In mid-2002, the Nationale Referenzzentrale für Salmonellen (NRCS)
noticed a regional cluster of human Salmonella enterica subsp. enterica
ser. Enteritidis phage type 5 (S. Enteritidis PT5) infections (1) in
a region consisting of two neighbouring districts (combined population
of the two
districts: 235 000; population of Austria: 8 031 000). From November
2001 to August 2002, 61 human isolates of S. Enteritidis PT5 were ascertained
in these two districts (figure 1).
There had been a small outbreak of S. Enteritidis PT5 in this region
in 1999. A least 8 people had been infected after eating cake that had
been left unrefrigerated for about five days. The eggs used in the preparation
of this cake had been traced back to a local egg producer. The reappearance
of S. Enteritidis PT5 in 2002 in the same two districts was a reason
to reinvestigate the farm involved in the 1999 outbreak.
Material and methods
The NRCS receives almost all human and non-human Salmonella isolates
from Austrian laboratories. No preselection is done by the dispatching
laboratories. The strains are accompanied by basic epidemiological data
which includes date of receipt by the primary laboratory, patient's
place of residence, etc. All S. Enteritidis strains are phage typed
according to international standards (2).
In Austria human outbreaks are investigated by local health authorities.
In case of outbreaks of major interest the NRCS collaborates actively
with local investigators.
In September 2002 a questionnaire asking about symptoms and a basic
food history in the three days prior to infection was sent to the patients.
At that time, the only available information about the S. Enteritidis
PT5 outbreak in 1999 in the same region was anecdotal. Therefore the
Österreichische Qualitätsgeflügelvereinigung (QGV) -
Austrian Poultry Health Service - APHS) was contacted to locate the
farm involved in the 1999 outbreak and to gather information about both
the 1999 and the current outbreaks. When the farm had been located,
voluntary bacteriological examinations of the farm started.
To exclude the possibility that the farm had been contaminated with
S. Enteritidis PT5 by the recently introduced hens, the APHS also gathered
all available information about the status of the rearing flocks of
the poultry breeders who had supplied the flocks to the farm.
When the evidence concerning the current outbreak was sufficient to
incriminate this farm, the farmer immediately stopped selling table
eggs from the farm.
To assess whether or not there had been a real increase of human Salmonella
infections in the region, the number of isolates ascertained during
the peak of the outbreak (calculated by adding together all isolates
identified during the months of July, August and September) were statistically
compared with the retrospective numbers from July to September in previous
years (1998 - 2001), assuming a Poisson distribution.
Analysis of the returned questionnaires did not give conclusive results
about the origin of the outbreak. Some human infections of S. Enteritidis
PT5 reported outside the two districts were related to travel in the
outbreak region. The returned questionnaires also revealed that one
person had died.
The farmer had destroyed his flocks following the 1999 outbreak, and
the poultry sheds had remained unoccupied for several weeks afterwards.
During this time, the poultry sheds were disinfected thoroughly using
formalin gas. During the years that followed, only vaccinated flocks
were introduced in the farm. An inactivated vaccine against S. Enteritidis
was used (TALOVAC SE, Lohmann Animal Health, Germany). In the summer
of 2001, however, the farmer decided to stop vaccinating his flocks.
The first flock not to be vaccinated against S. Enteritidis was introduced
into the farm in August 2001, and a second flock was introduced at the
end of April 2002. Both of these flocks comprised about 6 000 hens.
The output of table eggs from the farm was about 1.7 million eggs per
year (annual egg consumption in Austria is around 1 870 million eggs).
The hens were kept in cages. The eggs were sold locally by the Austrian
system of direct marketing (for example, in local market places, and
The untreated table eggs were withdrawn from sale at the end of September
2002 after S. Enteritidis PT5 was found in eggs from the farm. All eggs
from this farm were pasteurised thereafter before being used in industrial
In September 2002, in collaboration with the owner of the farm, bacteriological
samples were taken from the farm. S. Enteritidis PT5 was found in pooled
faecal samples, on eggshells, and in swabs from the room where eggs
were stored before leaving the farm.
There was no evidence for S. Enteritidis PT5 infection in hens from
the same rearing flocks as the outbreak flocks that were sold to other
farms. Most of these flocks were tested (independently from the described
outbreak) for Salmonella, but none of these samples tested positive.
The two rearing flocks were repeatedly tested for Salmonella (meconium
after hatching, pooled faeces in the tenth and fourteenth weeks of rearing,
swabs taken from the devices used for the transport of hens). These
bacteriological examinations also showed no growth of Samonella.
Retrospectively, the first sporadic cases of S. Enteritidis PT5 were
apparent by the end of 2001. A higher number of isolates from the two
districts were send to the NRCS from July 2002 onwards. From January
to the end of October 2002, 70 human isolates of S. Enteritidis PT5
were recognised in the two districts. Sixty eight isolates were derived
from faecal samples, one from a blood culture and one from an abdominal
wound swab. Thirty two patients were female, and 38 male. All age groups
were represented; the median age was 38 years. One patient is known
to have died from S. Enteritidis PT5 infection.
There had also been human infections with other types of Salmonella
in the two districts. In figure 2, S. Enteritidis PT5 infections are
compared to non-S. Enteritidis PT5 infections. When the total number
of isolates (S. Enteritis PT5 and non S. Enteritidis PT5) in the major
outbreak months (July-September 2002) was compared with the preceding
years, a statistically significant increase (p < 0.001) was observed.
The expected number of cases was 58.5, compared with the 132 actually
Untreated table eggs from the farm were withdrawn from sale at the end
of September 2002. In October 2002 only one isolate of S. Enteritidis
PT5 was reported in the two districts, although the overall number of
Salmonella isolates in Austria was high and comparable to the three
The problem of re-infection of new flocks placed in previously infected
chicken sheds is well known. The 2001 annual zoonoses report from Denmark
mentions nine egg producers with repeated infections after cleaning
and disinfection (3). After excluding the possibility that S. Enteritidis
PT5 had been introduced by primarily infected hens (S. Enteritidis PT5
was not found in the farms where the hens had been reared originally
from the same rearing flocks) it is most likely that the infection of
the flocks in 2002 resulted from S. Enteritidis PT5 contamination in
the environment of the farm. Presumably there had been niches in the
farm (poultry litter conveyer belts, rodents, etc.) where S. Enteritidis
PT5 had survived. As long as the flocks were vaccinated against S. Enteritidis,
however, the hens (and their eggs) were protected.
Vaccination of poultry may reduce the contamination of the environment
of a farm with Salmonella (4). It is tempting to believe that this effect
may eventually contribute to the eradication of Salmonella from a farm.
This outbreak demonstrates the consequences of the persistence of Salmonella.
Nevertheless, the near total disappearance in 2000 and 2001 of human
cases of S. Enteritidis PT5 in the region after the implementation of
vaccination is an example of the effectiveness of vaccination in preventing
human infections under field conditions (figure 1).
Laboratory confirmed Salmonella isolates represent only a small proportion
of all human infections. A recent report from the Netherlands states
that between 1996 and 2000, only 3729 (6.9%) out of 53 500 estimated
cases of human gastrointestinal salmonellosis were laboratory confirmed
(5). In the United States the degree of underreporting has been estimated
at around 38-fold (6). It is therefore realistic to assume that several
hundred infections occurred in the course of this outbreak in the space
of only a few months. This is noteworthy, because of the relatively
small amount of table eggs sold by this producer. It emphasises the
importance of eggs as vehicles for human infections.
Occurrence of 'extra' cases of S. Enteritidis PT5 infections, supported
by the statistical comparison of the total number of isolates in the
region from July to September 2002 with the corresponding figures of
the years 1998 - 2001, would probably have been preventable if the farmer
had continued to vaccinate his flocks or if Salmonella had been eradicated
from the farm after earlier clean up.