Investigating anthelmintic efficacy against gastrointestinal nematodes in cattle by considering appropriate probability distributions for faecal egg count data

Abstract

The Faecal Egg Count Reduction Test (FECRT) is the most widely used field-based method for estimating anthelmintic efficacy and as an indicator of the presence of anthelmintic resistant nematodes in cattle, despite never having been validated against the gold standard of controlled slaughter studies. The objectives of this study were to assess the normality of cattle faecal egg count (FEC) data and their transformed versions, since confidence intervals used to aid the interpretation of the FECRT, are derived from data assumed to be normally distributed, and violation of this assumption could potentially lead to the misclassification of anthelmintic efficacy. Further, probability distributions and associated parameters were evaluated to determine those most appropriate for representing cattle FEC data, which could be used to estimate percentage reductions and confidence limits. FEC data were analysed from 2175 cattle on 52 farms using a McMaster method at two different diagnostic sensitivities (30 and 15 eggs per gram (epg)) and a sensitive centrifugal flotation technique (SCFT) with a sensitivity of 1 epg. FEC data obtained from all egg count methods were found to be non-normal even upon transformation; therefore, it would be recommended that confidence or credible intervals be generated using either a Bootstrapping or Bayesian approach, respectively, since analyses using these frameworks do not necessarily require the assumption of normality. FEC data obtained using the SCFT method were best represented by distributions associated with the negative binomial and hence arithmetic means could be used in FECRT calculations.Where FEC data were obtained with less sensitive counting techniques (i.e. McMaster 30 or 15 epg), zero-inflated distributions and their associated central tendency were the most appropriate and would be recommended to use, i.e. the arithmetic group mean divided by the proportion of non-zero counts present; otherwise apparent anthelmintic efficacy could be misrepresented.

Field evaluation of anticoccidial efficacy: A novel approach demonstrates reduced efficacy of toltrazuril against ovine Eimeria spp. in Norway

“…The sensitivity of commonly used McMaster methods for performing Eimeria oocyst counts usually range between 5 and 50 OPG ( Reeg et al, 2005 ; Saratsis et al, 2011 ). The method used in our study has a theoretical sensitivity of 5 OPG, which, given the extremely high mean OPG values, is of sufficient sensitivity to ensure the oocyst count values will be high enough to minimise the proportion of false/excess zeros that may otherwise affect the distribution of counts and bias the final results ( Denwood et al, 2008 ; Love et al, 2017 ). In FECRT-calculations, diagnostic accuracy can be improved by increasing the sample size and lowering the detection limit ( Levecke et al, 2012 ).…”

Ivermectin treatment failure on four Irish dairy farms

“…It should be recognised that a number of factors such as sample size, the detection limit of the method used to determine FEC, the pre-treatment FEC values, the level of FEC aggregation within the treated group and the method used to generate confidence intervals can influence both the detection and interpretation of treatment efficacy/inefficacy [19, 20]. In an effort to mitigate against the influence of some of these factors on both test sensitivity and specificity, 15 calves were randomly selecting for sampling on day 0.…”

Investigating the Benefits of Target Selective Treatment by Average Daily Weight Gain Against Gastrointestinal Nematodes in Morada Nova Lambs

“…Although there was not a highlighted agreement between DWG and EPG, the weight measurement seems to be a more reliable parameter of the impact of infection than EPG counts. According to Love et al (2017), EPG counts, performed by different methods, failed in the correct decision-making for the control of parasites, and it also interferes negatively in the use of the anthelmintics through FECRT. Therefore, a TST not based on EPG count can be a differential, especially in ocks with a predominance of resilient animals, and also, may be an important tool in the selection of more susceptible animals to be discarded.…”