Table 1

Recommendations for faecal egg count reduction (FECR)-based testing of anthelmintic effectiveness (AE) in gastrointestinal nematodes of livestock, and inference of anthelmintic resistance (AR) using the FEC reduction test (FECRT), in the light of confounders on farms.

Action Comment
Understand AE versus AR AR is reduced efficacy, and hence effectiveness, that arises from heritable resistance to the drug; whereas reduced effectiveness can occur in susceptible populations due to other factors. Confusing poor AE (= therapeutic failure) with AR is misleading.
Exclude confounders Make every reasonable attempt to exclude confounders of FECR, by ensuring accurate drug delivery, to animals without issues likely to interfere with bioavailability, carrying representative parasite burdens, and using high technical standards at treatment, sampling and laboratory stages.
Document residual confounders Note those confounders that cannot be excluded and consider them in the interpretation of FECR results.
Heed level of effectiveness AR misclassification risk is most affected by FEC variation and confounders when FECR is close to 95%, but is lower further from this threshold; the level of FECR, and not only binary classification as resistant or susceptible (or, for AE, effective or not), is relevant to farm management decisions.
Repeat measurements Do not rely on a single FECRT to definitively diagnose the AR status of a herd or farm, since this can be misleading; results will vary according to time and other factors, so repeat the test when possible. Track AE alongside routine farm management and follow up low FECR with further investigations.
Identify parasite species Parasite species information before and after treatment, if available, will improve FECR reliability and enable targeted remedial measures in response to poor AE or AR.
Communicate limitations There are fundamental reasons for variation in FECR results. Denying them could fuel false expectations and devalue the test as the best available indicator of AR in most situations. Make management decisions in light of %FECR and uncertainty around the test result and not just apparent AR classification.
Nudge behaviour change Use FECR and other data as an opportunity to stimulate and engage with holistic approaches to parasite management, to preserve remaining anthelmintic efficacy.

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