Norwalk-like viral gastroenteritis: a large outbreak on a university campus.Introduction
Viral gastroenteritis viral gastroenteritis Intestinal flu Infectious disease A generic term for GE induced by viruses Clinical presentations 1. Epidemic VGE, most often caused by the Norwalk agent or Norwalk-like viruses Clinical N&V, diarrhea, abdominal pain, anorexia, is estimated to be the second most common illness in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. (1), with 30% to 40% of all cases of infectious diarrhea
Acute infectious syndrome of the stomach lining and intestines. Symptoms include diarrhea, vomiting, and abdominal cramps. may be attributable to the Norwalk family of viruses (3). In one review of 38 confirmed outbreaks of the Norwalk virus Nor·walk virus
Norwalk virus (nôr´wôlk),
n. , only four implicated im·pli·cate
tr.v. im·pli·cat·ed, im·pli·cat·ing, im·pli·cates
1. To involve or connect intimately or incriminatingly: evidence that implicates others in the plot.
2. food as the vehicle of transmission (4). There are few descriptions in the literature of large outbreaks where foodborne transmission is well documented (5-11), and even fewer where airborne transmission airborne transmission Epidemiology The transmission of pathogens by aerosol, which enter the body by the respiratory tract. See Aerosol. was suggested (12, 13). However, when a virus is identified as the etiologic agent in a foodborne outbreak, 90% of the time it is a member of the Norwalk group (14).
This article describes an investigation of a large outbreak of acute nonbacterial gastroenteritis occurring among a university student population where a suspected Norwalk-like virus Norwalk-like virus Virology Any of a group of viruses with biologic, clinical, and immunologic findings similar to those of the Norwalk agent(s). see Gastroenteritis, Hawaii agent, Norwalk agent(s), Otofuke virus, Snow Mountain virus was transmitted through a common food distribution system.
In November 1990, a large foodborne outbreak occurred on a university campus in Boulder, Colorado The City of Boulder (, Mountain Time Zone) is a home rule municipality located in Boulder County, Colorado, United States. Boulder is the 11th most populous city in the State of Colorado, as well as the most populous city and the county . The university had a total student enrollment of 25,176 with 6,008 students housed in 10 on-campus dormitories. Most food is prepared separately in each dormitory kitchen facility. Some meat and all bakery items, however, are prepared at a central kitchen facility and then distributed to each dormitory kitchen for serving.
On the evening of November 9, an unusually large number of students appeared at the student health center with symptoms of nausea, vomiting, and diarrhea. By morning, 144 students were treated and released Treated and Released is Joe Esposito's only album. Track listing
1. therapy. On the morning of November 10, the medical director of the student health center contacted the Boulder County (Boulder) Health Department concerning the outbreak, and an epidemiological investigation was initiated.
For purposes of this outbreak investigation, a case was defined as any university student or employee who developed, between November 8 and 12, an acute case of vomiting or diarrhea with at least one of the following symptoms: nausea, abdominal pain Abdominal pain can be one of the symptoms associated with transient disorders or serious disease. Making a definitive diagnosis of the cause of abdominal pain can be difficult, because many diseases can result in this symptom. Abdominal pain is a common problem. , chills, muscle aches, or fever.
Several sources were used to determine the extent of the outbreak. Accessing a preexisting pre·ex·ist or pre-ex·ist
v. pre·ex·ist·ed, pre·ex·ist·ing, pre·ex·ists
To exist before (something); precede: Dinosaurs preexisted humans.
v.intr. disease-reporting network within the county, all area hospitals, laboratories, and selected physicians' offices were surveyed regarding patients who met the case definition. This survey provided an estimate of the community's burden of illness as well as identified any university students who sought medical care off campus. Phone logs and admission records at the student health center provided a census of the number of students reporting illness during the outbreak period. Additionally, the university housing staff conducted a floor-by-floor survey of each dormitory at the request of the health department.
Fresh stool samples from 12 cases were screened for Salmonella, Shigella shigella
Any of the rod-shaped bacteria that make up the genus Shigella, which are normal inhabitants of the human intestinal tract and can cause dysentery, or shigellosis. Shigellae are gram-negative (see gram stain), non-spore-forming, stationary bacteria. S. , and Campylobacter Campylobacter
Genus of gram-negative spiral-shaped bacteria infecting mammals. Many species, especially C. fetus, cause miscarriage in sheep and cattle. C. jejuni is a common cause of food poisoning. Sources include meats (particularly chicken) and unpasteurized milk. at the Colorado Department of Health laboratory. Two of these were tested for the presence of Staphylococcus staphylococcus (stăf'ələkŏk`əs), any of the pathogenic bacteria, parasitic to humans, that belong to the genus Staphylococcus. The spherical bacterial cells (cocci) typically occur in irregular clusters [Gr. toxin. Seven of the 12 samples were sent to the Centers for Disease Control and Prevention Centers for Disease Control and Prevention (CDC), agency of the U.S. Public Health Service since 1973, with headquarters in Atlanta; it was established in 1946 as the Communicable Disease Center. and examined by direct electron microscopy for the presence of viral particles and by solid phase immune electron microscopy immune electron microscopy
The use of an electron microscope to examine viral specimens bound to specific antibody. (SPIEM) (15) against sera to Norwalk virus previously obtained in volunteer studies. In addition, three of the specimens were tested by SPIEM with homologous homologous /ho·mol·o·gous/ (ho-mol´ah-gus)
1. corresponding in structure, position, origin, etc.
1. convalescent con·va·les·cent
Relating to convalescence.
A person who is recovering from an illness, an injury, or a surgical operation.
1. pertaining to or characterized by convalescence.
2. antisera. Nineteen suspect food samples from several meals and various dormitories were submitted to the Colorado Department of Health laboratory for a Staphylococcus enterotoxin enterotoxin /en·tero·tox·in/ (en´ter-o-tok?sin)
1. a toxin specific for the cells of the intestinal mucosa.
2. a toxin arising in the intestine.
The central kitchen and several of the dormitory kitchens were inspected for proper food preparation practices and hygienic hy·gien·ic
1. Of or relating to hygiene.
2. Tending to promote or preserve health.
3. Sanitary. conditions shortly after the onset of the outbreak. Previous inspection reports were reviewed, and all central and dormitory kitchen staff were interviewed regarding any history of illness.
Case Control Study
A standardized questionnaire requesting basic demographic information, signs and symptoms of illness, and consumed food history was given to the students who sought medical attention and met the case definition. The questionnaire was also distributed at six randomly selected dormitories with a protocol to randomly select ill and non-ill students from each dormitory, not to include ill students who had previously returned a questionnaire after visiting the student health center. In total, 101 ill and 71 non-ill students completed the questionnaire.
The case control data provided for the calculation of food-specific attack rates, frequency of symptoms, and duration of illness. The overall attack rate was calculated from the floor-by-floor dormitory survey. Fisher's exact test Fisher's exact test
a statistical test for association in a two-by-two table based on the exact hypergeometric distribution of the frequencies within the table. was used to test statistical significance when comparing proportions (16).
The frequency of symptoms as reported by the affected students is shown in Table 1. Nearly three-quarters (73%) of the ill students were found to have recovered from their acute symptoms within 24 hours of initial onset. The epidemic curve [ILLUSTRATION FOR FIGURE 1 OMITTED] shows date and time of onset of symptoms among ill students, dormitory food staff, and central kitchen food staff. Importantly, the onset of illness among the five central kitchen staff members occurred prior to the peak incidence among ill students.
In total, 787 of 6,008 dormitory residents fit the case definition for illness during the three-day outbreak period (attack rate = 13.1%). Each of the 10 dormitories on campus reported ill students with attack rates ranging from 6.6% to 19.9%. Illness was reported among 13 of 60 dormitory food service workers and 5 of 10 central food service workers for attack rates of 21.7% and 50%, respectively. Surveillance for additional cases continued, but no evidence suggested the outbreak had continued beyond the initial three-day period.
Attack rates for the most implicated foods and meals are shown in Table 2. The large variety of foods offered at each meal and the apparent lack of accurate recall by the students complicated the calculation of food-specific attack rates. The menu item "deli and hamburger bar" actually consisted of a variety of food items including various bakery products and leftovers from previous meals.
Table 1. Frequency of Symptoms Reported Among III University Students, November 1990.
Nausea 92% Vomiting 89% Diarrhea 81% Abdominal pain 75% Muscle ache 73% Fever 55%
All laboratory results for Salmonella and Shigella were negative. One stool sample tested positive for Campylobacter. All submitted stool samples and food samples tested negative for Staphylococcus toxin. Direct electron microscopy revealed 30 nm small round structured virus particles (SRSV SRSV Small Round Structured Virus
SRSV Satellite Robot Simulator Vehicle (NASA) ) in three of seven stool specimens. None of these three reacted with the homologous convalescent antisera, and none of the seven samples reacted with the Norwalk antisera.
The sanitary inspection and review of past records indicated that the central and dormitory kitchens had been employing proper and hygienic food preparation practices. However, investigation found only one of the five ill central kitchen staff members had removed themselves from food preparation duties while symptomatic. This occurred in spite of a personnel policy for food service workers that discourages ill employees from attending work and provides sick leave compensation.
The epidemic curve clearly indicates a large scale, common source outbreak common source outbreak Epidemiology An outbreak that results from a group of persons being exposed to a common pathogen or toxin Public health A mass infection from a single contaminated source occurring over a three-day period. Surveys of each campus dormitory and interviews with food service personnel found the outbreak had affected 787 dormitory students and 18 food service workers.
While the attack rates were similar among the 10 campus dormitories, they were markedly higher among the central food service staff. In addition, the central food service staff was found to have symptom onsets approximately 24 to 48 hours prior to the peak onset for students. Only one of the five ill staff members took sick leave during their illness. The suspected foods were among those foods that are received or prepared in the central kitchen before distribution to the individual dormitories. We believe some food, particularly bakery items, may have been contaminated contaminated,
v 1. made radioactive by the addition of small quantities of radioactive material.
2. made contaminated by adding infective or radiographic materials.
3. an infective surface or object. in the central kitchen prior to distribution and eventual consumption.
Table 2. Food-specific Attack Rates Among Dormitory Students, November 1990.
Attack Rate Date of Meal and Food Item % ill % not ill P-value
November 8, Dinner 74% 54% .04 Baked Ham
November 9, Lunch 79% 46% .002 Deli Bar
The community water supplier provides domestic water to the university as well as other residents within the city. The water purveyor (World-Wide Web) Purveyor - A World-Wide Web server for Windows NT and Windows 95 (when available).
E-mail: <firstname.lastname@example.org>. routinely tests over 100 water samples each month, and no contamination was found before or during the outbreak. Had the outbreak been associated with the community water supply, it would have been expected to equally affect the non-university population, but repeated surveys using the preexisting disease-reporting network did not reveal a community-wide outbreak.
The negative bacteriological bac·te·ri·ol·o·gy
The study of bacteria, especially in relation to medicine and agriculture.
bac·te analyses, explosive spread, type and frequency of symptoms, incubation period incubation period
1. See latent period.
2. See incubative stage.
Incubation period , and duration of illness are all consistent with reports of Norwalk and Norwalk-like virus-mediated gastroenteritis in the literature (5-10, 17). Kaplan et al. found a high correlation between outbreaks fitting the Norwalk-like pattern of illness and serologic se·rol·o·gy
n. pl. se·rol·o·gies
1. The science that deals with the properties and reactions of serums, especially blood serum.
2. confirmation of a Norwalk-like viral infection viral infection,
n an infection by a pathogenic virus. A virus acts on the cell nucleus, taking over the genetic material within the nucleus and replicating itself. (3). Kaplan's proposed criteria for classifying an outbreak as due to a Norwalk-like virus are negative bacterial pathogen cultures, an incubation period between 24 and 48 hours, duration of illness 12 to 60 hours, and vomiting in [greater than or equal to]50% of the cases (3). Hedberg and Osterholm (18) recently expanded this definition to include an increased frequency of vomiting relative to fever rather than a simple vomiting frequency [greater than or equal to]50%. All of the data herein are consistent with both definitions and suggests that the spread of a Norwalk-like virus through contaminated food caused this outbreak.
The case-control study case-control study,
n an investigation employing an epidemiologic approach in which previously existing incidents of a medical condition are used in lieu of gathering new information from a randomized population. did not implicate im·pli·cate
tr.v. im·pli·cat·ed, im·pli·cat·ing, im·pli·cates
1. To involve or connect intimately or incriminatingly: evidence that implicates others in the plot.
2. a single food served at one particular meal. This is consistent with our belief that ill food workers from the central kitchen facility contaminated several food items, including bakery products, which were served during several meals. Various contaminated food items served at consecutive meals would obscure the findings of a case-control study. The serving of leftover foods in a buffet-style "deli-bar" would further dilute the analysis.
While attempts at visualizing the viral particles by direct EM and SPIEM were inconclusive, the findings herein are consistent with the proposed etiology (9, 19). In 24 serologically-confirmed Norwalk virus outbreaks, SRSV particles were only seen in five. In only two of these five did the particles aggregate upon exposure to the matching convalescent antisera (4). In samples from this outbreak, relatively few SRSV particles were seen. Immunoglobulin-resembling material, which frequently blocks the reaction to appropriate antisera in the laboratory, precoated many of the observed particles. It is also possible that improper handling may have compromised the integrity of the samples. The samples were collected nearly a week before they were prepared and studied at the CDC See Control Data, century date change and Back Orifice.
CDC - Control Data Corporation laboratories. Prompt refrigeration refrigeration, process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective. and analysis within 24 hours has yielded increased numbers in past studies (Humphrey, CDC, personal communication, 1992). Generally, [10.sup.6-7] particles/gram are needed in the sample for adequate visualization (10,17,20), and so optimal timing of collection and preservation are important. Detection of a suspected virus in implicated food is rarely attempted due to the difficulties with available methods and the inability to culture the common foodborne viruses (21).
A set of circumstances and food handling practices were in place that created a high potential for a large foodborne outbreak to occur. First, the Norwalk group of viruses is moderately hardy, able to survive both freezing (22) and mild heating (23), although most of the reported Norwalk virus foodborne outbreaks implicate uncooked food (5,6,7,8,11,19,24). The virus is also highly infectious, and outbreaks of Norwalk-like gastroenteritis are characterized by their explosive nature (25,26). The role of the infected food worker in the propagation of this illness has been well described (5,8,12,16,26). In the largest outbreak found in the research conducted, an estimated 3,000 persons became ill over a four-day period from one common source (6). Volunteer studies show that the infective dose can be quite low. From 10 to 100 ingested in·gest
tr.v. in·gest·ed, in·gest·ing, in·gests
1. To take into the body by the mouth for digestion or absorption. See Synonyms at eat.
2. particles can produce illness (17,19). Industry accepted standards for hand washing may not remove all infectious particles. Viral particles may be shed up to 48 hours after recovery from symptoms (8,19), and infection may be asymptomatic (28). Given the high infectivity, low dosage, and asymptomatic shedding that is possible, one infected food worker can contaminate con·tam·i·nate
1. To make impure or unclean by contact or mixture.
2. To expose to or permeate with radioactivity.
con·tam·i·nant n. large quantities of food in a short period of time without an overt disregard for good hygienic practices.
Second, the suspect food items were received and handled in a central kitchen facility for distribution to outlying dormitory kitchens. This food distribution channel creates a situation where an ill food worker could unwittingly infect many hundreds of students. It is clear why many of the described outbreaks occur in institutions, cruise ships, and other settings where a few people prepare food for a much larger group. Fortunately, this food distribution network also contributed to the outbreak's quick subsidence. The ill workers had quickly recovered by the time local health authorities were involved in the outbreak. Once there was no longer a common source for the virus, the opportunity for continued transmission via handled food was eliminated. Secondary transmission (direct-contact) of enteric viruses from primary cases is usually limited (29) and did not contribute to the continuation of this outbreak.
Third, while there is a personnel policy for sick leave to exclude ill food service staff from reporting to work, four of five ill central kitchen workers continued to work while symptomatic. While there is no direct evidence that it was these specific workers who eventually contaminated the suspected food, this disregard for existing policy definitely contributed to the potential risk of an outbreak. Given the persistence of the virus in stools during the convalescent phase, it has been recommended that workers not return to work until 48 hours after symptoms have resolved (7,10,30,31).
To reduce the potential for future outbreaks, personnel policies regarding time off for ill kitchen staff, or at least reassignment to duties uninvolved un·in·volved
Feeling or showing no interest or involvement; unconcerned: an uninvolved bystander.
Adj. 1. in food preparation, must be strictly followed. Also, proper food handling and hand washing techniques should be rigorously enforced. In states such as Colorado that have not fully adopted the FDA's food handling regulations (32,33), employers should consider voluntary compliance with those regulations that prohibit direct, bare-handed contact of ready-to-eat foods by food employees. The authors further recommend that food distribution systems in large institutions be modified to receive and prepare potentially hazardous foods at the local kitchen where people are served.
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tr.v. pro·tract·ed, pro·tract·ing, pro·tracts
1. To draw out or lengthen in time; prolong: disputants who needlessly protracted the negotiations.
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Hepatitis A is an inflammation of the liver caused by a virus, the hepatitis A virus (HAV). It varies in severity, running an acute course, generally starting within two to six weeks after contact with the virus, and lasting no )," PHLS Microbiol Digest.
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An integrated bodily response to an antigen, especially one mediated by lymphocytes and involving recognition of antigens by specific antibodies or previously sensitized lymphocytes. and Prevalence of Antibody to Norwalk Enteritis enteritis (ĕn'tərī`tĭs), inflammation of the gastrointestinal tract. Acute enteritis is not usually serious except in infants and older people, in whom the accompanying diarrhea can cause dehydration through the loss of fluids. Virus as Determined by Radioimmunoassay," J Clin Microbiol, 10:903-9.
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Robert A. Brockmann, M.S., Laboratory Services, Boulder County Health Department, 3450 N. Broadway, Boulder, CO 80304.