Faecal egg count reduction test

The fecal egg count reduction test was suggested in the World Association for the Advancement of Veterinary Parasitology guideline for estimating the reduction in fecal egg counts and its corresponding confidence interval. [1] The results of this test can be used to determine the anthelmintic resistance status of the animals.

Generally an analytical sensitivity of 50 is used, 15 is also possible if a greater sensitivity is desired. In order to reduce the counting variability, using groups of at least 10-15 animals was suggested, depending on the animal species of interest. In addition, the mean pre-treatment fecal egg counts should be at least 150 eggs per gram, otherwise the test can give unreliable results.

Mathematical formulation

Suppose a group of ${\displaystyle n_{T}}$ animals received anthelmintic treatment and a group of ${\displaystyle n_{C}}$ animals serves as control. The percentage reduction in fecal egg counts can be calculated as

${\displaystyle {\text{Percentage reduction }}=100\times \left(1-{\frac {{\bar {x}}_{T}}{{\bar {x}}_{C}}}\right)}$

where ${\displaystyle {\bar {x}}_{T}}$ and ${\displaystyle {\bar {x}}_{C}}$ denote the mean counts of the treatment and the control group. Assuming independence, the estimated asymptotic variance of the log ratio is given by

${\displaystyle {\widehat {\text{Var}}}\left(\log {\frac {{\bar {X}}_{T}}{{\bar {X}}_{C}}}\right)={\frac {s_{T}^{2}}{n_{T}{\bar {x}}_{T}^{2}}}+{\frac {s_{C}^{2}}{n_{C}{\bar {x}}_{C}^{2}}}.}$

where ${\displaystyle {\bar {X}}_{T}}$ and ${\displaystyle {\bar {X}}_{C}}$ denote the means of random samples, ${\displaystyle s_{T}^{2}}$ and ${\displaystyle s_{C}^{2}}$ denote the sample variance of the treatment and the control group counts. The variance can be used to construct an approximate 95% CI of the log ratio using the 97.5% and the 2.5% quantile of a Student's t-distribution with ${\displaystyle n_{T}+n_{C}-2}$ degrees of freedom. The CI for the log-ratio can be then transformed back to obtain the 95% CI for the estimated reduction.

Results interpretation

The World Association for the Advancement of Veterinary Parasitology guideline [1] states that for sheep and goats, the resistance is present if

(i) the percentage reduction in fecal egg counts is less than 95% and,

(ii) the corresponding lower 95% confidence limit is less than 90%.

If only one of these two criterion is met, then an anthelmintic resistance is suspected. Different thresholds have been suggested for other livestock.

Criticism and current alternatives

Firstly, the conventional counting techniques such as the McMaster method introduce additional variability in the counts which is not accounted for in the fecal egg count reduction test. [2] As a result, the estimated percentage reduction are less reliable especially for low counts. Secondly, the distribution of egg counts is typically aggregated within the host population. There were several attempts to propose more elaborate statistical models in the past years. [3][4] An emerging class of statistical model, namely Bayesian hierarchical models, has been proposed to overcome these problems. [5][6][7][8][9] Easy-to-use software [10][11] and website interface [12] based on those sophisticated but reliable statistical models are available to aid veterinary researchers.

Reference

1. Coles, G.C.; Bauer, C.; Borgsteede, F.H.M.; Geerts, S.; Klei, T.R.; Taylor, M.A.; Waller, P.J. (September 1992). "World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) methods for the detection of anthelmintic resistance in nematodes of veterinary importance". Veterinary Parasitology. 44 (1–2): 35–44. doi:10.1016/0304-4017(92)90141-U. PMID 1441190.
2. Torgerson, Paul R.; Paul, Michaela; Lewis, Fraser I. (2012-09-10). "The contribution of simple random sampling to observed variations in faecal egg counts" (PDF). Veterinary Parasitology. 188 (3–4): 397–401. doi:10.1016/j.vetpar.2012.03.043.
3. Torgerson, P. R.; Schnyder, M.; Hertzberg, H. (2005-03-31). "Detection of anthelmintic resistance: a comparison of mathematical techniques". Veterinary Parasitology. 128 (3–4): 291–298. doi:10.1016/j.vetpar.2004.12.009. PMID 15740866.
4. Dobson, R. J.; Hosking, B. C.; Jacobson, C. L.; Cotter, J. L.; Besier, R. B.; Stein, P. A.; Reid, S. A. (2012-05-04). "Preserving new anthelmintics: A simple method for estimating faecal egg count reduction test (FECRT) confidence limits when efficacy and/or nematode aggregation is high". Veterinary Parasitology. Special issue: Novel Approaches to the Control of Helminth Parasites of Livestock. 186 (1–2): 79–92. doi:10.1016/j.vetpar.2011.11.049. PMID 22154971.
5. Torgerson, Paul R.; Paul, Michaela; Furrer, Reinhard (2014-04-01). "Evaluating faecal egg count reduction using a specifically designed package "eggCounts" in R and a user friendly web interface" (PDF). International Journal for Parasitology. 44 (5): 299–303. doi:10.1016/j.ijpara.2014.01.005. PMID 24556564.
6. Levecke, Bruno; Anderson, Roy M.; Berkvens, Dirk; Charlier, Johannes; Devleesschauwer, Brecht; Speybroeck, Niko; Vercruysse, Jozef; Van Aelst, Stefan (2015-01-01). "Mathematical Inference on Helminth Egg Counts in Stool and Its Applications in Mass Drug Administration Programmes to Control Soil-Transmitted Helminthiasis in Public Health". In Basáñez, Roy M. Anderson and Maria Gloria (ed.). Advances in Parasitology. Mathematical Models for Neglected Tropical Diseases: Essential Tools for Control and Elimination, Part A. 87. Academic Press. pp. 193–247. doi:10.1016/bs.apar.2015.01.001. ISBN 9780128032565. PMID 25765196.
7. Denwood, M. J.; Reid, S. W. J.; Love, S.; Nielsen, M. K.; Matthews, L.; McKendrick, I. J.; Innocent, G. T. (2010-03-01). "Comparison of three alternative methods for analysis of equine Faecal Egg Count Reduction Test data". Preventive Veterinary Medicine. SVEPM 2009Papers presented at the 2009 annual meeting of the Society for Veterinary Epidemiology and Preventive Medicine (SVEPM). 93 (4): 316–323. doi:10.1016/j.prevetmed.2009.11.009. PMID 19962203.
8. Wang, Craig; Torgerson, Paul R.; Höglund, Johan; Furrer, Reinhard (2017-02-15). "Zero-inflated hierarchical models for faecal egg counts to assess anthelmintic efficacy". Veterinary Parasitology. 235: 20–28. doi:10.1016/j.vetpar.2016.12.007. PMID 28215863.
9. Wang, Craig; Torgerson, Paul R.; Kaplan, Ray M.; George, Melissa M.; Furrer, Reinhard (2018-08-16). "Modelling anthelmintic resistance by extending eggCounts package to allow individual efficacy". International Journal for Parasitology: Drugs and Drug Resistance. 8 (3): 386–393. doi:10.1016/j.ijpddr.2018.07.003. PMC 6091319. PMID 30103206.
10. "eggCounts". CRAN R Packages.
11. "bayescount". CRAN R Packages.
12. "Modelling Faecal Egg Counts". Retrieved 14 October 2016.