Anthelmintic resistance in cattle has been increasing over the past 10 years and resistance to commonly used drenches has been reported in the USA, Europe, South America and Australia (Sutherland & Leathwick, 2011). Anthelmintic resistance in small ruminants has a significant economic impact and is considered a significant component of the total estimated cost of nematodes to the sheep industry of $436 million per annum (Lane et al., 2015). Although the cost of nematodes in the cattle industry is lower than that for the sheep industry ($82 M per annum), anthelmintic resistance in cattle worms is likely to also have a significant economic impact (Lane et al. 2015).
Studies in New Zealand found resistance to macrocyclic lactones and benzimidazoles over two decades ago (Hosking & Watson, 1991; Vermut & Pomroy, 1996), and more recent figures indicate that resistance has increased significantly since then (Waghorn et al., 2006). In Australia, surveys have shown that resistance in cattle nematodes is common and involves all major anthelmintic classes (Rendall, 2010, Cotter et al., 2011). There have been very few drench resistance studies done in cattle west of the Tablelands in NSW. The aim of this study was to assess the effectiveness of commonly used drenches in central west NSW.
The central west region of NSW is a non-seasonal rainfall zone. Common worm species found in this area in cattle are Haemonchus placei, Cooperia spp., Ostertagia ostertagi, Trichostrongylus axei, Oesophagostomum radiatum and Bunostomum phlebotomum, as demonstrated by worm test results in the area over 2011-2012 and the pre-trial worm tests in this study.
Clinical worm burdens are infrequently diagnosed, but cattle are often drenched in early winter with a macrocytic lactone product, primarily for lice control. The most commonly sold drench products, as assessed by record interrogation of local rural merchandise stores in the central west region are moxidectin pour-on, eprinomectin pour-on and ivermectin pour-on.
The study was conducted in winter 2014. Suitable properties were assessed for inclusion in the Worm Egg Count Reduction Trial (WECRT) using a preliminary worm egg count (WEC) of at least ten individual, weaner-age animals. Four mobs with a WEC of >180 eggs per gram were included in the trial. Mean WECs on these four farms were 186, 216, 855 and 2529 eggs per gram respectively.
On each property animals were drafted randomly into groups of 15 animals per group, with drenches randomly allocated to each mob. Animals were weighed and ear-tagged according to the treatment they received. The dose rates administered were according to the manufacturer's instructions and the dose administered was calculated from each animal's individual bodyweight.
Anthelmintics used in the study were macrocyclic lactones or MLs (ivermectin, abermectin, moxidectin, doramectin) and combinations (levamisole + abamectin).
In addition to the treatment groups, each farm involved in the study had a control group of 15 animals that did not receive an anthelmintic product but were weighed and ear-tagged.
Each pour-on treatment group was run separately for 14 days after application to avoid cross-contamination due to grooming.
Fourteen days after treatment, faecal samples from each individual animal were collected and submitted to the laboratory for WEC.
The pre-trial WECs showed that a mix of worm species was found on the four farms, as outlined in Table 1.
|Larval culture (%)|
|Farm “O”||Farm “W”||Farm “H”||Farm “B”|
|WEC (epg)||% reduction with resistant species noted|
|Pretrial Control||Lev+ Aba||Mox||Iver||Doro||Mox||Doro||Aba|
Unable to culture spp
|Lev = Levamisole, Mox = Moxidectin, Iver = Ivermectin, Aba = Abamectin, Doro = Doramectin|
|PO = Pour-On, Inj = Injectable|
|“-“ indicates that drench was not trialed on that property|
WECRTs were only performed on four farms, which means interpretation must be made with caution. It is hoped that this pilot study will prompt further consideration of worm burdens in cattle and encourage other producers to perform WECRT on their farms.
It is also acknowledged that in cattle (unlike sheep), monitoring worm burdens using WECs is relatively insensitive. Undrenched weaner cattle were used in this trial, as they would be expected to have less immunity to internal parasites. WECs could therefore be expected to be a more reliable measure of the parasite burden than in older animals.
The WECRT confirmed the presence of abamectin and moxidectin resistant Cooperia spp. and ivermectin resistant Haemonchus placei. These three active ingredients are all macrocytic lactones (or MLs).
During a review of the worldwide literature, Hutchinson (2003) stated evidence for resistance in cattle worms was only slowly coming to light, but that resistance to MLs was likely to become established in Australia. Widespread ML resistance has already been described by Rhodes (2006) on the north island of New Zealand where 92% of farms were found to have evidence of resistance, and nationally, resistance was present in some 90% of tests (Waghorn et al. 2006). In 2010, Lyndal-Murphy et al. confirmed drug-resistant cattle worms in south east Queensland in Cooperia spp. and also Haemonchus placei, as the first Australian report of ML drench failure in these species. In 2010, Rendell documented resistance in cattle nematodes in 62% of 13 tests in south-west Victoria, and in Western Australia, Cotter et al. reported resistance in 17 of 19 properties tested. In both Australian studies, ML resistance was found chiefly in Cooperia spp., and resistance in Ostertagia ostertagi only to the benzimidazoles and levamisole.
The results of this pilot trial are similar to the published studies, with ML resistance on three out of four farms tested, and resistance in Cooperia spp. and Haemonchus placei.
There did, however, appear to be variation of resistance between properties. One property, Farm H, did not show any resistance to the three drenches used, whereas farm O had resistance to two drenches and did not have a drench with 100% reduction. This suggests a useful message to producers - there are no firm rules for anthelmintic resistance across regions, it can vary from farm to farm and so individual farm monitoring of resistance is warranted.
The emergence of ML resistant nematodes may be due to a number of management factors:
It is also interesting to consider the impact of route of administration via pour-on and injectable routes of administration. A study by George, 2011, of anthelmintic resistance in feedlots in Australia found pour-ons were less efficacious than the injectable and oral forms. Similarly, Leathwick and Miller, 2013, found that reductions in WEC was significantly greater following treatment with moxidectin oral, followed by injection, followed by pour-on.
This pilot study has demonstrated ML anthelmintic resistance in central west NSW that is similar to that found in other Australian studies. There are no distinct trends across the four properties included in the study, and the efficacy of anthelmintics varied between each farm. Producers are asked to consider anthelmintic resistance in cattle when making drenching choices, consider performing WECRTs every few years on individual properties to determine whether resistance is present, and to seek advice from their veterinarian to develop parasite control strategies that are suited to their enterprise.
Thank you to Brown Besier and Bruce Watt for their technical advice and assistance in preparing this paper.
Thanks are due to Merial Australia for funding these trials.