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Foodborne Outbreak of Diarrheal Illness Associated with Cryptosporidium parvum -- Minnesota, 1995
On September 29, 1995, the Minnesota Department of Health (MDH) received reports of acute gastroenteritis among an estimated 50 attendees of a social event in Blue Earth County on September 16. This report summarizes the epidemiologic and laboratory investigations of the outbreak, which indicate the probable cause for this foodborne outbreak was Cryptosporidium parvum.
Of the 26 persons who attended the function and who completed telephone interviews with MDH, 15 (58%) reported onset of diarrhea (three or more stools during a 24-hour period) within 14 days after attending the event (range: 1-9 days; median: 6 days). Symptoms included watery diarrhea (100%), abdominal cramps (93%), and chills (79%). The median length of illness was 4 days (range: 1/2 day-14 days). Three persons who sought medical care received outpatient treatment for acute gastroenteritis. Stool specimens obtained from two of these persons were negative for bacterial pathogens and for ova and parasites but were not tested for C. parvum. There were no other reports of cryptosporidiosis in the community at the time of this outbreak.
To identify risk factors for illness, MDH conducted a case-control study using the 15 ill and 11 well attendees. In addition, MDH collected stools from three ill persons, and these were cultured for Salmonella, Shigella, Campylobacter, and Escherichia coli O157:H7; examined for ova and parasites; and tested for C. parvum using acid-fast staining and direct-fluorescent antibody (DFA) methods.
Based on the case-control study, only consumption of chicken salad was associated with increased risk for illness (15 of 15 cases versus two of 11 controls; odds ratio= undefined). Water consumption at the event was not associated with illness.
The chicken salad was prepared by the hostess on September 15 and was refrigerated until served. The ingredients were cooked chopped chicken, pasta, peeled and chopped hard-boiled eggs, chopped celery, and chopped grapes in a seasoned mayonnaise dressing. The hostess operated a licensed day-care home (DCH) and prepared the salad while attendees were in her home. She denied having recent diarrheal illness and refused to submit a stool specimen. In addition, she denied knowledge of diarrheal illnesses among children in her DCH during the week before preparation of the salad. She reported changing diapers on September 15 before preparing the salad and reported routinely following handwashing practices.
Stool specimens from two of the persons whose illnesses met the case definition were obtained by MDH 7 days after resolution of their symptoms; one sample was positive for oocysts and Cryptosporidium sporozoites on acid-fast staining, but the DFA test was negative. The presence of oocysts containing sporozoites was confirmed by acid-fast tests at two other reference laboratories. Stool specimens obtained from a third person -- the spouse of a case-patient -- who did not attend the event but had onset of diarrhea 8 days after onset of diarrhea in his spouse was positive for C. parvum by acid-fast staining and DFA. All stools obtained by MDH were negative for bacteria and for parasites. No chicken salad was available for testing.
Reported by: JW Besser-Wiek, MS, J Forfang, MPH, CW Hedberg, PhD, JA Korlath, MPH, MT Osterholm, PhD, State Epidemiologist, Minnesota Dept of Health. CR Sterling, PhD, Univ of Arizona, Tucson. L Garcia, PhD, Univ of California at Los Angeles Medical Center. Div of Parasitic Diseases, National Center for Infectious Diseases; Div of Applied Public Health Training (proposed), Epidemiology Program Office, CDC.
Editorial Note: Known modes of transmission of C. parvum include consumption of contaminated surface or ground water (1,2), exposure to contaminated recreational water (3), animal-to-person contact (2), and person-to-person contact (2). Because outbreaks of cryptosporidiosis and asymptomatic carriage of Cryptosporidium have been documented in child-care settings (4), the food preparer in this outbreak may have contaminated the implicated salad after contact with an asymptomatically infected child in the DCH. The salad required extensive handling in preparation, was moist, and was served cold -- conditions conducive to initial contamination and preservation of infectious oocysts.
The outbreak of gastroenteritis described in this report was associated with eating chicken salad at a social function. Despite the small number of stools submitted for testing by ill persons who attended the event, the symptoms, incubation period, and the presence of C. parvum in the stool of an ill attendee all indicate that this was a foodborne outbreak of cryptosporidiosis.
Although foodborne transmission of C. parvum has been suspected previously, evidence supporting this mode has been limited to one report of a point source outbreak associated with raw apple cider (5) and reports of sporadic cases attributed to contaminated foods (6). The reported low infectious dose of C. parvum (ID50=132 organisms) suggests that transmission in food is possible (7). Cryptosporidiosis should be considered in the differential diagnosis of suspected foodborne gastroenteritis.
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