The effect of pasture molluscicide on small lungworm infections and the productivity of grazing lambs
Introduction
Small lungworm infections of sheep are often detected at post-mortem examinations and at abattoirs but are generally thought to be unimportant because they cause few obvious ante-mortem clinical signs. However, heavy lungworm infections may cause production loss, either directly or by worsening other respiratory diseases, such as pneumonia (Rose, 1959), and therefore prevention may be warranted when the prevalence is high. A very high prevalence of infection with small lungworms has been reported in sheep from farms in the southeast of South Australia (SA), an area with more than three million sheep, with 20–25 % of lamb consignments infected based on abattoir surveillance studies (ABS, 2017; Dal Grande et al., 2019). Lungworms make important contributions to the overall effects of nematode infections on mortality and morbidity in sheep in parts of Europe and Africa, and can cause significant economic loss in their own right (Pandey et al., 1984; Vina et al., 2013). However, the impact of the high prevalence of small lungworm infections observed in sheep from SA is currently unknown.
Strategies to prevent or manage lungworm infections in sheep have not been described or investigated in any detail in Australia, and currently sheep producers have no specific recommendations available to them for preventing or managing these infections. In contrast, programs for the control of gastro-intestinal nematodes (GINs) of sheep in southeastern Australia are well developed, with the use of anthelmintics typically integrated with other strategies such as grazing management (Anderson, 1972; Larsen, 2014). GIN control programs can partially control lungworm infections, especially large lungworms (Dictyocaulus filaria) (Larsen, 2014; Lopez et al., 2011), but previous studies suggest effective control of small lungworm infections will most likely require additional anthelmintic treatments (McCraw and Menzies, 1986), and at higher doses than those used for the GINs (Richard and Cabaret, 1992). Additionally, the high prevalence of small lungworm infections in southeast SA occurs despite sheep producers extensively using anthelmintics for the control of gut nematodes. Therefore, relying solely on anthelmintics may not achieve satisfactory or efficient control of small lungworms (Paraud et al., 2005), yet additional treatments specifically seeking to control lungworms has implications for the overall cost of anthelmintic use and potentially increases selection pressure for anthelmintic resistance in GINs (Kahn and Woodgate, 2012).
Integrated parasite management (IPM) uses a combination of approaches to achieve effective worm control, without solely relying on anthelmintics (Larsen et al., 2006; Woodgate and Besier, 2010). The small lungworm species present in Australia (Muellerius capillaris and Protostrongylus rufescens) require an intermediate host mollusc (Prietocella barbara, Cernuella virgate and Theba pisana) to complete their lifecycle. As well as serving as the intermediate hosts of potentially important sheep parasites, these snails infest many types of pastures and crops and can cause significant damage, especially during the establishment phase (Micic et al., 2008). The cost of damage and control of invertebrate pests including snails is conservatively estimated at several hundred million dollars each year in southern Australia (Micic et al., 2008). Consequently, targeting them provides an opportunity to use an IPM approach to small lungworm control. Additionally, the lifecycles of M. capillaris and P. rufescens are relatively similar (Soulsby, 1965), so the effectiveness of preventative measures should be similar for both small lungworm species.
Reducing the population of the intermediate host snails may provide an alternative, cost-effective means to break the small lungworm lifecycle, reducing infections in sheep and the direct effects of snails on pasture growth. Snail densities on pastures and crops can be reduced using stubble management, or via the application of molluscicides (Baker and Hawke, 1990; DeGraaf and Kimber, 2015; Poynter and Selway, 1966). Stubble management, such as rolling or cabling to squash molluscs must be done whilst there is no pasture or crop present, making it unsuitable for perennial pastures. Although most molluscicide control options have been mainly developed for cereal crops, several products are also registered for use on established perennials (Charwat and Davies, 1999), so these may be of benefit in the sheep grazing systems used in SA.
The aim of this study was to assess the effect of an intensive pasture molluscicide treatment applied to established lucerne pastures on a) the prevalence and severity of lungworm infections and b) the productivity of lambs grazing those pastures between weaning and slaughter. This pasture treatment was used in addition to routine anthelmintic control for GINs.1
Section snippets
Farm description
The farm selected for this study was 55 km west of Naracoorte in SA (37.001 °S, 140.085 °E), with an average annual rainfall of 602 mm (1879–2020) that followed a winter dominant pattern. The useable area of the farm was 2758 ha, typically stocked with 3000 Merino and first-cross (Border Leicester x Merino) ewes at an annual average stocking rate of about 6 dry sheep equivalents (DSE)/ha. The farm was selected because recent abattoir monitoring had detected a high prevalence of small lungworm
Measurements
Lambs commenced grazing the trial paddocks on 29 October 2019 and were then monitored at four farm visits from October to February. A total of 259, 258, 255 and 253 lambs were weighed, and faecal samples collected from 60, 60, 55 and 58 animals for WECs and Baermann tests at visits 1–4, consecutively. During the study, two lambs from the Control group and one from the Treatment group were removed for unrelated health reasons. At slaughter, measurements were made on 244 lambs, with untrimmed
Discussion
In the past 10 years the prevalence of small lungworm at slaughter has been extremely high in sheep in southeast SA (Dal Grande et al., 2019), prompting intense producer interest in the ramifications of this finding. Additionally, molluscicide has been used in this region to effectively control snails during pasture establishment (Baker and Hawke, 1990) and therefore a logical extension was for molluscicide to be used to control small lungworm (Morrondopelayo et al., 1992). However, despite
Conclusion
Despite recent intense interest in small lungworm due to the high prevalence of lesions detected at slaughter (Dal Grande et al., 2019), this trial found that the method of snail control using additional molluscicide applications did not improve lamb production. Our results emphasise that producers should focus on the well-established determinants of production, such as nutritional management by optimising pasture quality through the correct management of rotational grazing. Effective methods
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the Scobie and Claire Mackinnon Trust, Meat and Livestock Australia and Fair Dinkum Fertilisers which supplied molluscicide at a discounted price. The Australian Government Research Training Program Scholarship supported the involvement of Jenny Hanks in this study. The authors gratefully acknowledge the farm owners; Josh and Madeline Hancock for their time and involvement, and for making this study possible. Our colleagues from the Mackinnon project are also
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