Gastrointestinal nematode (GIN) parasites impose the largest animal health cost to the Australian sheep industry. The majority of this cost (70-80%) is associated with production loss which occurs despite control strategies implemented on farm by sheep producers. It is difficult for producers to visualize the magnitude of the production loss associated with subclinical infection and unfairly, their attention is drawn to the direct, but relatively minor cost of control. The total annual cost of GIN across the major sheep producing regions of Australia has been estimated to have increased from $3.32/head in 2001 to $3.93/head in 2010, principally because the value of wool and meat increased to a greater extent than the cost of treatment (Kelly 2011). Areas characterized by endemic infection from Haemonchus contortus experience a greater annual cost of GIN ($11.00/ewe suggested for Typical management) because of sporadically high rates of mortality.
The increasing cost of GIN brings into focus the importance of good control within the constraint of minimising further selection for anthelmintic resistance. At the same time, the endemic nature of anthelmintic resistance and its increasing severity (Besier and Love, 2003) highlights the importance of control practices that minimise further selection for resistance. A potential tension exists between the aims of good control of GIN and the task of minimising selection for anthelmintic resistance. This tension is the basis for greater complexity in worm control which initially led to investigations such as Integrated Parasite Management of Sheep project (Walkden-Brown et al.., 2004) and ultimately to the upgrading of WormBoss, which is the national worm control resource.
The key strategies available for GIN control include anthelmintics, genetic selection for increased host resistance, grazing management and improving host nutrition (especially protein nutrition). Other strategies are or may soon become available 'medicinal forages with particular reference to tannins, vaccines to boost host immunity with current reference to H. contortus and fungi (such as Duddingtonia flagrans) that predate GIN larval stages within the faecal pellet' but these will not be discussed in this paper.
The challenge for regional worm control programs is to determine the role of each strategy within an integrated approach and within a commercial context of sheep production. This was the aim of the IPMs project which built upon earlier efforts (i.e. WormKill; Wormkill Technical Committee, 1995) by placing greater emphasis on the aspects of (i) protection from infection; and (ii) detection of infection rather than; (iii) responding with an anthelmintic treatment. Protection from GIN infection is afforded by grazing management and selecting sires with negative Australian Sheep Breeding Values for worm egg count (WEC). Detection of infection is the result of monitoring WEC and using the results (both count and contributing species) as the basis for treatment.
Over a period of two years, Kelly et al. (2010) compared the performance of Merino sheep on six properties, in the Northern Tablelands of NSW, with half adopting an integrated approach (WormBoss regional program) and the remaining using regionally typical worm control practices. Typical practices involved little if any attention to protective strategies, infrequent detection of WEC and undetermined anthelmintic efficacy. Using the WormBoss program, anthelmintic frequency was reduced by nearly 30% while WEC and mortality were significantly lower (Table 1). Effects of worm control approach on individual production were small and not significant. The individual production values in Table 1 describe the change in production due to GIN infection (i.e. difference within farm between worm-free and Typical or between worm-free and WormBoss sheep) and not the absolute level of production which would be confounded with other property factors. For example, the cost of GIN was numerically greater (but not statistically significant) for greasy fleece weight with WormBoss (loss of 0.2 kg versus 0.14 kg for Typical) but absolute level of greasy fleece weight was 3.07-0.02 kg for Typical and 3.60-0.02 kg for WormBoss.
Adapted from Kelly et al. (2010).*Probability values indicating statistical significance. Differences between Typical and WormBoss are not significant when P>0.05; N/A = not applicable to be analysed;**Change denotes the difference between Typical and/or WormBoss sheep with their "worm-free" control sheep grazed in each flock. This represents the effect of GIN as distinct from simple differences between properties.
The data were used to calculate that the annual cost of GIN (2010 prices) with Typical and WormBoss practices was $11.09 and $5.82 per ewe respectively (Kelly 2011). The main factor that increased the cost of GIN with Typical management in the Northern Tablelands was the increased annual mortality of Merino ewes 'haemonchosis accounted for most of this increase' which resulted in less wool, sale sheep and lambs and a greater need for replacements. The cost of anthelmintic treatment and monitoring of WEC and drench efficacy accounted for 19% and 49% of the total cost of GIN with Typical and WormBoss respectively (Figure 1).
Without protective strategies such as grazing management and genetic selection it is not possible to reduce reliance on chemotherapy. The use of GIN resistant sires (on the basis of ASBV) leads to lower WEC, a reduction in the number of H. contortus infective larvae on pasture (Kahn et al., 2003) and presumably, eventually to fewer anthelmintic treatments. In practice, the potential for undesirable correlations between WEC and other production traits is managed by using index values or independent selection levels (MLA, 2010).
In the shorter term, grazing management can reduce the number of infective GIN larvae on pasture. The benefits of grazing management have been demonstrated in winter and summer rainfall regions of Eastern Australia (Niven et al., 2002; Bailey et al., 2009a; Colvin et al., 2008). While the procedures differ among these regions, they commonly rely on allowing a greater proportion of infective larvae on pasture to die between grazing events and/or avoidance of reinfection during a grazing period. A sound understanding of GIN ecology, especially temperature and rainfall permissive of development and factors controlling death rate of GIN larvae on pasture are key to effective grazing management strategies.
In the Northern Tablelands of NSW, year-round grazing management, in the form of planned or cell grazing, is effective at controlling GIN infection (especially from H. contortus) but this has yet to be widely adopted by producers. In a simpler approach, avoiding autumn (i.e. March and April) pasture contamination of spring-lambing paddocks with viable GIN eggs (mostly H. contortus) is a highly effective strategy for producing low worm-risk lambing paddocks. From May to August, development to H. contortus infective larvae is effectively prevented by temperature (Bailey et al., 2009a; Southcott et al., 1976). The combined effects of temperature and autumn management create a 6-month period of little, if any, recruitment to pasture H. contortus larvae (Bailey et al., 2009b).
Having implemented protective strategies for GIN control, anthelmintic treatment in WormBoss regional programs is based on both strategic treatments (e.g. prelambing, weaning or first summer depending on region) and WEC/coproculture results as the basis for tactical treatments. A series of Drench Decision Guides (DDG) have been developed for WormBoss regions (Figure 2) that provide a structured and evidence-based approach in the form of Boolean keys ( www.wormboss.com.au )
WormBoss (www.wormboss.com.au) was first launched in 2005 as the national sheep worm control resource and the upgrade was completed in November 2012. The upgraded WormBoss (completed by the Sheep CRC) now possesses a problem-solving focus with up-to-date information filling a supportive role. The tools that provide the problem-solving focus are (i) regional worm control programs; (ii) Drench Decision Guides and (iii) drench search database.
The basis of the worm control programs and DDGs have been described earlier in this paper. The searchable drench database allows searching by:
Results are ordered by number of active ingredients and comment is provided on GIN species targeted and broad regional drench resistance status.
There have been many people who have generously contributed to the technical content of the upgrade of WormBoss. The key contributors are Arthur Le Feuvre, Brown Besier (DAFWA), Deb Maxwell (Sheep CRC), Maxine Lyndal-Murphy (DAFF Qld), Stephen Love (NSW DPI) and Rob Woodgate (CSU). Other animal health advisors (both private and public) have also kindly contributed to various regional programs and Drench Decision Guides.