Eggs Not a Likely Source of Salmonella Contamination

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February 05, 2003 | 28,608 views

Is salmonella really a significant threat when eating eggs raw?

To understand either side of the issue, it’s helpful to first understand the infection. Salmonella, discovered in 1887 by American veterinarian Daniel Elmer Salmon, is a large genus of bacteria with more than 2,000 strains or serotypes.

Some strains, such as typhoid (S. typhi), only affect humans, while others strictly affect birds (S. pullorum and S. gallinarum) or rodents. The strains are widely adaptable to most vertebrates, however.

According to one professor who has studied salmonella for 40 years, the bacteria likely exist in many organisms as a part of the gut flora and may mistakenly make its way across the gut wall and into the organism. Once there, it can overcome many of the body’s defense mechanisms.

Salmonella poisoning often causes gastroenteritis, an irritation at the gut wall that can cause loose stools and flu-like symptoms. Some cases are not severe and may go unnoticed while others, as noted by the U.S. Centers for Disease Control and Prevention (CDC), caused by the S. enteritidis strain may, in rare cases, be deadly.

Most deaths occur among the elderly, infants or people with weakened immune systems. From 1985 to 1998 there were 79 deaths associated with the S. enteritidis "epidemic," which when equated to about five deaths per year is one-tenth the number of U.S. deaths caused by lightening each year.

CDC Assumptions

In 1988, the CDC noticed that a formerly dominant strain, S. typhimurium, was being replaced by the S. enteritidis strain, which is still troublesome today.

The CDC also noted that food poisoning in the northeastern United States had risen fivefold. Since a percentage of the food poisoning victims had eaten Grade A whole eggs, they reasoned that the eggs must have been the cause.

The increase, according to CDC speculations at the time, could have come from a particularly dangerous hen-adapted salmonella strain that, if present in the hen's reproductive systems, might be able to infect eggs underneath the shells. Therefore normal precautions, such as washing eggs, would not control it.

The CDC also said that this theory might be wrong. S. enteritidis was not new; its prevalence had simply risen. Further, according to CDC doctors, between 1973 and 1984 some 64 percent of salmonella outbreaks did not involve food.

Moreover, Americans eat about 225 eggs each in a year. It was difficult to pinpoint the food poisoning to eggs since it was likely that everyone had eaten an egg before getting sick.

The increase in S. enteritidis during the late 80s was seen not only in the northeastern United States but also in Europe, particularly in the United Kingdom, Scandinavia and Eastern Europe. Also, the strain appeared in Southern California by 1994.

The local outbreak was the S. enteritidis subtype, called "phage 4," which had been detected in European chicken flocks but which was previously unknown in U.S. poultry. Eggs were thought to be the probable cause, despite the fact that there was no obvious agricultural source.

According to one biologist with the school of veterinary medicine at UC Davis, the more obvious carriers of S. enteritidis are humans. Further, since S. enteritidis’ showed up in Southern California, researchers looked at Mexico as a possible source. According to Mexican authorities, S. enteritidis had been a leading human infection in the country for many years.

Human involvement seemed even more likely in 1994 when a researcher from UC Davis' Veterinary Diagnostic Laboratory System found identical S. enteritidis phage types from municipal sewage outfalls serving housing developments, one of which was near an affected chicken ranch. Upon further testing he found that the strain was also present in wildlife and feral cats.

This research implied that S. enteritidis was being transferred from humans to creeks to wildlife and then, probably through mice, to chicken feed and then to hens.

Similarly, research conducted in Britain found a connection between outbreaks and fields treated with sewage sludge.

If humans were significant carriers for S. enteritidis, and if it was widely dispersed through environmental bacterium, then on-farm controls would not necessarily stop S. enteritidis food poisoning.

Low-Risk or No-Risk

Salmonella infected eggs are a rarity, largely because hens with infected ovaries tend to stop laying eggs.

A more likely source of salmonella would be cross- contamination in the kitchen when eggs are broken. Once their yolks are spilled from behind layers of protective shell, membranes and albumin, they become an excellent medium for S. enteritidis if contaminated from another source and kept at the right temperature, according to one researcher.

S. enteritidis poisonings are more likely to occur in commercial kitchens than in homes, as indicated by CDC data. Commercial kitchens may be more likely to keep egg yolks around for omelet mix or French toast dipping. During the salmonella "epidemic" from 1985 to 1991, there were 380 reported outbreaks. Of these, the CDC found that only 40 occurred in private kitchens.

Pasteurization Offers No Cure

If cross-contamination is the problem, pasteurizing the eggs probably offers no protection. According to one British advisor, pasteurization could even cause more harm if people believe there is not risk with pasteurized eggs and therefore do not exercise basic hygiene measures.

This is an interestingperspective that may help to alleviate some of the widespreadassociation between raw eggs and salmonella. The major challengethat many people have to overcome before consuming uncookedeggs is their phobia of salmonella infection.

Salmonella infectionsare relatively rare. Aside from the issues discussed above,data from the U.S. Department of Agriculture shows that thesalmonella infection rate of eggs is only one egg in 30,000.

Further, itis not likely that the hens are transferring the infection,but rather humans. The comment I wrote on the CDC'srecent salmonella update is, therefore, not quite accurate.Screening programs to detect farms that are producing contaminatedeggs may not solve the problem since salmonella is endemicto our culture.

For more detailsand specific information on how to properly consume raw eggs,please review my recent article RawEggs For Your Health. If you search on the Internet for‘raw eggs’ using Google search engine, this articlecurrently comes up as the number one listing.


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