Setting stricter targets for Salmonella in laying hens at the farm level could cut the number of salmonellosis cases from that source in half, according to the European Food Safety Authority (EFSA).
European Union countries are required to reduce the proportion of laying flocks infected with certain types of Salmonella to 2 percent. EFSA experts estimate that if this target was 1 percent with the same testing and trade restrictions that salmonellosis cases in humans transmitted via laying hens would drop by 50 percent.
Salmonellosis is the second most common foodborne disease after campylobacteriosis in Europe. In 2017, member states reported 91,662 cases of Salmonella infections. The main source of infection for humans is consumption of contaminated foods of animal origin, including eggs and poultry meat.
A total of 3,637 foodborne outbreaks caused by Salmonella at EU level from 2014 to 2016 were recorded with 29,250 cases. One multi-country outbreak involved Salmonella Enteritidis linked to eggs from Poland, with cases ongoing since 2016. Current evidence points to the source of infection likely being at the level of laying hen farms. Whole genome sequencing (WGS) and epidemiological evidence suggests there is persistent contamination at laying hen operations in Poland.
Modeling based on 2016 data found the number of layer-associated human salmonellosis cases, which was 465,200, was estimated to be reduced by 53.38 percent. This decline would translate into a 6.2 percent decrease of the overall 4.08 million human salmonellosis estimated true cases.
A target of 1 percent is in place for breeding hens – at the beginning of the poultry production chain – for five types of Salmonella of human health significance. The EFSA opinion recommended maintaining the existing target for three of these and replacing the other two with more relevant types.
Reconsideration of the ﬁve current target serovars for breeding hens showed there is justiﬁcation for keeping Salmonella Enteritidis, Salmonella Typhimurium including monophasic variants, and Salmonella Infantis, while Salmonella Virchow and Salmonella Hadar could be replaced by Salmonella Kentucky and either Salmonella Heidelberg, Salmonella Thompson or a variable serovar in national prevalence targets.
However, a target that incorporates all serovars is expected to be more effective because relevant serovars in breeding ﬂocks vary between member state and over time. The EU’s member states must set up Salmonella National Control Programs (NCP) aimed at reducing prevalence of Salmonella serovars, which are considered relevant for public health, in certain animals.
Under the ﬁve serovar target, six member states failed in breeding ﬂocks at some point: Belgium in 2014, Bulgaria in 2015, Denmark in 2014, Greece in 2014, 2016, Poland in 2014, 2015, 2016, and Romania in 2014. However, if there had been an all Salmonella serovar target, 18 member states would have failed at least once from 2014 to 2016. Only the Czech Republic, Estonia, Finland, France, Portugal and Sweden did not exceed the 1 percent target in that time frame.
Culling flocks could be recommended for infections involving the top ﬁve serovars. Other containment methods such as improved egg hygiene, segregated hatching, competitive exclusion treatment and improved terminal hygiene following depopulation of infected ﬂocks could be applied for serovars considered less relevant.
EFSA experts said there is conﬂicting evidence on the occurrence of Salmonella in laying hens when raised in cage systems compared to non-cage systems.
“Overall, evidence points to a lower occurrence in non-cage systems compared to cage systems. Whether this is linked to the housing system as such or whether it is caused by the associated change of furniture, break in the historical infection cycle or the reduced stocking density, is unclear. The evidence that outdoor access or conventional vs. enriched cage systems affect Salmonella occurrence in laying hens at the EU level is inconclusive,” they said.
Evidence that outdoor access affects occurrence of Salmonella in broiler ﬂocks was also found to be inconclusive.
“There is conclusive evidence that an increased stocking density, larger farms and stress-inducing conditions result in increased occurrence, persistence and spread of Salmonella in laying hen ﬂocks. For broiler ﬂocks, the limited evidence available shows that stress, stocking density and increasing the number of ﬂocks per farm increases Salmonella susceptibility or infection rate. There is no data evaluating the link between welfare indicators and Salmonella occurrence in broilers,” according to the agency opinion.
Based on the differences between Salmonella and Campylobacter epidemiology and patterns of colonization, an impact of Salmonella control programs on the prevalence of Campylobacter in broiler ﬂocks at the holding and on broiler meat at the end of the slaughter process is not expected.
Recommendations from EFSA experts included WGS for comparing isolates from poultry breeding ﬂocks with those in commercial generations of birds and humans to provide evidence of a link between these areas and investigate the reasons for under-detection of Salmonella in ﬂocks of laying hens and to do ﬁeld investigations on effectiveness of Salmonella vaccination programs used in laying ﬂocks, and their protective effect.
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