The last two WormMails were reports of drench resistance or reduced drench efficacy, relating to ‘Zolvix’ (monepantel) and/or ‘Startect’ (abamectin+derquantel) The first was a statement from Elanco letting the industry know of confirmed resistance of Haemonchus to Zolvix (monepantel) on two commercial sheep farms in the New England region of NSW. I thought the Elanco statement was timely, balanced and put the matter into perspective. I am sure other companies would have done likewise.
Also in the other WormMail, which reported multi-drug including monepantel resistance of Haemonchus on a sheep farm in the Greater Sydney region – and reduced efficacy of Startect – the authors were likewise at pains to put the matter into perspective. Following is the last section of that article:
“Resistance to the older broad-spectrum actives (benzimidazoles, levamisole, macrocyclic lactones) is now quite common on Australian sheep farms. However, confirmed cases of reduced efficacy are still rare for the newer drenches (Startect, Zolvix).
As each farm has a different resistance profile, sheep producers should test to determine which drenches are effective and utilize all available options in line with best practice. See WormBoss (wormboss.com.au).”
Reports of resistance? – what do they mean?
Confirmed field cases of drench resistance are usually reported to inform others of something unusual. Often these reports are about early cases of resistance to a new drench active. The take home message generally is that producers should be aware and that they should monitor the efficacy of the product on their farm. It certainly does not constitute ‘buying advice’ along the lines of, “Resistance to Drench X has been reported, so it is no longer an option!” Or, “In this case report, drench A was better than drench B, so A is what I should use!”
Other reports are about resistance surveys. Many of these surveys do not test enough farms to reliably say that the prevalence of resistance of worm A to drench X in region Z is W% of farms. The New England (NSW) Closantel Resistance Survey conducted 15 years or so ago (senior investigator: Dr Joan Lloyd) is one of the few surveys ( that comes to mind) that tested sufficient farms to give a good estimate of the prevalence of resistance in the region. (As an aside: the results of that survey were in the same ballpark of lab data from the former Armidale Regional Veterinary Laboratory (Dept. of Agriculture, NSW). Also the late Dr Peter Greentree, a former Tenterfield District Vet, reliably predicted the prevalence in his district). So, lab data and observations of experienced people in the field may be biased, but sometimes it’s not too bad.
Another recent survey in NSW that had a good number of farms in the study, was that done by District Vet Dr Eliz. Braddon and colleagues in the former Lachlan Livestock Health and Pest Authority (Young, Forbes etc., New South Wales). Also see here. But smaller surveys (e.g. Walker E et al; Central West NSW, 2011 – see here) also yield valuable information,
Most surveys however give an idea of the likely prevalence, if not robust estimates, which is usually good enough. A recent one is the study published by Playford and others in the Australian Veterinary Journal in 2014.
Here is Table 1 from that paper:
To get the context and full information, read the paper.
The results in the table should of course be interpreted with due care. For example, note that for some drench products, there are only a small number of tests.
Also, the results summarised in table 1 are Australia wide. In the New England region of New South Wales (NSW), Haemonchus resistant to abamectin, moxidectin and closantel occur on roughly 80+% of farms (generally more than the figures in table 1). Likewise, the table shows that in 30% of tests, Haemonchus was resistant to levamisole. In the New England, however, data from Veterinary Health Research, Armidale suggests around 60% of farms now have Haemonchus resistant to levamisole. Other tables in the paper by Playford and others (2014) show the different results for each state.
In short, the situation varies from region to region and from farm to farm, even neighbouring farms.
Also, we know from recent tests that Zolvix and Startect are no longer operating at or near high efficacy (say, >98%) 100% of the time. Reduced efficacy is still rare, but it’s there.
So, when you look at results such as that displayed in Table 1, what you get is a rough idea of how likely a particular drench active or combinations of actives will be effective. But on your farm, sooner or later you have to test. Either that, of ‘fly in the dark’. The resistance profile of your property may be quite different from what is more or less likely on a more or less average Australian sheep farm.
And, given that resistance evolves, you have to keep on monitoring the drenches you use.
And, it’s not just about drenches. See ‘Your Program’ at wormboss.com.au .
Playford MC, Smith AN, Love S, Besier RB, Kluver P and Bailey JN, 2014. Prevalence and severity of anthelmintic resistance in ovine gastrointestinal nematodes in Australia (2009-2012). Aust Vet J 2014; 92: 64-71.