Outbreak of Staphylococcal Food Poisoning Associated with Precooked Ham -- Florida, 1997

On September 27, 1997, a community hospital in northeastern Florida notified the St. Johns County Health Department about several persons who were treated in the emergency department because of gastrointestinal illnesses suspected of being associated with a common meal. This report summarizes the investigation of the outbreak by the Florida Department of Health; the findings implicated staphylococcal intoxication as the cause of illness among some persons who attended a retirement party on September 26, 1997.

Self-administered questionnaires were distributed to the 125 attendees to document food histories, illnesses, and symptoms. A case was defined as nausea and/or vomiting in a person who attended the party or consumed food served at the party and who became ill within 8 hours after eating. Leftover food was collected and submitted for laboratory analysis. Food preparers were interviewed about the purchase and preparation of food served at the party.

Of the approximately 125 persons who attended the party, 98 completed and returned questionnaires. Of these, 31 persons attended the event but ate nothing, and none of them became ill; they were excluded from further analysis. A total of 18 (19%) persons had illnesses meeting the case definition, including 17 party attendees and one person who ate food brought home from the party. Ill persons reported nausea (94%), vomiting (89%), diarrhea (72%), weakness (67%), sweating (61%), chills (44%), fatigue (39%), myalgia (28%), headache (11%), and fever (11%). Onset of illness occurred at a mean of 3.4 hours after eating (range: 1-7 hours); symptoms lasted a median of 24 hours (range: 2-72 hours). Seven persons sought medical treatment, and two of those were hospitalized overnight. Illness was strongly associated with eating ham (risk ratio=26.8 {95% confidence interval=3.8-189.6}). Of the 18 ill persons, 17 (94%) had eaten ham. The ill person who had not attended the party had eaten only leftover ham. None of the other foods served at the party were significantly associated with illness (Table_1).

One sample of leftover cooked ham and one sample of leftover rice pilaf were analyzed by reversed passive latex agglutination to identify staphylococcal enterotoxin and were positive for staphylococcal enterotoxin type A. Samples of stool or vomitus were not obtained from any ill persons, and cultures from nares or skin were not obtained from the food preparers.

On September 25, a food preparer had purchased a 16-pound precooked packaged ham, baked it at home at 400 F (204 C) for 1.5 hours, and transported it to her workplace, a large institutional kitchen, where she sliced the ham while it was hot on a commercial slicer. The food preparer reported having no cuts, sores, or infected wounds on her hands. She reported that she routinely cleaned the slicer in place rather than dismantling it and cleaning it according to recommended procedures and that she did not use an approved sanitizer. All 16 pounds of sliced ham had been placed in a 14-inch by 12-inch by 3-inch plastic container that was covered with foil and stored in a walk-in cooler for 6 hours, then transported back to the preparer's home and refrigerated overnight. The ham was served cold at the party the next day. The rice pilaf was prepared the day of the party by a different person.

Reported by: K Ward, MSEH, R Hammond, PhD, D Katz, PhD, D Hallman, Florida Dept of Health. Foodborne and Diarrheal Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Staphylococcal food poisoning, caused by enterotoxin-producing strains of Staphylococcus aureus, is one of the most common foodborne illnesses (1). Sudden onset of nausea, vomiting, and diarrhea usually occurs 30 minutes to 8 hours after eating contaminated food; the incubation period may vary in relation to individual susceptibility, amount of toxin in the food, and amount of food ingested. Although the duration of illness is short and almost always self-limited, some deaths have been reported (2).

Although staphylococci are commonly found on environmental surfaces and in a wide variety of mammals and birds, humans are thought to be the primary source of organisms associated with staphylococcal food contamination. Organisms may be present in the nasal passages, throat, hair, and skin of healthy persons, and are abundant in cuts, pustules, and abscesses (2,3). Staphylococci grow in the temperature range of 45 F and 118 F (7 C and 48 C); rapid growth and enterotoxin production occurs between 68 F and 99 F (20 C and 37 C). Although growth usually is constrained by the presence of competing organisms, staphylococci thrive in high concentrations of salt and sugar that other organisms cannot tolerate. Staphylococcal enterotoxins are highly resistant to heat. Measures to prevent the growth of S. aureus are critical because normal temperatures used in cooking will not destroy the toxins, and foods containing staphylococcal enterotoxin usually look and taste normal (2,3).

Ham is the most commonly reported vehicle of transmission in staphylococcal food poisoning (1,4). The salt content of precooked, packaged hams is high, often as high as 3.5%, which provides an ideal growth medium for Staphylococcus (2). Although the exact source of contamination for the ham in this outbreak is unknown, the ham could have been contaminated by the food preparer's hands, even though she had no signs of staphylococcal infection. Only one third of food handlers from whom staphylococci are isolated have symptoms consistent with an active staphylococcal infection (4). The ham also could have been contaminated by contact with the slicer because the slicer had not been cleaned adequately. Slicing the ham when the ham was warm increased the surface area and provided a favorable temperature for replication of toxin-producing organisms. In addition, placement of a large quantity of warm, salty ham in a small, tightly closed container prevented rapid cooling and extended the time during which growth and toxin production occurred.

To reduce the incidence of staphylococcal gastroenteritis, potentially hazardous foods such as baked ham must be prepared and served appropriately. The amount of manual handling should be minimal, and food preparers should wash their hands thoroughly before handling food. Food contact surfaces and equipment such as slicers should be cleaned and sanitized. Ham should be sliced cold or, if served warm, immediately before serving to decrease the opportunity for replication of organisms introduced during slicing. Food should be eaten promptly after cooking or refrigerated immediately at a temperature less than or equal to 41 F (less than or equal to 5 C). To permit rapid cooling, food should be stored in small portions in containers that are shallow and loosely covered; this method facilitates adequate air flow and rapid transfer of heat from the food to the container (5).

References

1. Bean NH, Goulding JS, Lao C, Angulo FJ. Surveillance for foodborne-disease outbreaks -- United States, 1988-1992. In: CDC surveillance summaries (October). MMWR 1996;45(no. SS-5).

2. Bergdoll MS. Staphylococcal food poisoning. In: Cliver DO, ed. Foodborne diseases. San Diego, California: Academic Press, Inc, 1990:85-106.

3. Bergdoll MS. Staphylococcal intoxications. In: Riemann H, Bryan FL, eds. Food-borne infections and intoxications. 2nd ed. Orlando, Florida: Academic Press, Inc, 1979:444-90.

4. Holmberg SD, Blake PA. Staphylococcal food poisoning in the United States: new facts and old misconceptions. JAMA 1984;251:487-9.

5. Food and Drug Administration. Food code, 1997. Washington, DC: US Department of Health and Human Services, Public Health Service, 1997.

TABLE 1. Attack rates and risk ratios associated with buffet foods, by
food type -- Florida, September 26, 1997
==============================================================================
                            Attack rate
                                (%)
                     ---------------------------
Food                   Ate           Did not eat   Risk ratio    (95% CI*)
------------------------------------------------------------------------------
Ham                    65.4              2.4          26.8      (3.8-189.6)
Chicken                30.0             25.5           1.2      (0.5-  2.7)
Turkey                 38.9             22.4           1.7      (0.8-  3.8)
Rice pilaf             15.4             29.6           0.5      (0.1-  2.0)
Rolls                  47.1             20.0           1.4+     (0.8-  2.3)
Eggs                   34.8             22.7           1.5      (0.7-  3.3)
Salad platter          31.3             25.5           1.2      (0.5-  2.9)
Nuts                   25.0             27.1           0.9      (0.3-  3.3)
Cake                   23.5             28.0           0.8      (0.3-  2.2)
Cookies                11.8             32.0           0.4      (0.1-  1.4)
Punch                  18.4             37.9           0.5      (0.2-  1.1)
------------------------------------------------------------------------------
* Confidence interval.
+ Summary risk ratio after stratifying on ham consumption.
==============================================================================

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