title: Zero-inflated hierarchical models for faecal egg counts to assess anthelmintic efficacy.
abstract: The prevalence of anthelmintic resistance has increased in recent years, as a result of the extensive use of anthelmintic drugs to reduce the infection of parasitic worms in livestock. In order to detect the resistance, the number of parasite eggs in animal faeces is counted. Typically a subsample of the diluted faeces is examined, and the mean egg counts from both untreated and treated animals are compared. However, the conventional method ignores the variabilities introduced by the counting process and by different infection levels across animals. In addition, there can be extra zero counts, which arise as a result of the unexposed animals in an infected population or animals. In this paper, we propose the zero-inflated Bayesian hierarchical models to estimate the reduction in faecal egg counts. The simulation study compares the Bayesian models with the conventional faecal egg count reduction test and other methods such as bootstrap and quasi-Poisson regression. The results show the Bayesian models are more robust and they perform well in terms of both the bias and the coverage. We further illustrate the advantages of our proposed model using a case study about the anthelmintic resistance in Swedish sheep flocks.
authors.affiliation: Department of Mathematics, University of Zurich, Zurich, Switzerland.
authors.forename: Craig
authors.initials: C
authors.lastname: Wang
authors.affiliation: Section of Veterinary Epidemiology, Vetsuisse Faculty, University of Zurich, Switzerland.
authors.forename: Paul R
authors.initials: PR
authors.lastname: Torgerson
authors.affiliation: Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, Section for Parasitology, Uppsala, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Höglund
authors.affiliation: Department of Mathematics, University of Zurich, Zurich, Switzerland; Department of Computer Science, University of Zurich, Zurich, Switzerland.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Furrer
mesh_terms: D000818:Animals; D000871:Anthelmintics / Q000592:standards / Q000627:therapeutic use; D001499:Bayes Theorem; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology*; D005260:Female; D006374:Helminthiasis, Animal / Q000188:drug therapy / Q000469:parasitology*; D006376:Helminths / Q000187:drug effects / Q000502:physiology*; D015233:Models, Statistical; D010270:Parasite Egg Count / Q000662:veterinary*; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy / Q000469:parasitology*; D013548:Sweden
chemical_list: D000871:Anthelmintics
keywords: Anthelmintic resistance; Bayesian hierarchical model; Faecal egg count reduction test; Statistical analysis; Zero-inflated models