Anthelmintic resistance in ascarid and strongylid nematodes in Australian TB’s

A national survey of anthelmintic resistance in ascarid and strongylid nematodes in Australian Thoroughbred horses
Int J Parasitol Drugs Drug Resist. 2023 Nov 29:24:100517. doi: 10.1016/j.ijpddr.2023.11.006. Online ahead of print.
Authors
Ghazanfar Abbas 1 , Abdul Ghafar 1 , Emma McConnell 2 , Anne Beasley 3 , Jenni Bauquier 1 , Edwina J A Wilkes 4 , Charles El-Hage 1 , Peter Carrigan 5 , Lucy Cudmore 5 , John Hurley 6 , Charles G Gauci 1 , Ian Beveridge 1 , Elysia Ling 1 , Caroline Jacobson 2 , Mark A Stevenson 1 , Martin K Nielsen 7 , Kristopher J Hughes 8 , Abdul Jabbar 9
Affiliations

1 Melbourne Veterinary School, The University of Melbourne, Werribee, Victoria 3030, Australia.
2 Centre for Animal Production and Health, Murdoch University, Murdoch, Western Australia, Australia.
3 School of Agriculture and Food Sustainability, University of Queensland, Gatton, Queensland 4343, Australia.
4 Racing Victoria, Flemington, Victoria 3031, Australia.
5 Scone Equine Hospital, Scone, New South Wales 2337, Australia.
6 Swettenham Stud, Nagambie, Victoria 3608, Australia.
7 M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
8 School of Agricultural, Environmental and Veterinary Sciences, Charles Sturt University, Wagga Wagga, New South Wales 2650, Australia.
9 Melbourne Veterinary School, The University of Melbourne, Werribee, Victoria 3030, Australia. Electronic address: jabbara@unimelb.edu.au.

PMID: 38064906
DOI: 10.1016/j.ijpddr.2023.11.006

Abstract

This study quantified the extent of anthelmintic resistance (AR) in ascarid and strongylid nematodes against commonly used anthelmintics in Australian Thoroughbred horses. Faecal egg count reduction tests (FECRTs, n = 86) and egg reappearance period (ERP) tests were conducted on 22 farms across Australia. Faecal egg counts (FECs) were determined using the modified McMaster technique, and percent faecal egg count reduction (%FECR) was calculated using the Bayesian hierarchical model and hybrid Frequentist/Bayesian analysis method. The results were interpreted using old (published in 1992) and new (2023) research guidelines of the World Association for the Advancement of Veterinary Parasitology (WAAVP). The species composition of strongylid nematodes was detected utilising a DNA-metabarcoding method using pre- and post-treatment samples. Resistance was observed in strongylid nematodes to commonly used single-active and combination anthelmintics, including ivermectin (IVM %FECR range: 82%-92%; 95% lower credible interval (LCI) range: 80%-90%), abamectin (ABM: 73%-92%; 65%-88%), moxidectin (MOX: 89%-91%; 84%-89%), oxfendazole (OFZ: 0%-56%; 0%-31%) and its combination with pyrantel (OFZ + PYR: 0%-82%; 0%-78%). Resistance in Parascaris spp. was observed to IVM (10%-43%; 0%-36%), ABM (0%; 0%) and MOX (0%; 0%). When the new thresholds recommended by the WAAVP were used, AR was detected in six additional FECRTs for strongylids and three more tests for Parascaris spp., introducing resistance to OFZ and OFZ + PYR in the latter. Shortened ERPs (4-6 weeks) of strongylids were observed in 31 FECRTs in which AR was not detected at 2 weeks post-treatment for all the anthelmintics tested. Among cyathostomins, Cylicocyclus nassatus, Cylicostephanus longibursatus and Coronocyclus coronatus were the most prevalent species at 2 weeks post-treatment, whereas the main species appearing at five weeks following treatments with macrocyclic lactones were Cylicocyclus nassatus, Cylicostephanus longibursatus and Cylicocyclus ashworthi. After treatment with OFZ + PYR, the latter three, plus Coronocyclus coronatus and Cyathostomum catinatum, were detected at 5 weeks post-treatment. Overall, the study highlights the prevalence of AR in both ascarids and strongylid nematodes against commonly used anthelmintic products to control worms in Australian horses. The results indicate that ML combination products provided acceptable efficacy at 2 weeks. However, ERP calculations suggest that products work less effectively than previously measured. It is suggested to regularly monitor the efficacy of the anthelmintics and consider changing the worm control practices to better manage worms and AR in Australian horses.

Keywords: Anthelmintic resistance; Australia; Cyathostomins; Egg reappearance period; Horse; Macrocyclic lactones; Parascaris spp..

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