EFSA: factors associated with Salmonella pen positivity

A European Union-wide baseline survey on Salmonella in holdings with breeding pigs was carried out in 2008. Now EFSA publishes the results from analyses of the associations of 19 pen- or holding- level factors and Salmonella positivity of pens in holdings with breeding pigs. Also the results from correlation analyses between Salmonella prevalence in breeding and in production holdings, from analyses of the Salmonella serovar distribution across the European Union, and from analyses of an additional within-holding prevalence study carried out by five Member States are also presented.

Friday 29 July 2011 (7 years 2 months 20 days ago)
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These are results regarding the association of several pen- and holding level factors, on which data were collected, and Salmonella pen positivity:

• The risk for Salmonella pen positivity varied significantly between countries and between holdings within a country, even when adjusting for the effect of other associated risk factors. Thus, within countries there were holdings with a higher prevalence and holdings with a lower prevalence of Salmonella-positive pens.

• At EU level, the odds of Salmonella pen positivity in holdings with breeding pigs increased with the size of the holding, as well as with the number of pigs in the pen. Pens containing pregnant or farrowing and lactating pigs were less likely to be positive for Salmonella compared to pens housing maiden gilts. Pens where pigs were fed with commercial feed had higher odds of being positive compared to those where pigs were fed with home-mill mixed feed, or feed with maize, or feed from other sources. In addition to the source of the feed, the type of diet (formulation of the ration) was found to be significantly associated with Salmonella-positive pens. Pens where pigs were fed with pelleted feed had higher odds of positivity, while mash (meal/mash) or wet (porridge liquid) feed was found to have a protective effect.

• At EU level, in production holdings, pens with (fully) ‘slatted floors’ were significantly associated with lower odds of Salmonella positivity than pens having solid floors with otherthan-straw bedding or without bedding, or pens with partly slatted floors. No significant differences were found when comparing ‘solid floor with straw’ and the type of floor classified as ‘other’ with (fully) ‘slatted floors’. Pens with a floor type ‘outdoor in field or paddock’ had more than three times higher odds of Salmonella positivity than the ‘fully slatted floor’. Conversely, in breeding holdings, the category ‘other’ type of floor put pens significantly at risk for Salmonella positivity when compared to pens with ‘slatted floor’ and no other associations were found between the type of floor and the Salmonella positivity.

• The pooled swab sample was a more sensitive sampling method for detecting Salmonella positivity compared to the pooled composite faecal sample.

• The analyses showed that 56% of the unexplained variance in the prevalence of Salmonella positive pens in holdings with breeding pigs could be either attributed to uninvestigated holdingspecific factors and/or to the clustering of Salmonella linked to its infectious character. It was not possible to estimate the association of these factors with Salmonella pen positivity and their potential confounding role on the effect of factors on which data were available.

• The results of the analyses of Population Attributable Fractions suggested that the type of feed used is a major determinant of Salmonella pen positivity in holdings with breeding pigs. Feeding pigs on a porridge/liquid diet may be considered as a valuable control measure for Salmonella, leading to a reduction of the Salmonella pen positivity. It was also suggested that intervention strategies targeting other factors such as floor type and holding size may contribute to reduce Salmonella in holdings with breeding pigs. Conversely, control measures aimed at reducing the number of pigs per pen might not have a worthwhile impact on the Salmonella prevalence.

• Heterogeneity in the distribution of Salmonella serovars between Member States indicated that some serovars tend to occur in pig production in specific geographic regions within the EU. This spatial distribution, as well as the variation in prevalences across the countries might be related to differences in policies for trade in pigs, feeding and housing practices.

• The analyses of the Salmonella serovar distribution revealed agreement between the most frequently reported serovars in breeding pigs, those isolated in slaughter pigs and some serovars involved in human cases. This supports the role of pigs and pig meat as a potential source of Salmonella infection in humans; even though it is acknowledged that other food producing animal species and food thereof also play a role as sources of infection.

Salmonella Typhimurium is the second most common serovar in humans in the EU and its relative importance has increased during recent years. This serovar was common in pigs, and relatively more common than in poultry, suggesting that the pig reservoir is likely to be an important source of some human S. Typhimurium infections.

• Even though the baseline surveys on Salmonella in slaughter pigs and in holdings with breeding pigs were conducted one year apart, the descriptive comparison made between the Salmonella Member States prevalence figures of both surveys disclosed a significant positive correlation, indicating that Salmonella has a potential to disseminate from breeding pigs to fattening and then slaughter pigs. This link was further evidenced by the strong association found between the prevalence of Salmonella-positive breeding holdings and the prevalence of Salmonella-positive production holdings in countries suggesting dissemination of Salmonella from breeding pigs to fattening and then slaughter pigs.

• The results of the within-holding prevalence study indicated that, due to a non-perfect diagnostic test sensitivity, the observed EU-level prevalence of Salmonella-positive holdings with breeding pigs was roughly 80% of the estimated true EU-level prevalence. But this proportion varied between Member States. The diagnostic sensitivity of pooled and individual faecal samples was estimated to be 92% and 87%, respectively.

EFSA

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