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Current Trends Update: Surveillance of Outbreaks -- United States, 1990

Although disease outbreaks are often preventable public health problems, no uniform national system exists in the United States for surveillance of outbreaks. In 1988, CDC's Epidemiology Program Office participated with the epidemiology programs in four states (Maryland, New York, Oklahoma, and Washington) in a 5-month trial of an electronic system for the timely surveillance of outbreaks (1). This report updates the initial trial by summarizing a pilot project of surveillance of outbreaks in nine states--Maryland, Mississippi, Missouri, New York, Oklahoma, Rhode Island, South Dakota, Vermont, and Washington.

The project's objective is to develop and evaluate a simple, timely electronic system of surveillance of outbreaks. From July 1, 1990, through June 30, 1991, the epidemiology programs in the participating states are conducting surveillance of all reported outbreaks (or epidemics) by using a standardized computer form to record the data and electronic telecommunications to transmit the records weekly to CDC. In July 1991, each participating state will complete an evaluation of the system. For this pilot project, an outbreak is defined as "a recent or sudden excess of cases of a specific disease or clinical syndrome. For a foodborne or waterborne outbreak, ngreater than or equal to 2 persons ill; for other outbreaks, ngreater than or equal to 3 persons ill."

From July through December 1990, the nine states reported 233 outbreaks involving 6241 cases. The number of reported outbreaks per month ranged from 33 in December to a peak of 52 in August. Fifty-eight (25%) outbreaks occurred following private gatherings or in the general community; 53 (23%) were associated with restaurants, resorts, or hotels; 47 (20%) with camps, schools, or child day care centers; 43 (18%) with medical-care settings, nursing homes, or other long-term care facilities; and 32 (14%) with other settings. Of the 176 (76%) outbreaks for which a suspected mode of transmission was reported, 102 (58%) were foodborne. State epidemiologists and/or their staff provided consultation for 122 (52%) outbreaks and either provided on-site assistance or investigated another 76 (33%). For 20 (9%), investigators were assisted by CDC epidemiologists who were either based at CDC headquarters or directly assigned to state or local health departments.

For each of 131 (56%) outbreaks, greater than or equal to 10 cases were reported. These outbreaks accounted for 5754 reported cases and nine deaths. For 12 outbreaks, greater than or equal to 100 cases were reported. The largest (468 cases) was an outbreak of acute gastrointestinal illness of unknown etiology (AGI) among persons attending a catered turkey dinner in November at their workplace in Oklahoma. The most commonly suspected causes of these larger outbreaks were AGI (55 (42%) outbreaks), Shigella (16 (12%)), and Salmonella (14 (11%))--of which Salmonella enteritidis was the suspected causative agent in nine outbreaks. In 55 (42%) of these 131 outbreaks, foodborne transmission was suspected; of these, 24 (44%) were associated with restaurants, resorts, or hotels. The rate of reported large outbreaks per million population for the 6-month period ranged from 0.8 in Mississippi to 14.1 in Vermont (Table 1) (2).

The pilot system also collects data on single case reports that may require epidemic control or prevention measures and on toxic exposures that may require similar control/prevention measures, even when no illness is reported. Among 64 case reports, the most commonly suspected cause of illness was hepatitis A virus (32 (50%)). Twenty-seven hepatitis A cases occurred among commercial food handlers; of these, 21 (78%) were reported to the state health department within 2 weeks of onset. Of the 18 such cases that were reported from New York and Oklahoma, nine required and received follow-up epidemic prevention measures, including immune globulin prophylaxis for co-workers, recommendations for prophylaxis for patrons, or announcements to educate the public about risks from exposure. Seven environmental exposures were reported; the highest number of illnesses (six) resulting from such exposures occurred in Missouri. These illnesses were associated with possible carbon monoxide exposure in an office building with a faulty furnace exhaust system. Reported by: C Groves, J Taylor, D Dwyer, E Israel, MD, State Epidemiologist, Maryland State Dept of Health and Mental Hygiene. B Brackin, M Currier, FE Thompson, MD, State Epidemiologist, Mississippi State Dept of Health. MF Bright, M Fobbs, HD Donnell, Jr, MD, State Epidemiologist, Missouri Dept of Health. P Drabkin, SF Kondracki, GS Birkhead, DL Morse, MD, State Epidemiologist, New York State Dept of Health. SJ McNabb, GR Istre, MD, State Epidemiologist, Oklahoma State Dept of Health. M Rittmann, TT Gilbert, BA DeBuono, MD, State Epidemiologist, Rhode Island Dept of Health. L Schaefer, KA Senger, State Epidemiologist, South Dakota State Dept of Health. CJ Greene, RL Vogt, MD, State Epidemiologist, Vermont Dept of Health. M Chadden, JM Kobayashi, MD, State Epidemiologist, Washington Dept of Health. Div of Field Epidemiology and Div of Surveillance and Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The national system of notifiable disease reporting (presented weekly in MMWR Tables I, II, and III) provides surveillance data on many diseases that can cause outbreaks or epidemics; except for certain specific problems (e.g., foodborne outbreaks (3)), no national surveillance for outbreaks exists. A system that focuses surveillance efforts on outbreak detection may enhance efforts to identify preventable factors that increase the risk for outbreaks (4), even when the causative agent of an outbreak is not identified. Surveillance for outbreaks that occur in settings where outbreaks are preventable (e.g., restaurants) may assist in monitoring the effectiveness of outbreak control and prevention measures. For example, before this project began, a retrospective study in Washington indicated that low scores on routine inspections in restaurants were predictive of increased risk for foodborne outbreaks (5). Surveillance can also provide information on both the burden of disease from outbreaks and the resources required by state and local health departments to respond to outbreaks. In addition, data from surveillance of sentinel case reports or environmental/occupational exposures may assist in determining whether reporting is timely enough for epidemic prevention and/or control measures.

In this pilot project, the rates of larger outbreaks per million population vary substantially by state; reasons for this variation may include the relatively small number of outbreaks detected and differences in the completeness of reporting. Surveillance for a more extended period could provide a basis for estimating an "expected" incidence of outbreaks, which could then assist states in assessing completeness of reporting or adequacy of prevention and control measures.

Evaluations of the pilot project by each state will further assess strengths and limitations of this approach to outbreak surveillance. In particular, the assessment can examine the basic attributes of the system (6), such as timeliness of reporting and its relation to the need for information on final case counts, etiologic agents, and modes of transmission. Because simplicity of conducting surveillance must be considered in the context of limited resources, a national system of outbreak surveillance could be integrated with the existing electronic system of notifiable disease surveillance.


  1. CDC. Surveillance for epidemics--United States. MMWR 1989;38:694-6.

  2. Bureau of the Census. State population and household estimates: July 1, 1989. Washington, DC: US Department of Commerce, Bureau of the Census, 1990. (Current population reports; series P-25, no. 1058).

  3. Bean NH, Griffin PM, Goulding JS, Ivey CB. Foodborne disease outbreaks, 5-year summary, 1983-1987. In: CDC surveillance summaries, March 1990. MMWR 1990;39(no. SS-1):15-57.

  4. St Louis ME, Morse DL, Potter ME, et al. The emergence of grade A shell eggs as a major source of Salmonella enteritidis infections: new implications for the control of salmonellosis. JAMA 1988;259:2103-7.

  5. Irwin K, Ballard J, Grendon J, Kobayashi J. Results of routine restaurant inspections can predict outbreaks of foodborne illness: the Seattle-King County experience. Am J Public Health 1989;79:586-90.

  6. CDC. Guidelines for evaluating surveillance systems. MMWR 1988;37(no. S-5).

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