Confounding factors affecting faecal egg count reduction as a measure of anthelmintic efficacy Author Morgan, Eric R. School of Biological Sciences, Queen’s University Belfast, 19, Chlorine Gardens, BT 9 5 DL Belfast, United Kingdom Author Lanusse, Carlos Laboratorio de Farmacología, Centro de Investigación Veterinaria de Tandil (CIVETAN) (UNCPBA-CICPBA-CONICET), Author Rinaldi, Laura Department of Veterinary Medicine and Animal Production, University of Naples Federico II, Via Delpino, 1, 80137 Naples, Italy Author Charlier, Johannes Kreavet, Hendrik Mertensstraat 17, 9150 Kruibeke, Belgium Author Vercruysse, Jozef Faculty of Veterinary Medicine, University of Gent, Salisburylaan 133, 9820 Merelbeke, Belgium text Parasite 2022 Paris, France 2022-04-07 29 20 1 13 http://dx.doi.org/10.1051/parasite/2022017 journal article 10.1051/parasite/2022017 1776-1042 PMC8988865 35389336 12524472

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.