market at almost the same time. The challenge to sheep producers and their advisors is to maximise the long term benefits of these actives. The WECRTs presented to you tell two stories. The first being how desperately 50% 0f those properties sampled need a new alternative but for the other 50% who still have 3 or more options as individual actives or as combinations controlling worms remains a simple but constant challenge.
Paul Tudor from Pfizer Animal Health.
Steve Love for his input, his excellent work used and referenced in this paper and his healthy cynicism.