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Anthelmintic resistance survey in the Central West LHPA

Evelyn Walkera, Neil Charmanb, Jillian Kellya, and Susan McClurea
a District Veterinarian, Central West LHPA, Dubbo
b Veterinarian, Pfizer Animal Health Australia

Posted Flock & Herd April 2013


Internal parasites are ranked the highest costing disease in the Australian sheep industry costing $369 million per year followed by flystrike ($280M), lice ($123M), and post weaning mortality ($76M)1. Losses in production will continue to rise as sheep worms become more resistant to commonly used available chemicals. Approximately 90% of sheep farms have benzimidazole (BZ) resistance followed by 80% with levamisole (LEV) resistance and 60% with resistance to BZ/LEV combinations2. Resistance of Haemonchus contortus (H. contortus) to the macrocyclic lactones (MLs) (ivermectin, abamectin and moxidectin) has been reported in 70% of farms in Northern NSW with up to 30% of these farms having moxidectin-resistant H. contortus 2,3.


The Central West region of NSW is generally a non-seasonal rainfall zone. Traditionally many farmers in this region drench only once or occasionally twice during summer months. However, unusually wet conditions in the summers of 2010-2012 in Central Western NSW resulted in unprecedented H. contortus burdens. Deaths due to high worm burdens, repeated need for drenching, and ineffective drenches were common reports and findings at local district veterinary offices. As a result, a drench resistance survey of various sheep farms commenced in the summer of 2011.


Ten sheep properties within the Central West Livestock Health and Pest Authority (LHPA) were selected based on farmer interest in determining their resistance status and presence of suitable worm egg counts in weaner lambs (ages 4-9 months). A property was included in the faecal egg count reduction test (FECRT) when preliminary worm egg counts on a mob sample had an overall mean ≥250 and had a mix of different worm species present. Individual lambs were mixed in the yards, and then randomly allocated to different treatment groups according to the order they come into the race. Every animal was identified with a uniquely colour coded ear tag which corresponded to a different treatment group. The animals were then drafted, weighed and treated according to their corresponding ear tag colour. On one farm (Farm 3), individually numbered ear tags and block randomisation was used for treatment allocation.

Every farm had a control (untreated) group and three or more treatment groups from various drench classes. See table 1 for detailed description of drenches used. All properties had a moxidectin treatment group. On Farms 1,2,4,6,7,8,9 and 10; ten animals were in each treatment group. On Farm 3, fifteen animals were in each treatment group. On Farm 5, seven animals were in the control group (3 had died in the control group) and 10 animals were in the other treatment groups.

All drenches were given orally at the recommended dose rate. The recommended dose weight was calculated according to sheep bodyweight. Sheep were weighed on electronic scales. Sheep on Farms 1, 2, 4, 8 and 9; sheep were drenched with a drench gun according to the heaviest bodyweight in the mob. On Farms 3, 5, 6, 7, and 10; sheep were drenched with a syringe according to individual body weights. Faeces were collected individually from each animal 10-14 days post-treatment into labelled jars. Faeces were submitted to the Elizabeth Macarthur Agricultural Institute (EMAI) for Farms 1, 2, 3, 4, 8 and 9 and Veterinary Health Research (VHR) for Farms 5, 6, 7, and 10 for individual faecal egg count and group larval cultures. Drench efficacy was calculated based on the following formulae:

Overall % reduction =

Species % reduction = 100

Table of anthelmintics
Table 1: Oral anthelmintics used on farms


H. contortus constituted greater than 50% of larval cultures for the control animals throughout the testing period of November 2011 to June 2012. The only exceptions were Farm 6 and Farm 10 where Trichostrongylus spp. (43% and 94% respectively) were the predominant worm species (Graph 1). Farm 10 had the lowest levels of overall worm burden (average WEC of 380 in control group) presumably because the last drench (monepantel) given five months prior was highly efficacious, the paddocks moved into were clean, and as a result there were low levels of H. contortus persisting in the sheep and/or contaminating the environment going into early winter at the time of testing.

Chart of larval cultures
Graph 1: Comparison of larval cultures post treatment (control group) for each corresponding farm

Four farms had a benzimidazole (BZ) treatment group. The overall worm egg count reduction percent (rWEC) for the BZ groups ranged from 19 to 83% (Table 2). All farms had a levamisole (LEV) treatment group alone or a benzimidazole/levamisole (BZ/LEV) combination treatment group. The rWEC for the LEV groups ranged from 85 to 100% (Table 2).

In contrast, the rWEC for ML treatment groups when given alone or when combined differed significantly (Table 3). Abamectin (ABA) rWEC ranged from 0 to 87%. Moxidectin (MOX) rWEC ranged from 81 to 100%. Derquantel/abamectin (DERQ/ABA) rWEC ranged from 99 to 100%. Properties with moxidectin-resistant H. contortus also demonstrated abamectin resistance. Nine out of ten farms also had a naphthalophos (NAP) treatment group alone or a naphthalophos/ fenbendazole/levamisole (NAP/BZ/LEV) combination treatment group. The overall rWEC for the NAP groups ranged from 34 to 100%. (Table 4.)

Table of FECs
Table 2: Strongyle egg count reductions (%) on farms given LEV or BZ treatments alone or a BZ/LEV combination

FECRT = faecal egg count reduction test; WEC = worm egg count (eggs per gram);

CL = confidence limit; rWEC% = % reduction in (strongyle) worm egg count post-treatment

Table of egg count reductions
Table 3: Strongyle egg count reductions (%) on farms given ML treatments alone or combined

FECRT = faecal egg count reduction test; WEC = worm egg count;

CL = confidence limit; rWEC = % reduction in (strongyle) worm egg count post-treatment

Table of egg count reductions
Table 4: Strongyle egg count reductions (%) on farms given OP treatments alone or combined

FECRT = faecal egg count reduction test; WEC = worm egg count;

CL = confidence limit; rWEC = % reduction in (strongyle) worm egg count post-treatment

Properties with anthelmintic resistance were defined as those where the drench failed to reduce FECs by 95% or more4. Each of the ten farms tested had resistance to one or more chemical groups. Resistance of H. contortus was the most common. (See graph 2). BZ and ABA H. contortus resistance was present on all farms tested. MOX-resistant H. contortus was present on half the farms. Interestingly, no resistance of H. contortus to LEV, naphthalophos (NAP), or DERQ/ABA was detected. Trichostrongylus and Teladorsagia spp. did not exhibit resistance to any MLs.

Chart of drench efficacy
Graph 2: Percentage of farms with less than 95% drench efficacy post-treatment


These FECRTs confirmed the presence of ABA and MOX-resistant H. contortus in the Central West region of NSW. The emergence of ML resistance in our area could be due to a number of factors including environment (e.g. unusually wet summer conditions or possibly refugia), management factors (e.g. lack of drench rotation in previous years, lack of worm egg count monitoring and farmer preference for MLs), and/or sheep factors (importation of drench resistance). Only one farm in this particular study drenched every year for the last 3 years with MOX only. One farm in this study believes he imported his drench resistance.

Most farms understood the concept of drench rotation, but it seemed there was some confusion between understanding ABA versus MOX. It was not uncommon for some to unknowingly use both MLs in the same year. Some farms even had a history of using combined injectable MOX plus vaccination, but did not realise this also needed to be in rotation. Other possibilities for ML resistance, is that many farms preferred to use MLs because they assumed the older drench classes (e.g. levamisole) did not work since it had been around for so long. Further studies in the form of questionnaires that address farmers' knowledge of anthelmintic classes, faecal egg count monitoring, drench resistance, and sheep management practices would be useful in understanding the factors that contributed to ML resistance in our area.

MOX-resistant H. contortus has been previously reported in 2003 in the New England region of northern NSW with 30% of farms affected3 compared to 50% of farms in this current study. However, Nielsen has more recently estimated that moxidectin-resistant H. contortus was likely present in 50% of New England properties in 2010 and now currently may be present on up to 74% of New England farms (R. Nielsen, personal communication, 2013). The dramatic rise of ML resistance in the New England region over such a short time period should therefore be heeded as a warning to Central West sheep producers to adopt sustainable sheep worm control practices now. In this study, LEV, NAP, DERQ/ABA were effective in the control of Haemonchus spp. including ML-resistant H. contortus strains.

Other studies have reported similar findings with NAP and NAP/LEV combinations being highly efficacious against Haemonchus spp.5 including MOX-resistant H. contortus.6 DERQ/ABA, has also been shown in other studies, as well as this study, to be efficacious against ML-resistant H. contortus.7

This paper is the first published report of ML-resistant H. contortus occurring in the Central West region of NSW with 100% of farms exhibiting abamectin resistance and 50% of farms further demonstrating moxidectin resistance.

Our area must therefore adopt strategies to manage our emerging MOX-resistant H. contortus. Producers should actively be encouraged to perform FECRTs to test drench efficacy for different worm species. Depending on the results of regular FECs and larval cultures, reliance on narrow, mid-spectrum, and/or new drench classes especially in summer when H. contortus is the predominate spp. could be used as an alternative to ML drenching.


The authors of this article wish to thank Central West sheep producers, Daniel Guest, Dr Greg McCann, Dr Alan Taylor, Dr Stephen Love, Rhett Robinson, Scott Sullivan, Jason Gavenlock, and Lucas Scales for their assistance in this study, and Pfizer Animal Health Australia for financial support.


  1. Sackett D, Holmes P, Abbott K, Jephcott S & Barber M (2006) Assessing the economic cost of endemic disease on the profitability of Australian beef cattle and sheep producers. Final Report of project AHW-087. Sydney: Meat and Livestock Australia
  2. Love S (2011) Drench resistance and sheep worm control. New South Wales Department of Primary Industries. Primefact 478 2nd edition www.dpi.nsw.gov.au February 2011. Viewed 28 Dec 2012
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  4. Coles GC, Bauer C, Borgsteede FHM et al. (1992) World Association for the Advancement of Veterinary Parasitology (WAAVP): Methods for the detections of anthelmintic resistance in nematodes of veterinary importance. Veterinary Parasitology 44:35-44
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  7. Little PR et al. (2011) Efficacy of a combined oral formulation of derquantel-abamectin against the adult and larval stages of nematodes in sheep, including anthelmintic-resistant strains Veterinary Parasitology 181:180-193


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