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Lesson 6: Investigating an Outbreak

Summary, References, and Websites

Outbreaks occur frequently. Not every outbreak comes to light, but of those that do, public health agencies must decide whether to handle them without leaving the office, or spend the time, energy, and resources to conduct field investigations. The most important reason to investigate is to learn enough about the situation to implement appropriate control and prevention measures. Other reasons include taking the opportunity to advance knowledge about the disease, agent, risk factors, interventions and other scientific issues; responding to public, political, or legal concerns; evaluating a health program's effectiveness and weaknesses; and to provide training.

Outbreaks are almost always unexpected events. Sometimes they are the subject of media attention and public concern, so investigators feel pressured to work quickly to find answers. When multiple agencies are involved, coordination and communication become even more essential but are more complicated than usual. Often the investigation takes place in the field, far from the conveniences and routines one counts on in the office. Under these circumstances, it is essential to have a systematic plan for conducting the investigation.

The steps listed in Table 6.2 comprise one such plan. Note that the order of the steps is conceptual, and investigators may decide that a different order is best suited for any given outbreak. To summarize, these are the steps of an outbreak investigation:

  • Planning for field work, establishing the existence of an outbreak, and verifying the diagnosis are usually the first steps, sometimes done in that order, sometimes done in reverse order, sometimes done simultaneously. (Steps 1–3)
  • After the diagnosis has been confirmed investigators create a workable case definition, then go out and look for additional cases. Information about these cases is organized either in a line listing or in a computer database that allows staffers to check for duplicate records, update records as additional information comes in, and perform descriptive epidemiology. (Steps 4-6)
  • Descriptive epidemiology — organizing the data by time, place, and person — is essential for characterizing the outbreak, identifying populations at risk, developing hypotheses about risk factors, and targeting control/prevention strategies. An epidemic curve — a histogram of number of cases by time of onset of illness — provides a handy visual display of the outbreak's magnitude and time trend. (Step 6)
  • Hypotheses, based on what is known about the disease, descriptive epidemiology, and what others have postulated, must be developed before conducting any kind of epidemiologic study (what are you going to study if you don't know what you are looking for?). (Step 7)
  • While not every outbreak requires an analytic study, those that do are usually addressed by either a cohort study or a case-control study. Both types of study attempt to identify associations between exposures (risk factors or causes) and the disease of interest. In a cohort study, best suited for an outbreak in a well-defined population such as guests at a wedding, investigators usually attempt to enroll everyone, determine exposures and outcomes, calculate attack rates, and compare attack rates with a risk ratio or relative risk to identify associations. In a case-control study, which is well suited for outbreaks without a well-defined population, investigators usually enroll all of the case-patients plus a sample of persons who did not get ill, then ask about exposures and compute an odds ratio to look for associations. (Step 8)
  • If needed, hypotheses can be refined and re-evaluated. In many investigations, while the epidemiologists are conducting their epidemiologic investigations, environmental health specialists and laboratorians are conducting studies and tests of their own. Ideally, this multidisciplinary approach points to a single conclusion. (Steps 9 and 10)
  • While implementing control and prevention measures is listed as Step 11, it is the primary goal of most outbreak investigations and usually occurs early in the investigation. Such measures can be implemented as soon as any link in the chain of disease transmission that is susceptible to intervention can be identified. If the source and mode of transmission is known, disease control measures need not wait. However, there is no guarantee that these measures will work, so continued surveillance is essential. (Steps 11 and 12)
  • Finally, communicating what was found and what should be or was done in a written report provides key public health, scientific, and legal documentation. (Step 13)


  1. Olsen SJ, MacKinon LC, Goulding JS, Bean NH, Slutsker L. Surveillance for foodborne disease outbreaks — United States, 1993–1997. In: Surveillance Summaries, March 27, 2000. MMWR 2000; 49(No. SS-1):1–59.
  2. Keene WE, Hedberg K, Herriott DE, Hancock DD, McKay R, Barrett T, Fleming D. A prolonged outbreak of Escherichia coli O157:H7 infections caused by commercially distributed raw milk. J Infect Dis 1997;176:815–8.
  3. Marx M. Diarrheal illness detected through syndromic surveillance after a massive blackout, New York City. Presented at 2003 National Syndromic Surveillance Conference.
  4. Swaminathan B, Barrett TJ, Hunter SB, Tauxe RV. PulseNet: the molecular subtyping network for foodborne bacterial disease surveillance, United States. Emerg Infect Dis 2001; 7:382–9.
  5. Preston R. West Nile mystery. The New Yorker, October 18–25,1999;90–107.
  6. Bush LM, Abrams BH, Beall A, Johnson CC. Index case of fatal inhalational anthrax due to bioterrorism in the United States, N Engl J Med 2001;345:1607–10.
  7. Bender AP, Williams AN, Johnson RA, Jagger HG. Appropriate public health responses to clusters: the art of being responsibly responsive. Am J Epidemiol 1990;132:S48–S52.
  8. Fiore BJ, Hanrahan LP, Anderson HA. State health department response to disease cluster reports: a protocol for investigation. Am J Epidemiol 1990;132:S14–22.
  9. Washington State Department of Health. Guidelines for investigating clusters of chronic disease and adverse birth outcomes [monograph on the Internet]. Olympia, Washington; 2001 [cited 2006 Sep 19]. Available from:
  10. Goodman RA, Buehler JW, Koplan JP. The epidemiologic field investigation: science and judgment in public health practice. Am J Epidemiol 1990;132:9–16.
  11. Galil K, Lee B, Strine T, Carraher C, Baughman AL, Eaton M, et al. Outbreak of varicella at a day-care center despite vaccination. New Engl J Med 2002;347:1909–15.
  12. Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population. Pediatrics. 2004;113:455–9.
  13. Hopkins RS, Juranek DD. Acute giardiasis: an improved clinical case definition for epidemiologic studies. Am J Epidemiol 1991;133:402–7.
  14. Fischer TK, Gentsch J, Ashley D, et al. Evaluation and utility of a novel diagnostic method in the investigation of an unusual outbreak of rotavirus diarrhea among children — Jamaica, 2003. Presented at: 53rd Annual EIS Conference, CDC, Atlanta, Georgia, April 19–23, 2004.
  15. Raupach JC, Hundy RL. An outbreak of Campylobacter jejuni infection among conference delegates. Commun Dis Intell 2003;27:380–3.
  16. Kuusi M, Nuorti JP, Maunula L, Miettinen I, Pesonen H, von Bonsdorff C-H. Internet use and epidemiologic investigation of gastroenteritis outbreak. Emerg Infect Dis 2004;10:447–50.
  17. Pryor JH, Martin MT, Whitney CG, Turco JH, Baumgartner YY, Zegans ME. Rapid response to a conjunctivitis outbreak: the use of technology to leverage information. J Am Coll Health 2002;50:267–71.
  18. Caldwell GG. Twenty-two years of cancer cluster investigations at the Centers for Disease Control. Am J Epidemiol 1990;132:S43–S47.
  19. Schulte PA, Ehrenberg RL, Singal M. Investigation of occupational cancer clusters: theory and practice. Am J Public Health 1987;77:52–6.
  20. Cartwright RA. Cluster investigations: are they worth it? Med J Aust 1999;171:p. 172.
  21. Centers for Disease Control and Prevention. NIOSH health hazard evaluation program. Cincinnati, Ohio: Department of Health and Human Services (NIOSH) Publication No. 2000-132: p. 3
  22. Palmer SR. Epidemiology in search of infectious diseases: methods in outbreak investigation. J Epidemiol Comm Health 1989;43:311–4.
  23. Last JM. A dictionary of epidemiology, 4th ed. New York: Oxford U Press, 2001:129.
  24. PAHO. Case definitions: meningococcal disease and viral meningitis. Epidemiol Bull 2001;22(4):14–6.
  25. Centers for Disease Control and Prevention. Eosinophilia-myalgia syndrome — New Mexico. MMWR 1989;38:765–7.
  26. Centers for Disease Control and Prevention. Eosinophilia-myalgia syndrome and L-tryptophan-containing products — New Mexico, Minnesota, Oregon, and New York, 1989. MMWR 1989;38:785–8.
  27. Centers for Disease Control and Prevention. Public health dispatch: outbreak of listeriosis — northeastern United States, 2002. MMWR 2002;51:950–1.
  28. Jernigan DB, Raghunathan PL, Bell BP, Brechner R, Bresnitz EA, Butler JC, et al. Investigation of bioterrorism-related anthrax, United States, 2001: epidemiologic findings. Emerg Infect Dis 2002;8:1019–28.
  29. Heyman DL, ed. Control of communicable diseases manual, 18th ed. Washington, DC: American Public Health Association, 2004.
  30. Peterson LR, Marshall SL, Barton-Dickson C, Hajjeh RA, Lindsley MD, Warnock DW, et al. Coccidioidomycosis among workers at an archaeologic site, northeast Utah. Emerg Infect Dis 2004;10:637–42.
  31. Snow J. Snow on cholera. London: Humphrey Milford: Oxford U Press, 1936.
  32. Tan C. A preventable outbreak of pneumococcal pneumonia among unvaccinated nursing home residents — New Jersey, 2001. Presented at Northeast Regional Epidemic Intelligence Service Conference, March 14, 2002, New York City.
  33. Lukacs SL, Hsu V, Harper S, Handzel T, Hayslett J, Khabbaz R, et al. Anthrax outbreak averted: public health response to a contaminated envelope on Capital Hill–Washington, DC, 2001. Presented at 51st Annual Epidemic Intelligence Service Conference, April 22–26, 2004, Atlanta.
  34. Ramsey AH, Belongia EA, Gale CM, Davis JP. Outcomes of treated human granulocytic ehrlichiosis cases. Emerg Infect Dis 2002;8:383–401.
  35. Mahoney FJ, Hoge CW, Farley TA, Barbaree JM, Breiman RF, Benson RF, McFarland LM. Communitywide outbreak of Legionnaires' disease associated with a grocery store mist machine. J Infect Dis 1992;165: 736–9.
  36. Torok TJ, Tauxe RV, Wise RP, Livengood JR, Sokolow R, Mauvais S, et al. A large community outbreak of salmonellosis caused by intentional contamination of restaurant salad bars. JAMA 1997;278:389–95.
  37. Hedberg CW, Fishbein DB, Janssen RS, Meyers B, McMillen JM, MacDonald KL, et al. An outbreak of thyrotoxicosis caused by the consumption of bovine thyroid gland in ground beef. N Engl J Med 1987;316:993–8.
  38. Jacobus CH, Holick MF, Shao Q, Chen TC, Holm IA, Kolodny JM, et al. Hypervitaminosis D associated with drinking milk. New Engl J Med 1992;326:1173–7.
  39. Blank S, Scanlon KS, Sinks TH, Lett S, Falk H. An outbreak of hypervitaminosis D associated with the overfortication of milk from a home-delivery dairy. Am J Public Health 1995;85:656–9.
  40. Jani AA, Barrett E, Murphy J, Norton D, Novak C, Painter J, Toney D. A steamship full of trouble: an outbreak of Salmonella Typhimurium DT 104 gastroenteritis at a holiday banquet — Virginia, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference; 2004 Apr 19–23; Atlanta.
  41. Centers for Disease Control and Prevention. Outbreak of histoplasmosis among industrial plant workers — Nebraska, 2004. MMWR 2004;53:1020–2.
  42. Becker KM, Moe CL, Southwick KL, MacCormack JN. Transmission of Norwalk virus during a football game. N Engl J Med 2000;343;1223–7.
  43. Taylor DN, Wachsmuth IK, Shangkuan YH, Schmidt EV, Barrett TJ, Schrader JS, et al. Salmonellosis associated with marijuana: a multistate outbreak traced by plasmid fingerprinting. New Engl J Med 1982;306:1249–53.
  44. MacDonald KL, Spengler RF, Hatheway CL, Hargrett NT, Cohen ML. Type A botulism from sauteed onions. JAMA 1985;253:1275–8.
  45. Nash D, Mostashari F, Fine A, Miller J, O'Leary D, Murray K, et al. The outbreak of West Nile virus infection in the New York City area in 1999. N Engl J Med 2001;344:1807–14.
  46. Roehrig JT, Nash D, Maldin B, Labowitz A, Martin DA, Lanciotti RS, et al. Persistence of virus-reactive serum immunoglobulin M antibody in confirmed West Nile virus encephalitis cases. Emerg Infect Dis 2003;9:376–9.
  47. Klee AL, Maldin B, Edwin B, IPoshni I, Mostashari F, Fine A, et al. Long-term prognosis for clinical West Nile Virus infection. Emerg Infect Dis 2004;10:1405–11.
  48. Fraser DW, Tsai TF, Orenstein W, Parkin WE, Beecham HJ, Sharrar RG, et al. Legionnaires' disease: description of an epidemic of pneumonia. N Engl J Med 1977;297:1189–97.
  49. Bopp DJ, Saunders BD, Waring AL, Waring AL, Ackelsberg J, Dumas N, et al. Detection, isolation, and molecular subtyping of Escherichia coli O157:H7 and Campylobacter jejuni associated with a large waterborne outbreak. J Clin Microbiol 2003;41:174–80.
  50. Division of Tuberculosis Elimination [Internet]. Atlanta: CDC; [updated 1999 Oct; cited 2006 Sep 19]. Self study modules on Tuberculosis, Module 7: Confidentiality in Tuberculosis Control: Background. Available from:


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Incubation periods for foodborne disease