EU research on microbial quality of eggs raises questions

Results and conclusions must be carefully evaluated.

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Dr. K. de Reu of the Institute for Agriculture and Fisheries Research in Melle, Belgium, reviewed current EU research on the microbial quality of eggs derived from a range of housing systems at the 2010 EGGS! program organized by VIV and sponsored by WATT.

Shell contamination varies
During the past five years field studies have been conducted in the EU to evaluate shell and interior quality under both experimental and field conditions from hens housed in conventional cages, enriched colony cages, aviaries and floor systems. Because of the differences in experimental design, comparison among trials and interpretation of results is difficult.
Based on the sources cited it was possible for Dr. de Reu to draw the following conclusions:

  • There is no significant difference in the level of shell contamination between eggs produced in conventional cages and eggs laid in the nest boxes of enriched cages.
  • Eggs laid in other than the nest box area of enriched cages are significantly more contaminated than eggs laid in the nest area.
  • Floor eggs have a significantly higher level of contamination than nest eggs in floor systems irrespective of whether communal nests or aviaries are installed. 
  • Eggs derived from furnished cages have a higher level of coliform organisms on their shells compared to conventional cages. The specific study cited did not differentiate between eggs laid in the nest area or the floor area.
  • Eggs from aviaries had higher levels of contamination than eggs from conventional cage systems although more recent studies have not disclosed any differences in the quantity of Gram-negative bacteria recovered.
  • In a field study comparing seven flocks divided between aviaries and floor systems, eggs from floor systems had higher levels of contamination than the shells of eggs from conventional cages.

Early studies on broiler breeders during the 1970s in the U.S. confirmed that eggs derived from units with litter floors have significantly higher levels of aerobic bacteria than eggs laid on wire floors. From 85% to 100% of floor eggs derived from non-caged systems have either fecal or litter contamination of shells and are regarded in the EU as unsuitable for human consumption.

The level of bacteria in the environment of flocks influences shell contamination. On average bacterial level of air from aviary houses was up to 100 times higher than the air in houses with either conventional or furnished cages.

An interesting observation comprised a review of the level of shell damage from alternative systems. Studies conducted in 1999 documented from 5% to 14% of eggs with shell cracks from conventional cages. This is higher than levels of damage recorded by U.S. in-line complexes. The proportion of cracked eggs ranged from 0 to 24% in the EU trials with the highest levels in furnished cages. Damage was attributed to incorrect adjustment of the egg saver wires which prevents damage as the egg rolls from the cage floor on to the collecting tray.

Salmonella status evaluated
Field studies were conducted in Belgium to determine the level of Salmonella enteritidis (SE) contamination in relation to housing systems. The most recent data cited involved a prevalence study conducted in 2006 which demonstrated a salmonella recovery rate of 0.8% with 90% of the egg isolates identified as SE. Projecting this prevalence rate to the U.S. would result in the daily production of over 1.6 million eggs contaminated with SE. It is obvious that contamination rates in Belgium and Holland are far higher than in the U.S. It is known that SE contamination in flocks in southern Europe is even greater than in northern Europe, invalidating any comparisons relating to the EU and the U.S.

Data from limited field surveys were presented suggesting that the highest levels of recovery of SE were obtained from hens in conventional cages. This is completely counterintuitive since cage systems inhibit coprophagy and these flocks should in fact have had lower levels of contamination. The EU field studies were obviously flawed as there was no indication of SE vaccination status, whether hens were in fact intestinal carriers or whether flocks had been stressed to induce vertical transmission.

The conclusion that “it is highly unlikely that a move from conventional cages to alternative cage systems and non-caged housing systems will result in an increase in salmonella infection and shedding, rather the opposite is expected” is not supported by either the validity of the surveys or by common sense. Many factors influence both intestinal colonization and vertical shedding.

Unfortunately this EU data will be misinterpreted and will be used to oppose cage systems to the determent of the U.S. industry. An ill-informed public will regrettably be confronted with fallacious data and opinions from opponents of confined housing, suggesting that caged systems are more likely to produce eggs contaminated with SE.

Egg washing reduces SE
The second significant consideration invalidating direct comparisons between the EU and the U.S. is the reality that our eggs are washed in accordance with guidelines specified by USDA-AMS. Regulators in the EU have traditionally disfavored washing of eggs within the EU although the practice is common in Scandinavian countries. Given the high levels of SE surface contamination in addition to the presence of paratyphoid Salmonella spp (serogroups A, B and C), a number of epidemiologists, researchers and industry representatives are calling for a reevaluation of egg washing which has been shown to be beneficial from a public health perspective.

Carefully evaluate results  

The take home message from the VIV EGGS! program is that results and conclusions from experiments and field studies must be carefully evaluated.

Interpretation should incorporate considerations such as:

  • housing systems,
  • industry structure,
  • egg quality assurance programs,
  • the National Poultry Improvement Plan,
  • universal adoption of efficient vaccination and management practices based on the UEP Five Star program and the proposed FDA Final Rule on suppression of salmonella.
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