Intestinal parasites in swine in the Nordic countries: multilevel modelling of Ascaris suum infections in relation to production factors

Abstract

In Denmark, Finland, Iceland, Norway and Sweden, 413 sow herds were randomly selected for sampling. Faeces from pigs of 7 age groups/categories were examined for helminth eggs (11233 individual samples), and an accompanying questionnaire was completed at each visit. In total, 1138 pigs on 230 farms were found to be positive for Ascaris suum. Considerable differences in the occurrence of A. suum could be observed directly for several of 20 independent variables at the herd or category level. However, given that univariate analyses may be severely affected by confounding of covariates resulting in spurious inference, additional multivariate analyses were undertaken. An ordinary logistic regression on Ascaris positive/negative farms showed that Denmark had the highest frequency of infected herds, while Iceland and Finland had the lowest frequencies and that herds using ‘late weaning’ and ‘Class 2’ drugs (pyrantel, levamisole) were most often infected. Because many herds were found to be totally negative for A. suum, mixed hierarchical logistic-normal regression models (both the penalized quasi-likelihood and the Markov Chain Monte Carlo methods) were developed for both a full (all herds) and a reduced (the 230 infected herds) data set using either a cut-off of >0 eggs per gram (epg) or >200 epg to counter for false-positive egg counts. Estimates for identical models, but where the animal level variance was constrained to the binomial assumption, were also calculated. Significant covariates were robust to model development with ‘Age group’, ‘Country’, ‘Weaning age’, ‘Water system’ and simple interactions between the latter two and ‘Age group’ being significantly associated with the occurrence of A. suum, while all variables concerning anthelmintic drug, anthelmintic strategy, floor type, bedding, dung removal, washing and disinfection were not. These findings are discussed in the light of the complex relationship between A. suum and its pig host.