A relevant long-term impact of the circulation of a potentially contaminated vaccine on the distribution of scrapie in Italy. Results from a retrospective cohort study |
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| Authors: | Bertolini Silvia Maurella Cristiana Bona Cristina Ingravalle Francesco Desiato Rosanna Baioni Elisa Chiavacci Laura Caramelli Maria Ru Giuseppe |
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| Institution: | 1.Epidemiologia e Osservatorio Epidemiologico, Istituto Zooprofilattico Sperimentale del Piemonte, Liguria e Valle d''Aosta (IZSPLVA), Via Bologna 148, 10154, Turin, Italy |
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| Abstract: | ABSTRACT: A sudden increase in the incidence of scrapie in Italy in 1997 was subsequently linked to theuse of a potentially infected vaccine against contagious agalactia. The relative risk for theexposed farms ranged between 6 and 40. The aim of this study was to assess the long-termimpact of exposure to the potentially scrapie-contaminated vaccine on the Italian classicalscrapie epidemic. We carried out a retrospective cohort study, fitting mixed-effects Poissonregression models, dividing national geographic areas into exposure categories on the basis ofthe vaccine circulation levels. We took into account the sensitivity of the surveillance systemapplied in the different areas. The population attributable fraction (PAF) was used to assessthe impact on the total population of farms associated with the effect of circulation of thevaccine. The provinces where the vaccine was more often sold were noted to have a higherlevel of disease when compared to those provinces where the vaccine was sold less often(incidence rate ratio IRR]: 2.7; 95% confidence interval CI]: 1.1-6.5). The populationattributable fraction was high (68.4%). Standardization techniques allowed to account for thepotential of geographical variability in the sensitivity of the Italian surveillance system.Although the number of the directly exposed farms was limited, an important long-termimpact of the vaccine circulation could be quantified in terms of secondary outbreaks likelydue to the exchange of animals from directly exposed flocks. |
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