Egg Industry Presentations

Assessing the Significance of Salmonella Heidelberg in Egg-Laying Flocks, Richard K. Gast

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Dr. Richard Gast of the Egg Safety and Quality Research Unit, USDA-ARS Russell Research Institute in Athens Georgia recently addressed the UEP Annual Meeting in San Diego. His presentation is reproduced with permission, for the benefit of readers of EGG-CITE.

Effectively there has been no major outbreak of egg-borne S. Heidelberg (SH) reported to FoodNet since 1986. In this year an outbreak in New Mexico involving 91 patients was associated with consumption of eggs at a convention breakfast. Data collected during the period 1996 to 1997, associated consumption of eggs outside the home with sporadic SH infection. During 1993 to 2001, 21% of 101 investigated outbreaks of SH were attributed to consumption of eggs or egg-containing foods with 25% of outbreaks attributed to poultry meat.

Infection of consumers with this pathogen has been attributed mainly to consumption of ground turkey meat and from other non-egg sources.

In August 2010 SH was isolated from environmental samples derived from the Wright County Farms involved in the S. Enteritidis recall. There is obviously some measure of concern that the FDA may extend the Egg Safety Rule to include SH (Please refer to the editorial posted on October 24th). Given current information on the epidemiology of Salmonella infection in consumers, this would appear to be premature and unsupported by fact. Although SH is the fifth most frequently confirmed pathogen in consumers representing 3.5% of serotypes identified, cases are generally not associated with egg-borne transmission.

Although SH can be isolated from the environment of commercial laying flocks in the U.S. and Canada, the question of vertical transmission is an important consideration in determining the potential of SH to precipitate disease outbreak among consumers. Although FDA considers that SH can be transmitted vertically and is a food-borne pathogen the Agency apparently does not intend to include the serotype in the Egg Safety Rule. This is supported by the epidemiology of SH. Despite the large number of cases of S. Enteritidis (SE) confirmed and traced back to the Wright County complex in the 2010 outbreak there were no cases of SH documented, despite the presence of this serotype in the environment of flocks.

Dr. Gast has conducted trials on contrived infection using susceptible hens involving administration of selected strains of SH under laboratory conditions. It must however be recognized the level of infection used in these studies is significantly higher than what might be encountered under commercial conditions and the results may not be necessarily applicable to the practical situation. It is evident that SE can be recovered from the cecum, spleen, liver and to a lesser extent the oviduct and ovaries of hens which are artificially infected. Various strains of SH can also be obtained from organs in approximately the same proportion as SE. In contrast there is a higher rate of recovery of SE from egg pools compared to SH. Recovery of SH in either the yolk or albumen declined sharply with logarithmic reduction of the pathogen administered orally.

It is noted that various strains of SH will multiply in egg yolks over a 36 hour period if held at 86 F. This is a significant justification for maintaining a cold chain not exceeding 45 F from as close to lay as possible (36 hours in the Egg Safety Rule) through to point of sale. Even under controlled laboratory conditions with high infective doses, SH was associated with a far lower level of infection of the reproductive tract than SE, consistent with the fact that trace-back of SH has never been documented despite the few sporadic cases of presumably egg-borne infection among consumers.

From the perspective of prevention, the modalities currently applied to suppress SE would contribute to a reduction of SH infection. Unfortunately there is no NPIP "S. Heidelberg Clean" program for breeding flocks and hatcheries. Accordingly it is not possible to place pullet chicks known to be free of infection. The mutant live Salmonella Typhmimurium vaccine administered to pullets should suppress colonization of the intestinal tract with SH. It is evident that inactivated SE emulsion vaccines should incorporate SH antigen to stimulate humoral (circulating antibody) immunity in order to protect against organ infection and colonization. In the short term, high levels of biosecurity and vaccination against ST coupled with the use of probiotics and prebiotics and avoiding stress should be considered as positive measures to prevent infection.

The entire PowerPoint presentation by Dr. Gast as reproduced should be reviewed in relation to the epidemiology of S. Heidelberg in the context of the U.S. egg production industry.

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