Salmonella Enteritidis in commercial layer flocks in Europe: legislative background, on-farm sampling and main challenges

During 2006 there were 160,645 reported human cases of salmonellosis in the then 25 Member States of the European Union (equivalent to an incidence of 35.4 cases per 100,000 population (EFSA, 2007a), making Salmonella the second most commonly reported gastrointestinal zoonotic infection across the EU. However, this figure is likely to be a considerable underestimate of the true incidence of disease. In the EU there are five serovars that account for the majority of cases of Salmonella in humans. These serovars are S. Enteritidis (SE), S. Typhimurium (ST), S. Infantis, S. Hadar, and S. Virchow, and were designated by the European Commission (EC) as 'serovars of public health significance'. However, in Europe, SE is by far the predominant serovar (59-62%), followed at a distance by ST (13-17%) (EFSA, 2007a; The rise of SE over the last few decades can be rightly described as an epidemic. The number of human cases of SE started to increase dramatically in most countries of western Europe during the mid -to late eighties, and a similar phenomenon had been observed in some US regions a few years before This 'SE epidemic' involved mostly phage type (PT) 4, although in some European countries and the USA PT8 was initially predominant. In the UK, for example, at the peak of the SE epidemic (1996/97) PT 4 isolates made up 75% of all SE human isolates. Animal surveillance data suggested a rapid spread in commercial layers across the EU and beyond. The rapid spread of PT 4 worldwide suggests that the epidemic may have originated from infected grandparent breeding stock, and was subsequently amplified through hatcheries and disseminated globally. A notable exception to this phenomenon in western Europe was seen some Nordic countries with very strict Salmonella control programmes.