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The distribution of the pathogenic nematode Nematodirus battus in lambs is zero-inflated

Published online by Cambridge University Press:  14 July 2008

M. J. DENWOOD ,
M. J. STEAR ,
L. MATTHEWS ,
S. W. J. REID ,
N. TOFT  and
G. T. INNOCENT
M. J. DENWOOD*
Affiliation:
Comparative Epidemiology and Informatics, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
M. J. STEAR
Affiliation:
Veterinary Genes and Proteins Group, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
L. MATTHEWS
Affiliation:
Comparative Epidemiology and Informatics, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
S. W. J. REID
Affiliation:
Comparative Epidemiology and Informatics, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
N. TOFT
Affiliation:
Danish Meat Association, Vinkelvej 11, DK-8620 Kjellerup, Denmark
G. T. INNOCENT
Affiliation:
Comparative Epidemiology and Informatics, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK
*
*Corresponding author: Comparative Epidemiology and Informatics, Institute for Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK. E-mail: m.denwood@vet.gla.ac.uk
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Summary

Understanding the frequency distribution of parasites and parasite stages among hosts is essential for efficient experimental design and statistical analysis, and is also required for the development of sustainable methods of controlling infection. Nematodirus battus is one of the most important organisms that infect sheep but the distribution of parasites among hosts is unknown. An initial analysis indicated a high frequency of animals without N. battus and with zero egg counts, suggesting the possibility of a zero-inflated distribution. We developed a Bayesian analysis using Markov chain Monte Carlo methods to estimate the parameters of the zero-inflated negative binomial distribution. The analysis of 3000 simulated data sets indicated that this method out-performed the maximum likelihood procedure. Application of this technique to faecal egg counts from lambs in a commercial upland flock indicated that N. battus counts were indeed zero-inflated. Estimating the extent of zero-inflation is important for effective statistical analysis and for the accurate identification of genetically resistant animals.

Keywords

Nematodirus battusOvis ariesdistributionBayesian MCMCfaecal egg countZero-inflated negative binomial
Type
Original Articles
Information
Parasitology , Volume 135 , Issue 10 , September 2008 , pp. 1225 - 1235
Copyright
Copyright © 2008 Cambridge University Press

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