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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



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.

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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.


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%.

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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.

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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).

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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.

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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.

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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.

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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

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Table 1 Itinerary of the M/V Nantucket Clipper, January 23-30, 2000
Date of Cruise Port of Call
January 23, 2000Crown Bay, St. Thomas
January 24, 2000White Bay, Jost Van Dyke
January 25, 2000Tortola, British Virgin Isles
January 26, 2000Leverick Bay, Virgin Gorda
January 27, 2000Salt Island Bay, Norman Island
January 28, 2000The Caves, Norman Island
January 29, 2000Cruz Bay, St. John
January 30, 2000Crown Bay, St. Thomas


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Table 2 Characterization of Clinical Illness (n= 77 passengers, n=21 crew ), M/V Nantucket Clipper, January 23-30, 2000
SymptomIll PassengersIll Crew (%)Total Ill (%)
Diarrhea60 (79)20 (95)80 (82)
Vomiting52 (68)16 (71)68 (70)
Nausea46 (61)15 (76)61 (70)
Abdominal Cramps 24 (32)12 (57)36 (37)
Headache27 (36)11 (52)38 (39)
Fever28 (37)8 (38)36 (37)
Muscle Aches 29 (38)13 (62)42 (43)
Bloody Stool 2 (3)0 (0)2 (2)


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Table 3 Unadjusted gastroenteritis attack rates among passengers associated with exposure to selected risk factors, Nantucket Clipper cruise, January 23-30,2000
Risk FactorNo. 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 ice94/97 (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 Lounge77/97 (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


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