On April 2, 1996, Field Epidemiology Training Program Saudi Arabia learned of a gastroenteritis outbreak among American school teachers from Riyadh, Saudi Arabia who had attended an international American teachers' conference, March 20-23, in Cairo, Egypt, and took cruises on the Nile after the conference. A case of gastroenteritis was defined as diarrhea (more than 3 loose stools for one day or more) in a conference attendee or traveling companion with onset in Egypt or within 3 days of returning to Saudi Arabia.
Information on post conference activities in Egypt was obtained for 152 teachers and their companions. After the conference, 72 took cruise boat A from March 24 to 28 from Luxor to Aswan, and 58 (81%) reported onset of gastroenteritis during or for 3 days after the cruise. In comparison 3 of 36 (8.3%) teachers who engaged in other tourist activities in Egypt after the conference reported gastroenteritis (p<0.01, Chi square test, 1 df). Four teachers took other Nile cruise boats at the same time and three became ill (AR=75%). Teachers (44) who returned to Saudi Arabia immediately after the conference did not develop gastroenteritis.
Cruise boat A took the teachers from Luxor to Aswaa. Passengers shared two-bedded cabins with private bathrooms. All meals were provided on the boat and were served in a common dining room. Main dishes and drinks were placed on each table for passengers to serve themselves. Salads and desserts were sometimes placed on a separate table for passengers to serve themselves. In addition to a punch and bottled water served at the table without ice, passengers could buy carbonated and alcoholic drinks with or without ice. The boat stopped at tourist sites along the Nile but no meals were served on shore. Passengers could however buy snacks and drinks on shore.
Gastroenteritis among passengers on cruise boat A was characterized by diarrhea (100%) lasting 1 to 12 days (median 4) with fever (93 %), abdominal cramps (94%) and vomiting (90%). Shigella boydii type 2 was isolated from the stool of two of six passengers who still had diarrhea six days after returning to Saudi Arabia.
A point source outbreak occurred on March 25 and 26 followed by continuing incidence of new cases until March 30 (Figure 1). Among 74 foods served on March 24 and 25, fried fish served on the evening of March 24 was associated with illness developing during the point source outbreak on March 25 and 26 (RR=2.1, 95% CI=1.2-3.5, P=0.002, Chi Square test, Yates correction). Ice added to drinks at any time during the trip was associated with gastroenteritis (Mantel Haentzel weighted RR=1.7, 95% CI 1.4-2.1, p< 0.01) and all 14 persons who reported using ice developed gastroenteritis. For gastroenteritis developing during the point source outbreak the association with ice was independent of eating fried fish (Mantel Haentzel weighted RR=1.7, P<0.01). Salad items served at any time during the four day cruise were not associated with illness.
Teachers who traveled on three other cruise boats from March 24 to 28 also reported outbreaks of gastroenteritis among other passengers and crew. We assessed possible secondary transmission by counting the passengers who shared a cabin with a case-patient for the continuing phase of the outbreak. During the first 36 hours 56% of people who shared a cabin with a case-person. Rates during the next 36 to 71 hours were 60%, from 72 to 107 hours 50%, from 108 to 143 0%, and from 144 to 180 hours 50%.
Editorial note:
The comparison between teachers attending different activities after the conference clearly shows that the outbreak of gastroenteritis that affected cruise boat A passengers was specific to an exposure on that boat. The food specific attack rates implicate fish served for the first dinner aboard the boat as the vehicle of transmission for the point source phase of the outbreak. During both the point source phase of the outbreak and the continuing phase, ice was clearly implicated as the source of transmission. Evidence for person to person transmission or for transmission from infected persons to salads after the point source outbreak is very weak.
Most outbreaks of shigellosis have been attributed to foodborne [1,2) or waterborne [3] sources. Shigella organisms are rarely isolated from water sources therefore identification of a waterborne source is usually based on epidemiologic evidence.
Without an investigation aboard the ship we can only guess how the fish and the ice were responsible for transmission. Since outbreaks also affected all other teachers who took other cruise boats, it is possible that there was a common source for all cruises leaving Luxor. Moreover, ice is an excellent preservative for Shigella. It would protect the Shigella from competition from other organisms, and it would keep the Shigella from exhausting nutrients in the water. Since fish is commonly stored on crushed ice, the Shigella may have been transmitted to the fish via the ice. However, the short incubation period (12 to 36 hours) suggests heavy contamination of the fish. This indicates that there must have been mishandling of the fish after cooking. Other possibilities such as an infected food handler in the kitchen are less likely. Cold foods have been found cause Shigella gastroenteritis outbreaks [2], they did not appear to be responsible for transmission in this outbreak.
References
- Jewell JA, Warren RE, Buttery RB. Foodborne shigellosis. Cornmun Dis Rep CDR Rev 1993;3:R42-R44.
- Hedburg CW, Levine WC, White KE. An international food-borne outbreak of shigellosis associated with a commercial airline. JAMA 1992;268;3208-12.
- Samonis G, Elting L, Skoulika E, et al. An outbreak of diarrhoea! disease attributed to shigella sonnei. Epidemiol infect 1994;112:235-45.
