Friday, June 6, 1997, the Bureau of Epidemiology received
a telephone call from a physician in a Salt Lake City clinic,
reporting an outbreak of gastrointestinal illness among a
group of tourists from the United Kingdom. Upon interviewing
the four patients visiting the clinic, it was found that the
common symptoms of the patients were nausea, vomiting, abdominal
cramps, and explosive diarrhea. A subsequent interview
with the tour guide revealed that more than half of this group
of tourists had been ill. The index case had onset of
symptoms on May 29, 1997, i.e., one day after these tourists
arrived in the United States. Since this group of tourists
was scheduled to leave Salt Lake City that afternoon for Pocatello,
Idaho, and then to Jackson, Wyoming, Dr. Bao-Ping Zhu, EIS
Officer at the Department of Health, followed this tourist
group to conduct further investigations of this outbreak.
In this investigation, a case was defined as a person who:
(1) was riding in the tour bus between May 29 and June 6;
and (2) had vomited at least once during the period, or had
diarrhea (i.e., loose stools or bowel movements at least twice
during any 24-hour period). Cases were identified through
interviews with all persons riding the tour bus, including
the 45 tourists, the tour guide, and the bus driver, using
the case definition described above.
Information provided by the tour guide (which was later verified
during the interviews of the tourists) showed that, among
the 45 tourists, 39 had taken the same airplane from the United
Kingdom to Newark, New Jersey, on May 27. From Newark,
these 39 tourists boarded another airplane to Denver, Colorado.
They arrived in Denver late in the afternoon on May 28, 1997.
The remaining six tourists made their own travel arrangements
and arrived in Denver on May 28 on separate airlines.
Since three of the six tourists who had made their own travel
arrangements and arrived in Denver, Colorado on separate airlines
later also developed the illness, it appeared that the pathogenic
agent causing this outbreak was not contracted on the airplanes.
Therefore, efforts were focused on potential common food exposures
after these tourists had arrived in Denver. Because
the index case had onset of symptoms on the evening of May
29, and the only common meal shared before the onset of the
index case was the breakfast buffet served by Hotel A in Denver
between 7:00 a.m. and 8:00 a.m. on May 29, the investigation
was concentrated on food items served at that breakfast buffet.
In addition, information was collected about the food items
served on the airplanes and at a similar breakfast buffet
at the same hotel on the morning of May 30. A questionnaire
was constructed based on the information provided by the tour
guide and by the four patients visiting the Salt Lake City
clinic. The menus of the breakfast buffet on May 29
and May 30 and of the food items served on the airplanes between
the United Kingdom and Denver were obtained from Hotel A and
Airline X, respectively, to help the tourists recall their
food items. All persons involved agreed to participate
in the investigation, yielding a response rate of 100%.
Among the 47 persons riding the tour bus, 45 were tourists,
one was the tour guide, and one was the bus driver.
Twenty-five (53.2%) were men, and 22 (46.8%) were women.
The mean age of these persons was 65.6 years (range=51-83
years). The mean age for the cases was 66.3 years, and
that for the non-cases was 64.5 years. All persons were
Of the 47 persons involved, 28 had either vomited or had at
least two episodes of diarrhea during a 24-hour period, thus
meeting our case definition. The attack rate was 63.6%.
Among the 28 cases, nausea, vomiting and diarrhea were the
most common symptoms, each being reported by more than 70%
of the cases (Table 1). No
person had bloody diarrhea. Low grade fever was reported
by 35.7% of the cases. Seven (25.0%) had visited a health
care provider, and two persons (7.1%) were hospitalized.
Several cases developed a second episode of symptoms 1-3 days
after the first attack.
The index case had onset on the evening of May 29.
On the following day there were no cases. Four cases
had onset on May 31, followed by another day without cases.
During the next three days starting on June 2, the number
of cases dramatically increased, when there were eight, ten,
and four cases, respectively. No new cases emerged after
June 4, and by June 8, all patients had recovered. Assuming
the index case had the same illness as the other cases, the
breakfast buffet served by Hotel A on May 29 would be the
most logical common exposure. Therefore, the risk of
becoming ill in relation to the consumption of food and beverage
items served at that breakfast buffet (Table
2) was assessed. Three persons were excluded from
the analysis because they developed some symptoms but did
not meet the case definition. Of all food and beverage
items examined, only the breakfast potato was significantly
associated with the risk of developing illness (p=0.04).
However, the association
was rather weak, with the relative risk (RR) being only 1.6.
An interesting case was the bus driver. According to
the driver, he only started to have meals together with the
tourists on May 31. Thus he had no opportunity of being
exposed to the food or beverage items at the breakfast buffets
on either May 29 or May 30. However, on June 3 he also
became ill. Another relevant piece of information was
the seating patterns of the tourists on the bus. Instead
of staying in the same seat every day, the tourists systematically
rotated their seats twice a day. That is, if a person
sat in the front row in the morning, he or she would be rotated
to the last row in the afternoon, while the persons sitting
in the second row in the morning would be rotated to the front
row in the afternoon; and so on. Also worth mentioning
was the fact that there was a bathroom on the bus. According
to the tourists, during the first few days of the tour several
persons had used the bathroom when they were ill, although
they had been discouraged from using the bathroom later on
when more and more persons became ill.
The investigator's impression of the tour bus's interior was
that both the floor and the seats were remarkably clean.
The bathroom was not inspected; however, according to both
the tourists and the bus driver, the bathroom had been cleaned
frequently throughout the tour.
In addition to interviewing the tourists, the food services
management of Hotel A was contacted on June 9. According
to the hotel's management, no food handlers preparing the
breakfast buffet on May 29 had been ill prior to this outbreak.
Also, several employees of the hotel also ate at the breakfast
buffet on May 29, but none became ill afterwards.
Three cases submitted stool samples to the State Public
Health Laboratory, where the samples were tested for Salmonella,
Shigella, E. coli 0157:H7, and Campylobacter
spp. None of these pathogenic bacteria were found.
What pathogen has caused this outbreak? Among the four
types of pathogens that can cause gastrointestinal illnesses
(i.e., bacteria, viruses, toxins and parasites), it appears
that toxins and parasites can be readily excluded as the cause
of this outbreak, because intoxication (both chemical and
biological) usually has an acute and violent manifestation
and a short (<8 hours) incubation period, while parasite
infections (such as giardiasis) is usually characterized by
chronic diarrhea lasting several weeks, and patients rarely
have vomiting. From the distribution of cases' symptoms,
it appears that this outbreak was also unlikely to have been
due to bacterial pathogens. In most bacterial gastrointestinal
infections, fever and abdominal cramps are the most common
symptoms. In addition, in many bacterial gastroenteric
infections (such as those caused by E. coli, Shigella,
and Campylobacter), patients frequently have bloody
stool. In this outbreak, however, only 35.7% of the
cases had self-perceived low grade fever; just 50% of the
cases had abdominal cramps; and none of the patients had bloody
On the other hand, the following characteristics regarding
the distribution of symptoms among cases appears to be in
accordance with the hypothesis that the outbreak was an epidemic
viral gastroenteropathy caused by Norwalk virus or a similar
agent: that nausea, vomiting, and watery diarrhea each affected
more than 70% of the cases; that fever, abdominal cramps,
and myalgia were relatively infrequent; and that no cases
had bloody diarrhea. Unfortunately, the stool samples
submitted by the three cases were not properly preserved for
virus testing, nor were there acute and convalescent sera
collected from any of the cases, making it impossible to verify
Regarding the mode of transmission, although the breakfast
potato was significantly associated with the risk of
developing the gastrointestinal illness, this outbreak is
unlikely to have been due to a common food exposure for the
following three reasons. First, only a weak (albeit
statistically significant) association (RR=1.6) was observed
between exposure to the implicated food item (i.e., breakfast
potato) and risk of illness, whereas generally food-borne
outbreaks are characterized by large relative risks.
Second, 47.6% of the persons who did not eat breakfast potatoes
also developed the illness, including the bus driver, who
had no chance to have been exposed to the breakfast items
on either day. Third, since breakfast potatoes are usually
fully cooked, it was unlikely for this food item to be the
carrier of pathogenic agents, unless it was contaminated by
a food handler who was shedding the pathogens immediately
before it was served. According to the manager of food
services at Hotel A,
no employees preparing the breakfast buffet had been sick;
also, several employees had eaten at the breakfast buffet
on May 29, but none had been sick afterwards.
On the other hand, neither person-to-person nor airborne exposure
can be ruled out as the modes of transmission in this outbreak.
All the tourists stayed in close proximity and they systematically
rotated their seats twice a day during the entire tour.
Both of these factors had the potential to facilitate person-to-person
transmission. On the other hand, airborne transmission
could also have happened, given that the tour bus was a confined
environment, the tourists stayed in the bus for a prolonged
period of time, and several of the cases had used the bathroom
on the bus when they were vomiting or having diarrhea.
Previous investigations of Norwalk disease outbreaks have
suggested that airborne or contact transmission of the Norwalk
agent was possible in a confined environment, such as in a
The incubation period of Norwalk agent disease is usually
24 to 48 hours. It is possible that the index case whose
onset date was May 29 may have transmitted her illness to
the four cases whose onset date was May 31. These four
cases in turn may have transmitted their illness to all the
In summary, this outbreak appears to have been an epidemic
viral gastroenteropathy caused by Norwalk agent. Epidemiologic
evidence suggests that person-to-person or airborne were the
most likely modes of transmission for this outbreak.
We recommend that in the future, preventive measures be adopted
promptly after the emergence of the first case. These
measures could include educating passengers about proper hand-washing
after using the bathroom and advising symptomatic passengers
to limit close contacts with others. In addition, in
the setting of a possible outbreak, local health departments
should be notified immediately.
1. Benenson AS. Control of Communicable Diseases
Manual. (16th Ed.) Washington, DC: American Public
Health Association, 1995; p.201.