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Epidemiologic Notes and Reports Common-Source Outbreak of Giardiasis -- New Mexico

In April 1988, the Albuquerque Environmental Health Department and the New Mexico Health and Environment Department investigated reports of giardiasis among members of a church youth group in Albuquerque. The first two members to be affected had onset of diarrhea on March 3 and 4, respectively; stool specimens from both were positive for Giardia lamblia cysts. These two persons had only church youth group activities in common. Routine surveillance identified no other cases associated with the church youth group.

The youth group had dinner once a week at the church; food was prepared by parents of group members. The number of attendees at each meal varied, and no record of who attended was kept. A survey of all families attending the church sought to identify any family members who had eaten at any youth group dinners in March and any who had had diarrhea since February 1, 1988. One hundred forty-eight persons who attended at least one youth group dinner in March were interviewed about food they had eaten at the meal(s); the 42 persons reporting diarrheal illness were interviewed about details of their illness.

A case was defined as diarrhea and/or abdominal cramping with onset after February 1, 1988, lasting greater than 7 days and/or a stool specimen positive for Giardia cysts. Twenty-two (15%) persons met the case definition. Onset of illness occurred from March 3 to March 30 (Figure 1), and illness lasted 1-32 days (median: 20 days). Twenty-one (19%) of 108 persons who ate the youth group dinner on March 2 developed an illness meeting the case definition, compared with one (3%) of 40 who did not eat that meal (relative risk (RR)=7.8, 95% confidence interval (CI)=1.1-55.9, p=0.02).

For the 21 ill persons who had eaten the March 2 dinner, the most frequent symptoms reported were fatigue (95%), diarrhea (91%), abdominal cramps (57%), bloating (57%), and weight loss (67%). Patients ranged in age from 11 to 58 years (median: 39 years); 14 (67%) were female; 15 (71%) sought care from a physician. Fourteen (67%) patients submitted stool specimens for ova and parasite examination; 10 (71%) specimens were positive for Giardia cysts. Seven of the stool specimens were also tested for Shigella, Salmonella, Campylobacter, and Yersinia, and all were negative. One ill person attended a day-care center, one had household contact with a day-care center attendee, and none had consumed surface water. The foods served at the dinner on March 2 included tacos (with meat, onions, tomatoes, lettuce, cheese, salsa, sour cream, and tortillas), corn, peaches, cupcakes, soft drinks, coffee, and tea. No food samples were available for microbiologic testing. Persons who became ill were more likely to have reported eating lettuce (RR=8.1, CI=1.1-57.3), salsa (p less than 0.01), onions (RR=4.2, CI=1.9-9.1), or tomatoes (RR=3.5, CI=1.4-8.8) or drinking tea/coffee (RR=5.5, CI=2.3-13.4). Water consumption was not associated with illness. Lettuce, onions, and tea/coffee were most strongly associated with illness by logistic regression analysis.

Except for the commercially prepared salsa, the implicated foods were prepared in the church kitchen. The lettuce and tomatoes were rinsed at the kitchen's main sink; the outer leaves of the lettuce were removed; and the lettuce, tomatoes, and onions were chopped on the same cutting board, which was not washed between items. The dinner was prepared by eight women whose children were in the youth group; all ate the meal. Although the woman who prepared the lettuce and tomatoes taught preschool and had a child in preschool, neither she nor her child was ill when the meal was prepared. None of the eight food preparers reported symptoms at the time of meal preparation; however, five became ill with diarrhea after March 8. Three had stool specimens positive for Giardia cysts.

The church is on the municipal water system. A survey of possible connections between the church's potable water system and the sanitary sewer system identified five potential cross-connections. However, water samples taken at the time of the cross-connection survey had adequate chlorine levels and were negative for coliform bacteria. On April 4, after the investigation began, the church stopped using municipal water for consumption and began catering meals. After elimination of all cross-connections, every outlet was flushed simultaneously for 3 hours. No new cases occurred after the remediation measures were completed. Reported by: DJ Grabowski, MS, KJ Tiggs, JD Hall, DrPH, HW Senke, AJ Salas, Albuquerque Environmental Health Department; CM Powers, JA Knott, Bernalillo County District Health Office; LJ Nims, Scientific Laboratory Div; CM Sewell, DrPH, Acting State Epidemiologist, New Mexico Health and Environment Dept. Div of Field Svcs, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: In this apparent point-source outbreak of giardiasis, the most likely vehicle of transmission was taco ingredients. Although all the ill persons ate the commercially prepared salsa, salsa was unlikely to have transmitted Giardia cysts because the cysts would not remain viable after the pasteurization and canning processes. Two explanations for the contamination are possible. First, if the potable water was contaminated, the lettuce and tomatoes could have been contaminated when washed. Because the lettuce, tomatoes, and onions were all cut on the same board, cross-contamination could have occurred. However, because plumbing changes were made before completion of the epidemiologic investigation, this hypothesis could not be tested. Second, if the woman who prepared the lettuce and tomatoes was infected and excreting Giardia cysts, she could have contaminated the vegetables during preparation. However, this mode is less likely because this woman had acute onset of diarrhea 10 days after the meal, suggesting a new infection at that time.

Only two reported outbreaks of giardiasis have been associated with food: canned salmon (1) and noodle salad (2). In both outbreaks, contamination occurred when food was mixed with bare hands. Waterborne outbreaks of Giardia are well documented, and persons consuming untreated surface water are at increased risk for developing giardiasis (3). Person-to-person transmission is also well known in day-care and institutional settings (4). Public health officials should consider foodborne transmission when investigating outbreaks of giardiasis.


  1. Osterholm MT, Forfang JC, Ristenen TL, et al. An outbreak of foodborne giardiasis. N Engl J Med 1981;304:24-8.

  2. Petersen LR, Cartter ML, Hadler JL. A food-borne outbreak of Giardia lamblia. J Infect Dis 1988;157:846-8.

  3. Craun GF. Waterborne giardiasis in the United States: a review. Am J Pub Health 1979;69:817-9.

  4. Pickering LK, Woodward WE. Diarrhea in day care centers. Pediatr Infect Dis 1982;1:47-52.

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