Bacillus cereus Food Poisoning (page 2)
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© Kenneth Todar, PhD
EPIDEMIOLOGY
B. cereus food poisoning occurs year-round and is without any
particular geographic distribution. The short-incubation form is most
often associated with rice dishes that have been cooked and then held
at warm temperatures for several hours. It is often associated with
Mexican and Chinese restaurants, but in one reported outbreak, macaroni and
cheese made from powdered milk turned out to be the source of the bacterium.
"Mac and cheese"- you can't get much more American than that.
Long-incubation B. cereus food poisoning is frequently
associated with meat or vegetable-containing foods after cooking. The
bacterium has been isolated from 50% of dried beans and cereals and
from 25% of dried foods such as spices, seasoning mixes and potatoes. One
outbreak of the long-incubation form was traced to a "meals-on-wheels"
program in which food was held above room temperature for a prolonged period
before delivery to consumers.
The short-incubation or emetic form of the disease is diagnosed by
the isolation of B. cereus from the incriminated food. The
long-incubation or diarrheal form is diagnosed by isolation of the
organism from stool and food. Isolation from stools alone is not sufficient because 14% of
healthy adults have been reported to have transient gastrointestinal
colonization with B. cereus. Because B. cereus gastroenteritis is generally a
benign, self-limited illness, antimicrobial agents are of no value in management. Since the
bacteria grow best at temperatures ranging from 40 to 140°F, infection may be prevented if
cold food is refrigerated and if hot food is held at greater than 140°F before serving.
CASE STUDY:
Summary and Analysis of a Report of Bacillus cereus Food Poisoning Associated with Fried
Rice at Two Child Day Care Centers -- Virginia, 1993 (from CDC).
Summary:
On July 21, 1993, a regional public health facility received reports of
acute gastrointestinal illness that occurred among children and staff
at two jointly owned child day care centers following a catered lunch.
The catered lunch was served on July 21 to 82 children aged less
than or equal to 6 years, and to nine staff; dietary histories were obtained
for 80 persons. 67 ate the catered lunch. A case was defined as
vomiting by a person who was present at either day care center on July
21. Fourteen (21%) persons who ate the lunch became ill, compared with
none of 13 who did not. Symptoms included nausea (71%), abdominal
cramps or pain (36%), and diarrhea (14%). Twelve of the 14 cases occurred among
children aged 2.5-5 years, and two occurred among staff. The median incubation
period was 2 hours (range: 1.5-3.5 hours). Symptoms resolved a median of 4 hours
after onset (range: 1.5-22 hours).
Chicken fried rice prepared at a local restaurant was the only food
significantly associated with illness; illness occurred in 14 (29%) of
48 persons who ate chicken fried rice, compared with none of 16 who did
not.
The rice had been cooked the night of July 20 and cooled at room
temperature before refrigeration. On the morning of the lunch, the rice
was pan-fried in oil with pieces of cooked chicken, delivered to the
day care centers at approximately 10:30 a.m., held without refrigeration, and served at
noon without reheating.
Following the outbreak, health officials recommended to day care
staff and restaurant food handlers that the practice of cooling rice or
any food at room temperature be discontinued, food be maintained at
proper temperatures (i.e., below 41 F {5 C} or above 140 F {60 C}), and
a thermometer be used to verify food temperatures.
Analysis:
The emetic ("short incubation") form of the disease, which occurred in
this outbreak, is mediated by a highly stable toxin that survives high
temperatures and exposure to trypsin, pepsin, and pH extremes; the
diarrheal syndrome is mediated by a heat- and acid-labile enterotoxin
that is sensitive to proteolytic enzymes.
The diagnosis of B. cereus food poisoning can be confirmed
by the isolation of greater than or equal to 105 B. cereus
organisms per gram from epidemiologically- implicated food.
Underreporting of such outbreaks is likely because illness associated
with B. cereus is usually self-limiting and not severe. In addition, findings of a recent survey
about culture practices for outbreaks of apparent foodborne illness indicate
that 20% of state public health laboratories do not make B. cereus
testing routinely available.
Fried rice is a leading cause of B. cereus emetic-type foodpoisoning
in the United States. B. cereus is frequently present in uncooked rice,
and heat-resistant spores may survive cooking. If cooked rice is
subsequently held at room temperature, vegetative forms multiply, and
heat-stable toxin is produced that can survive brief heating, such as
stir frying. In the outbreak described in this report, vegetative forms of the organism
probably multiplied at the restaurant and the day care centers while the rice
was held at room temperature.
The day care staff and restaurant food handlers in this report were
unaware that cooked rice was a potentially hazardous food. This report
underscores the ongoing need to educate food handlers about basic
practices for safe food handling.
Bacillus cereus colonies on blood agar.
END OF CHAPTER
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