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

Jj Carrique-Mas,Rh Davies

doi:10.1590/s1516-635x2008000100001

Abstract

BACKGROUNDDuring 2006 there were 160,645 reported human cases ofsalmonellosis 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 reportedgastrointestinal zoonotic infection across the EU. However, this figureis likely to be a considerable underestimate of the true incidence ofdisease. In the EU there are five serovars that account for the majorityof cases of Salmonella in humans. These serovars are S. Enteritidis (SE),S. Typhimurium (ST), S. Infantis, S. Hadar, and S. Virchow, and weredesignated by the European Commission (EC) as ‘serovars of publichealth significance’. However, in Europe, SE is by far the predominantserovar (59-62%), followed at a distance by ST (13-17%) (EFSA, 2007a;Fisher, 2004). The rise of SE over the last few decades can be rightlydescribed as an epidemic. The number of human cases of SE started toincrease dramatically in most countries of western Europe during themid - to late eighties, and a similar phenomenon had been observed insome US regions a few years before (Saeed, 1999). This ‘SE epidemic’involved mostly phage type (PT) 4, although in some European countriesand the USA PT8 was initially predominant. In the UK, for example, atthe peak of the SE epidemic (1996/97) PT 4 isolates made up 75% of allSE human isolates. Animal surveillance data suggested a rapid spreadin commercial layers across the EU and beyond. The rapid spread of PT4 worldwide suggests that the epidemic may have originated frominfected grandparent breeding stock, and was subsequently amplifiedthrough hatcheries and disseminated globally. A notable exception tothis phenomenon in western Europe was seen some Nordic countrieswith very strict Salmonella control programmes.Epidemiological investigations demonstrated that contaminated eggsproduced by infected laying hens were the main source of humaninfection with SE (Coyle et al., 1988; Gillespie & Elson, 2005; Gillespieet al., 2005; Rabsch et al., 2001). SE shows an affinity for internal organsof the chicken and (in the case of laying hens) for the ovary and oviduct,leading to internal contamination of eggs prior to being laid (Guan etal., 2006; Humphrey et al., 1989; Poppe, 1999). However, the egg mayalso become contaminated in the shell when the poultry houseenvironment is highly contaminated with SE. In the EU, washing of eggsis not permitted, and irradiation is not regarded as acceptable by thegeneral population.Because the European Union functions as a single market, theEuropean Commission has played a major role in harmonising and co-ordinating monitoring and control programmes and ultimately aimingto reduce the prevalence of Salmonella in primary production of poultryacross member states (MS). A key piece of EU legislation is the EuropeanCommission (EC) Regulation No. 2160/2003, which requires that MS

Highlights

During 2006 there were 160,645 reported human cases of salmonellosis in the 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
These were that: 1) antimicrobials cannot be used to control Salmonella; 2) Mandatory vaccination is to be used against SE of commercial layers in countries in which the SE prevalence is greater than 10%; 3) live vaccines can only be administered to laying flocks in rear, and that the manufacturer must provide a method of distinguishing the vaccine from a field strain
For example, sampling faeces alone may miss birds that have passed the peak of infection but which are still producing contaminated eggs (Riemann et al, 1998). The sampling of both faeces and dust is recommended to increase the sensitivity of detection

Summary

BACKGROUND

During 2006 there were 160,645 reported human cases of salmonellosis in the 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. Main challenges to the control of Salmonella in commercial laying houses in the EU The presence of SE in laying houses in the UK (and in many other EU countries) in most cases is probably a consequence of the historical introduction of this serovar with replacement birds during the period when parent breeding flocks were most likely to be infected In these cases, SE has subsequently persisted over a number of production cycles without having been detected. In the UK, out of 15 houses that were detected to be positive with SE in the 2004/2005 EU baseline survey, 12 still remained contaminated with SE (in most cases with the same phage type ) when the followon flock was sampled (Carrique-Mas et al, 2008) This is not surprising given that past monitoring programmes have not been very sensitive, and cleaning and disinfection standards and rodent control have not been adequate, partly as a consequence of market pressures. Rapid elimination of SE within the life of a flock has been observed when the rodent populations have been eliminated

CONCLUSIONS

Findings

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