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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.
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