Investigation Summary: Outbreak of GI Illness Aboard the Cruise Ship, M/V Nantucket Clipper
Investigation Summary: Outbreak of GI Illness Aboard the Cruise Ship, M/V Nantucket Clipper,
January 23-30, 2000 and February 6-13, 2000, St. Thomas, US Virgin Islands
Background
- On January 20, 2000, New World Ship Management Company notified the Vessel Sanitation Program (VSP),
National Center for Environmental Health (NCEH), Centers for Disease Control and Prevention (CDC), that
10 of 85 passengers, and 9 of 35 crew aboard the M/V Nantucket Clipper had reported to the ship's captain
with diarrhea or vomiting. The ship had taken on passengers five days earlier in St. Thomas, U.S. Virgin
Islands. At the time of the report, the ship was sailing in the British Virgin Islands. The first cases
were documented by the captain on Wednesday, January 19th, the fourth day of a 7-day cruise. VSP staff
recommended public health interventions and heightened surveillance activities to the ship management
company for immediate implementation.
- On January 25, 2000, the Vice President of New World Ship Management Company notified VSP that 14
of 97 passengers, and 3 of 35 crew aboard the second cruise of the M/V Nantucket Clipper had reported to
the Captain or crew with diarrhea or vomiting. By January 26, 2000, the sixth day of the cruise, 19
passengers and 5 crew were ill, for a combined attack rate of 18% (Table 1).
- On the basis of the continued occurrence of diarrheal illness among passengers and crew during the
second in a series of 7-day cruises aboard the M/V Nantucket Clipper, VSP initiated an epidemiologic and
environmental investigation. VSP staff flew to St. Thomas Virgin Islands to board the vessel on January
29, 2000 in St. Thomas, U.S. Virgin Islands, the first US port of the vessel's itinerary after it set sail
on January 23, 2000.
- Objectives
- The objectives of the investigation were to; 1) identify the etiologic agent associated with the
outbreak; 2) determine the source(s) of spread of illness among passengers and crew; 3) formulate
interventional strategies to control the outbreak and prevent a recurrence.
Methods
- Epidemiologic Investigation
- Case Definition
- A case of gastroenteritis was defined as diarrhea (three or more loose stools in a 24-hour period) or
vomiting in a passenger or crew member aboard the M/V Nantucket Clipper with onset on or after January 23,
2000.
- Food Menus and Passenger Activities
- Ship menus were obtained from New World Ship Management for the M/V Nantucket Clipper during the week
January 23-30,2000. VSP staff interviewed the company by telephone to review food preparation and handling
practices and to match items listed on menus with food-production records. VSP staff also reviewed, by
phone, water and ice handling practices as well as daily passenger and crew activities. The chef and hotel
manager were subsequently interviewed in person once members of the investigative team were aboard the ship.
- Passenger Survey
- A standardized questionnaire, based on ship menus, food-preparation practices, and daily activities
was administered to all passengers and crew on the morning of January 29, 2000. Questionnaires were
administered in person by VSP’s staff epidemiologist and the environmental health officer. Interviewees
were asked about demographic information, and risk factors for illness (e.g. shore excursions, food and
water consumed), and were asked to select, from a list of food items, the foods they ate during of the
cruise. The questionnaires were used to characterize passenger demographics, timing of illness, attack
rates, symptomatology, and risk factors for illness.
- Case Interviews
- Several case-patients among passengers and all case-patients among crew members were interviewed
in-person to review exposure histories and identify individual risk factors for acquiring gastroenteritis
prior to the onset of illness.
- Records Review
- VSP staff reviewed the ship's medical log for information about gastrointestinal symptoms among
passengers and crew during the cruise and to verify attack rates. Because of the small size of the vessel
and the absence of medical staff, the log was maintained by the Captain and First Officer of the M/V
Nantucket Clipper.
- Statistical Methods
- Analysis of disease onset was based on returned questionnaires from both ill passengers and crew
members. People who reported symptoms but whose illness did not meet the case definition were excluded
from the cohort study. Descriptive and cohort analyses were based on the entire passenger and crew data
set.
- Data from questionnaires were entered and analyzed using Epi Info version 6.02 software. Baseline
characteristics of ill and well passengers and crew (e.g. symptomatology; and water, food, and ice
consumption) were described by frequencies and percentiles for categorical variables (e.g., number of
people with diarrhea), and ranges of values for continuous variables (e.g., age). Exposure variables were
coded as dichotomous variables. In univariate analysis, risk ratios with 95% confidence intervals were
used to test for differences in illness risk between ill and well people. Covariates identified as
significant in univariate analyses were included in multivariate regression models.
- Multivariate analyses were performed using SAS version 6.12 software, by using multiple logistic
regression to evaluate the independent association of risk factors with gastrointestinal illness. A model
was derived to best characterize the probability of gastrointestinal illness while controlling for other
potential risk factors. To obtain a reduced model that best characterized the probability of illness, a
sequential, forward-stepwise elimination procedure was performed. Confounding variables were retained in
the model if the beta coefficient for exposure was changed by > 10%.
- Laboratory Investigation
- On January 30, 2000, VSP staff collected serum and stool samples from ill passengers and crew members
for bacterial and viral testing. Whole stools were transported in specimen cups to CDC and were tested for
small round structured virus (SRSV) using electron microscopy (EM) and for Norwalk-like virus using
polymerase chain reaction (PCR) analysis. Bacterial swabs were transported in Cary-Blair medium to CDC and
examined for bacterial pathogens (Campylobacter, Salmonella, Shigella, Vibrio parahaemolyticus, toxigenic
Escherichia coli, and Yersinia enterocolitica).
- Environmental Investigation/Sanitation Inspection
- VSP also conducted a review of housekeeping procedures that included evaluating infection-control
measures, routine cabin and public space cleaning practices, laundry facilities, and sanitization of
snorkeling gear. The inspection involved a review of food storage, preparation, cleaning, and service
practices in the galley and bars. The free residual chlorine concentration in the potable water holding
tanks and distribution system was measured and a visual inspection and record review was performed to
identify possible direct or indirect cross-connections between potable and non-potable water stores.
Results
- Epidemiologic Investigation
- Descriptive epidemiology
- Completed questionnaires were returned by 93 (96%) of 97 passengers and by 34 (97%) of 35 crew. Of
respondents, 77 (83%) of passengers and 21 (62%) of crew members reported having had symptom(s) that met
the case definition of gastroenteritis. Diarrhea was reported in 80 (82%) of cases, vomiting in 68 (70%),
nausea in 61 (63%), muscle aches in 42 (43%), bloody stools in 2 (2%), headache in 38 (39%) abdominal
cramps in 36 (37%), and fever in 36 (37%;[Table 2]). The duration of illness was 2 days or less for 59
(71%) of the cases, the onset of illness peaked on the third day of the cruise. The median age of ill
passengers was 65 years (range 10-72 years); the median age of ill crew members was 24 years (range 21-72
years); 49 (50%) of all case subjects were female.
- Cohort study
- In an unadjusted analysis, persons who used the ice bucket in their cabins, obtained ice from the
main lounge, ate the breakfast served on January 24, 2000, and who consumed bread served at breakfast on
January 24, 2000, were more likely to become ill with gastroenteritis (Table 3). When we adjusted by
multiple logistic regression for risk factors determined statistically significant by univariate analysis,
we found that only consuming breakfast on January 24, 2000 was statistically significant, (OR = 3.9, CI:
1.50,10.31).
- Laboratory Investigation
- Five of eight fresh stool specimens were postivie for SRSV by electron microscopy. Seven of eight
specimens were positive for Norwalk-like virus by RT-PCR using a “region B” primer set. Bacterial swabs
of specimens were negative for Shigella spp., Vibrio spp., Campylobacter jejuni, Yersinia spp., E Coli
0157: H7 and enterotoxigenic E. Coli.
- Environmental Investigation
- Record keeping aboard the ship was good. The ship's medical log accurately reflected numbers of ill
passengers and crew. From potable-water chlorine analyzer records, we determined potable water chlorine
levels were appropriate (between 1.0 and 3.0 ppm) at the distribution-system level during the entire
cruise, never dropping below the 0.2 ppm minimum requirement. Analysis of samples collected and tested by
VSP from the potable water distribution system revealed high chlorine concentrations in the potable water
holding tanks and distribution system and no evidence of coliforms or E. coli bacteria.
- There were good procedures in place for cabin cleaning, public area (surface) cleaning, and laundry
practices. The galley facility had significant space limitations, which created problems with proper
separation of dirty and clean activities, proper and frequent handwashing, space for proper cleaning
around equipment, and a good operational flow through the space. The single, two-stage ice machine was
constructed in a way that allowed debris and mold to collect on surfaces directly above the waterfall so
that contact with water and ice was a strong probability. This machine interior was extremely difficult
to clean properly due to its design. The ice machine has since been replaced with a new machine.
- Discussion
- SRSV, which is commonly referred to as Norwalk or Norwalk-like virus cause acute, self-limited
gastroenteritis characterized by diarrhea, nausea, vomiting, abdominal cramps, muscle aches and fever.
The incubation period and the average duration of illness is from 24 to 48 hours. The epidemic curve for
this outbreak among passengers supports a common source of exposure, with the peak onset of illness on the
fourth day of the cruise for crew, and the sixth day of the cruise for passengers, findings that are
consistent with SRSV.
- Questionnaire data revealed a statistical association between eating breakfast on January 24, 2000,
sharing a cabin with an ill person, and illness for both passengers and crew. The strength of the
association persisted when adjusted for multiple risk factors. No individual food items at that meal were
identified as causative. Potential sources of spread of SRSV include consumption of contaminated food
(e.g., seafood and ready-to-eat foods such as salads), water, ice, fomites, and person-to-person spread.
Airborne transmission has been hypothesized but not established in this setting. Because illness was
widely distributed among passengers and crew over the duration of the cruise, modes of transmission likely
involved both person-to-person spread and other unidentified modes, including the possibility of an
unidentified point source of contamination. Consumption of non-bottled water and ice aboard the ship was
not associated with gastroenteritis in this investigation.
- Several factors obscured the ability to properly identify a single point source of contamination.
First, the high attack rates resulted in relatively low numbers of well passengers and crew, a situation
that limited statistical power. Second, it is likely that many passengers who reported being well at the
time of the survey subsequently became ill or experienced subclinical infections that resulted in
misclassification bias. Third, many crew were ill the week before this investigation a factor which most
likely contributed to person-to-person transmission and misclassification.
Recommendations and Follow-up Activities
- From January 30 to February 6, 2000 the vessel was voluntarily removed from service to carry out
recommendations provided by VSP to New World Management for immediate implementation. The vessel returned
to service on February 6, 2000. On Feb 10, VSP investigators visited the vessel in St. Johns U.S. Virgin
Islands. Upon boarding the vessel and talking with the ship’s Captain, VSP learned that several passengers
had become ill during the cruise and 14 had reported ill on the sixth day of the current cruise. VSP
staff initiated an immediate investigation of the apparent ongoing outbreak. An additional 21 cases were
reported on the seventh day of the cruise and after VSP held discussions with the vessel’s management,
the cruise for the week of Feb.13-20 was canceled and the vessel was once again taken out of service.
- From February 6-13, 2000, 45 of 82 (54.9%) passengers and 2 of 34 (5.9%) crew members on the cruise
ship, M/V Nantucket Clipper, were ill with acute gastroenteritis during a 7-day round-trip from St.
Thomas, U.S. Virgin Islands. The incidence of self-reported illness peaked on Saturday, February 12,
2000, the sixth day of the trip. The illness was characterized by diarrhea, vomiting, headache, abdominal
cramps, and muscle aches lasting approximately 1-2 days for most people. Four stool specimens were
collected from ill passengers. Laboratory analysis confirmed the presence of Norwalk-like virus in all
four specimens. Univariate analysis revealed no associated risk factors for illness.
- Elaine H. Cramer, MD, MPH
- Medical Epidemiologist
- Vessel Sanitation Program
- National Center for Environmental Health
- Centers for Disease Control and Prevention
Tables
Table 1 Itinerary of the M/V Nantucket Clipper, January 23-30, 2000
| Date of Cruise |
Port of Call |
| January 23, 2000 |
Crown Bay, St. Thomas |
| January 24, 2000 |
White Bay, Jost Van Dyke |
| January 25, 2000 |
Tortola, British Virgin Isles |
| January 26, 2000 |
Leverick Bay, Virgin Gorda |
| January 27, 2000 |
Salt Island Bay, Norman Island |
| January 28, 2000 |
The Caves, Norman Island |
| January 29, 2000 |
Cruz Bay, St. John |
| January 30, 2000 |
Crown Bay, St. Thomas |
Table 2 Characterization of Clinical Illness (n= 77
passengers, n=21 crew ), M/V Nantucket Clipper, January 23-30, 2000
| Symptom |
Ill Passengers |
Ill Crew (%) |
Total Ill (%) |
| Diarrhea |
60 (79) |
20 (95) |
80 (82) |
| Vomiting |
52 (68) |
16 (71) |
68 (70) |
| Nausea |
46 (61) |
15 (76) |
61 (70) |
| Abdominal Cramps |
24 (32) |
12 (57) |
36 (37) |
| Headache |
27 (36) |
11 (52) |
38 (39) |
| Fever |
28 (37) |
8 (38) |
36 (37) |
| Muscle Aches |
29 (38) |
13 (62) |
42 (43) |
| Bloody Stool |
2 (3) |
0 (0) |
2 (2) |
Table 3 Unadjusted gastroenteritis attack rates among passengers associated with exposure to selected risk factors, Nantucket Clipper cruise, January 23-30,2000
| Risk Factor |
No. Ill / Exposed (%) |
No. not Ill / No. not Exposed (%) |
Risk Ratio (CI) |
p-value |
| Consumed tap water |
85/93 (91) |
1/29 (3) |
0.95 (0.86, 1.04) |
0.685 |
| Consumed any drinks with ice |
94/97 (97) |
1/29 (3) |
1.00 (0.93, 1.08) |
1.000 |
| Shared Cabin |
96/98 (98) |
12/29 (41) |
1.67 (1.23, 2.27) |
0.000 |
| Use of ice bucket in cabin |
46/98 (47) |
23/28 (82) |
2.63 (1.16, 5.98) |
0.0108 |
| Obtained ice from the Main Lounge |
77/97 (79) |
14/26 (54) |
1.47 (1.02, 2.13) |
0.0171 |
| Consumed breakfast on January 24, 2000 |
77/91 (85) |
12/29 (41) |
3.44 (1.14, 10.40) |
0.0184 |
| Consumed bread with breakfast on January 24, 2000 |
32/90 (36) |
26/39 (67) |
1.44 (1.05, 1.98) |
0.0069 |
