NOVEMBER 2022


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the egg counting technique employed. Recent work has demonstrated the importance of the magnitude of eggs counted prior to applying the multiplication factor to convert the results to eggs per gram (EPG) of faeces (Dobson et al., 2012; Levecke et al., 2018). Depending on the technique used, an egg count of 200 EPG could represent as little as two or four eggs encountered or as many as 200 eggs counted under the microscope. A higher magnitude of eggs counted pretreatment will mean that treatment efficacy can be estimated with greater accuracy, so the egg counting technique employed will have a big impact on the reliability of the FECRT. Some veterinary practices and diagnostic services have adopted a practice of not counting above a certain egg count threshold. While this can be a justifiable pragmatic approach for some purposes, it prevents estimation of anthelmintic efficacy. A low post-treatment egg count could reflect anything from a very high to a significantly reduced efficacy depending on the pretreatment level. If the pretreatment egg count is not determined, however, this assessment cannot be made. Beyond efficacy testing, faecal egg counts are also


important for estimating strongylid egg shedding levels in mature horses. Several studies have documented that mature horses maintain consistent strongylid egg shedding levels over time, and tend to return to the same level after anthelmintic treatment (Misuno et al., 2018; Nielsen et al., 2006; Scheuerle et al., 2016). Furthermore, as a general pattern, a minority of horses is typically shedding the large majority of the total egg output in a herd as several studies have documented that 10–30% of horses contribute 80% of the total egg output (Lester et al., 2013; Nielsen et al., 2018b; Relf et al., 2013). Commonly used strongylid egg shedding categories in mature horses are low (<200 EPG), moderate (200–500 EPG) and high (>500 EPG) (Nielsen et al., 2019). Thus, there is value in identifying these higher shedders in a surveillance-based program, and targeting them with additional treatments, while leaving the low or negative shedders with a minimum of anthelmintic treatments (Nielsen et al., 2019). A third use of faecal egg count testing is for monitoring


of presence of ascarid parasites. The global anthelmintic resistance profile in Parascaris spp. is very different from that of the cyathostomins. For example, macrocyclic lactone resistance is widespread in equine ascarids, whereas this drug class is generally the most efficacious option for cyathostomin control (Peregrine et al., 2014). With resistance reported in both parasite categories to the other drug classes as well, a single all-round dewormer no longer exists and testing is therefore required in foals and young horses to determine the parasite categories present. Ascarid infections follow a characteristic pattern with most foals getting infected during the first weeks of life, egg counts peaking at 4–5 months of age, and burdens being eliminated from their hosts over the following few months (Donoghue et al., 2015; Fabiani et al., 2016; Nielsen et al., 2021). A proportion of yearlings sometimes experience a smaller second wave of infection at approximately 8– 12 months of age (Donoghue et al., 2015), and faecal testing will provide crucial information about the presence and subsequent elimination of ascarid infection. This information, in turn, will guide the veterinarian and their


clients to appropriately choose an anthelmintic with efficacy against ascarids, when encountered. Despite the relative simplicity of the faecal egg count


principle, their use and interpretation are often confusing to veterinary practitioners and their clients. There are several misconceptions surrounding faecal egg counts, and important biological and statistical aspects of this type of data are not always considered and presented correctly when interpreting the results. The aim of this article is to explain diagnostic performance parameters for faecal egg count


testing, address common


misconceptions associated with their use, and outline principles for a meaningful use of faecal egg counts in equine veterinary practice.


Egg count variability


A commonly made criticism about the diagnostic value of faecal egg counts is that they are highly variable. I have often encountered people claiming that egg count magnitude varies substantially during the day, and that there is pronounced variability between days. Over the years, many veterinarians and parasitologists have stated that a reliable egg count can only be determined by sampling the same horses for several consecutive days. It is true that an egg count determined on one day or at a certain time of day is likely to be different from another egg count determined from the same horse at a different time point. However, this does not necessarily mean that there is variation between time points or consecutive days, as observed variations could also be a reflection of variability between counts determined on the same sample. We therefore decided to do a study to properly identify and quantify sources of equine faecal egg count variation. We examined six horses with different strongylid egg count levels over five consecutive days. Each horse was sampled at the same four time points on each day: 6.00 am, 12.00 pm, 6.00 pm and 12.00 am. Three subsamples were taken from each sample and three repeated counts were determined on each subsample for a total of 1080 counts in the study (Carstensen et al., 2013). An analysis of variance found no differences between egg counts determined at the four different time points. Quantification of variance sources revealed that 89.01% of the variance in the data set was between the six horses, but only 0.10% was between the 5 days. The remaining variance was between subsamples (6.15%) and between repeated counts on the same sample (4.76%). Taken together, this study demonstrated that variation between time points and consecutive days is negligible, and that the majority of variance in faecal egg count data is between and within subsamples. In other words, there is little or no diagnostic gain by sampling horses across time points or consecutive days, despite those frequently made statements. The variation within and between subsamples emphasised the importance of method precision when determining faecal egg counts, which will be covered in the section entitled accuracy and precision.


Correlation with worm counts


It should be emphasised that neither ascarid nor strongylid faecal egg counts reflect the size of the intestinal mature © 2021 EVJ Ltd.


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