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Questions Over the Attribution of the Salmonella Braenderup “Outbreak” to NC Egg Farm

05/20/2018

Based on simple arithmetic, the apparent outbreak of Salmonella Braenderup attributed by the FDA to a large integrated farm in coastal NC is questioned. A total of 35 cases among consumers were identified between mid-November 2017 and mid-April 2018. Assuming 80 percent hen-day production, over a duration of 150 days with 2.4 million eggs per day, 360 million eggs would have been distributed in states with a population approximating 50 million. Accepting 35 diagnosed cases as a numerator and 360 million egg-days as a denominator, the probability of acquiring infection from an egg packed by the implicated farm is 1 in 1x10-7. Of the 25 patients interviewed, 16 reported consuming an egg dish at a restaurant.

The critical epidemiologic considerations to positively identify the farm as the source of an infection would be

  • Demonstrating homogeneity among the isolates derived from patients using whole genome sequencing. (WGS)
  • Demonstrating the presence of the same WGS pattern Salmonella Braenderup from the affected farm. This would be from manure drag swabs or on the surface of shells after washing or from egg contents assaying egg pools.

An on-farm inspection by relatively inexperienced FDA investigators which demonstrated the presence of flies and rodents does not represent even circumstantial support for the action taken by FDA given the prevalence of flies and rodents at “acceptable” levels on many farms with high-rise houses.

The FDA would be well advised to consult their own data relating to the 2010 outbreak of SE of long-standing infection in flocks operated by the DeCoster family in Iowa. The epi curve reproduced below characterized the outbreak from two farms with less than 1.5 million hens. There is no comparison in the attack rates between the 2010 Iowa outbreak and the 2018 North Carolina case nor the apparent relative prevalence of Salmonella as determined from on-site assays.

In the event the owners of the farm are now consigning eggs to breaking and pasteurization. It will be of interest to monitor the incidence rate of Salmonella Braenderup not only in the areas where the farms previously distributed shell eggs, but also in the population in the U.S. monitored by existing foodborne infection databases.