Epidemiologic Investigations

Wisconsin Division of Public Health Disease Surveillance Manual (EpiNet, February 2005)

A. Disease Outbreaks

1. Introduction and Background
An outbreak is the occurrence, in a community or region, of individuals who are identified as cases of an illness in excess of what is normally expected. The number of cases in an outbreak will vary according to the infectious agent, the size of the population exposed, and the time and place of occurrence. The outbreak is relative to the usual frequency of the disease in the same area among the specified population at the same time period. Defining an outbreak is directly related to the surveillance activities performed at the local, state and national levels.

2. Procedure for Epidemiologic Investigations
This procedure for the investigation and determination of the existence of an outbreak is consistent regardless of the disease being investigated.

The steps in this procedure include:

a) Verify Diagnosis
Analyze clinical histories of cases and have laboratory tests performed to confirm or refute the diagnosis and determine the type of etiologic agent associated with the illness (e.g., bacterial, viral, fungal, chemical, or other).

b) Establish the Existence of an Outbreak or Epidemic
Establish the existence of an outbreak by comparing available information about new cases with the incidence of the disease in a comparable geographic region during a comparable time period in preceding years.

c) Relate the Outbreak to Time, Place and Person
Conduct a survey of known or selected cases and become familiar with the community situation. Interview these cases to determine their common experiences, such as when they became ill (time), where they became infected (place), and who they are (person). Make case counts and relate these to the appropriate population to determine those groups at risk. Contact those with information on the illness or environmental circumstances contributing to the outbreak. A pattern can often be recognized in the results of these procedures.

d) Formulate a Tentative Hypothesis
Formulate a tentative hypothesis to explain the most likely cause, source, and distribution of the cases. This hypothesis is based on data and facts that have been collected and can only be tentative; no conclusions can be reached at this point in the investigation. The tentative hypothesis directs the course of an investigation and is tested by data gathered during the investigation. Develop several hypotheses if necessary. A series of hypotheses may evolve during an investigation. First, facts are examined, broad hypotheses are formulated, more facts are gathered, then a specific hypothesis is formulated. Next, additional facts to test the acceptability of the hypothesis are gathered. The cycle is continued as necessary.

e) Plan a Detailed Epidemiologic Investigation
Determine from the collected data what additional information is needed and what resources are available to test the hypothesis. Develop or obtain interview forms, gather specimen collection kits, and alert and train people involved in the investigation.

f) Conduct the Investigation
Interview non-ill (controls) who are similar or who had similar time or place experiences to those ill; gather appropriate community and environmental information; investigate potential sources of the responsible agent and factors that contributed to the outbreak; and collect specimens and samples.

g) Analyze and Interpret Data
Conduct laboratory tests and summarize field investigations. Compare and interpret all information collected and results of tests conducted. Construct epidemic curves, calculate rates, develop appropriate tables and charts, apply statistical tests, and interpret the cumulative data.

h) Test Hypotheses and Formulate Conclusions
Accept or reject the hypothesis on the basis of the available data and appropriate calculations. For a hypothesis to be acceptable the patterns of disease in the host must fit the nature of the agent, its source, its mode of transmission, and the contributory factors that allowed the outbreak to occur. If the hypothesis is rejected, another hypothesis must be developed and additional information must be gathered to test this new hypothesis.

i) Put Control Measures Into Operation
Devise effective control measures based upon the evidence uncovered. Use the information collected during the investigation for controlling not only the current situation but also for preventing future problems in the community. Initiate or intensify surveillance of the disease and agent. If imminent danger exists, control measures may be initiated after a tentative hypothesis has been formed. If the hypothesis proves to be wrong, corrected measures can be taken at an appropriate point.

j) Prepare a Report of the Investigation
Investigations should be summarized as soon as completed and a report should be sent to the BCD/CDES. The final outbreak investigation report should include the types of information listed in the sample on the following pages.

3. Requesting Outbreak Assistance
There is no standard formula that will answer the question of when outbreak assistance should be requested.

As a general rule, the DPH does not provide on-site assistance to LHDs for routine communicable disease outbreaks. However, if an outbreak is suspected and the LHD needs technical, material or personnel assistance for an epidemiologic investigation, the health officer should contact the state epidemiologist. Consultation with the state epidemiologist should always be sought when: (1) the pattern of illness appears to be unusual, (2) the etiology of an unusual disease occurrence is not known, or (3) severe disabling (hospitalization, death) consequences are occurring.

4. Local Health Department Responsibilities:

The LHD is expected to:

a) Conduct the initial investigation of a suspected outbreak.

b) Immediately notify the BCD/CDES of any suspect disease outbreak under investigation. Epidemiologic consultation by telephone should be sought early in the investigation.

c) Request assistance of the state epidemiologist if needed to control further spread of the outbreak.

d) Assume local costs (other than fee exempt testing) of the investigation. The DPH will cover expenses incurred by DPH staff when on-site.

e) Obtain laboratory specimens and conduct interviews and other related investigation efforts as requested by the state epidemiologist.

f) Prepare and submit a final outbreak investigation report.

5. State Epidemiologist Responsibilities:

The state epidemiologist is expected to:

a) Provide consultation and technical assistance for local and state regional staff for the outbreak investigation.

b) Provide guidelines for the investigation and control of the specific outbreak consistent with state and national objectives, current policy, and current medical literature.

c) Determine whether a particular outbreak warrants further investigation and the nature and extent of additional information and laboratory data required.

d) Identify and arrange additional staff and material resources from the DPH in the event an outbreak exceeds the staff and resource capacity of the LHD and the BPH. DPH staff reassigned to assist in the outbreak investigation will report to the state epidemiologist until assistance is no longer needed.

e) Provide vaccines in accordance with established protocol for hepatitis A and B, measles, mumps, rubella, diphtheria, tetanus, pertussis, polio, varicella, and medications for treating and preventing tuberculosis. Also provide standard immune globulin in outbreaks of hepatitis A if the state epidemiologist approves the need.

f) Initiate all control measures necessary that a LHD does not initiate.

6. Role of the Centers for Disease Control and Prevention (CDC)

CDC staff provides on-site assistance only upon the request of the DPH. In Wisconsin the state epidemiologist makes the request. The CDC staff serves as consultants to the state epidemiologist unless an alternative agreement is made. Occasionally, upon prior agreement with the state epidemiologist, CDC will conduct an independent investigation.

B. Components of an Outbreak Investigation Report

1. Background:
Brief statement as to what initiated the outbreak investigation, where the outbreak occurred, people involved, demographic setting, number of people exposed, number of people ill, symptoms, etc.

2. Methods:
List the control or prevention methods employed, how were cases identified and data analyzed. List the instruments used such as line lists, epidemic curves, attack rate tables, questionnaires or other instruments deemed pertinent to the outbreak investigation.

3. Results
Describe what the outbreak revealed, laboratory reports, epidemic curves, line lists, attack rate table, sanitarians report, or other pertinent information gathered during the investigation.

4. Discussion
Describe the effectiveness of the control measures, any unusual outcomes or findings, or important elements learned during the investigation.

C. EPINET - Wisconsin Disease Surveillance Manual

All communicable disease case reports should be followed up for the purposes of prevention and control in a manner consistent with those methods listed in the most current edition of Control of Communicable Diseases Manual, Chin J, (ed), 17th ed. 2000, Washington, D.C.

Disease follow-up investigation forms are included in this manual and are a valuable tool for the investigator to interview a case or suspect case of disease. The follow-up investigation form outlines a minimum of demographic, clinical, epidemiologic and laboratory information needed to begin assessing public health implications of the case. Each follow-up report form provides a framework to determine the epidemiologic circumstances of the illness. Some follow-up forms have worksheets available to better assist the investigator.

In the United States, requirements for reporting diseases are mandated by state laws or regulations, and the list of reportable diseases in each state varies. In Wisconsin, Wisconsin State Statute 252.05 and Wisconsin Administrative Code HSS 145 require reporting of Communicable Diseases. The list of reportable diseases in Wisconsin appears on the back of the DPH 4151 and is included in this manual. Additionally, state health departments report cases of selected diseases (Table 1) to the Centers for Disease Control and Prevention (CDC) National Notifiable Disease Surveillance System (NNDSS), on a weekly basis. These data are published weekly in the Morbidity Mortality Weekly Report (MMWR) and an updated final report is published annually in the Summary of Notifiable Diseases.

With the EPINET manual, we have tried to adhere as closely as possible to the case definitions developed by the CDC and the Council of State and Territorial Epidemiologists (CSTE), published in “Case Definitions for Infectious Conditions Under Public Health Surveillance”, MMWR 1997; 46(RR-10). As uniform case definitions are adopted by the states, the incidence of reported diseases in different geographic areas may be more meaningfully compared at the local, state, and national levels.

D . Diseases Reportable at the State and National Level

1. Wisconsin Disease Surveillance Category I: The following diseases are of urgent health importance and shall be reported IMMEDIATELY by telephone to the patient’s local health officer upon identification of a case or a suspected case. Complete and mail an Acute and Communicable Diseases Case Report (DOH 4151) within 24 hours. Public health intervention is expected as indicated in the footnotes. See Wisconsin Administrative Code HSS 145.04 (3) (a).

Anthrax 1,4,5
Botulism 1,4
Botulism, Infant 1,2,4,5
Cholera 1,3,4,5
* Diphtheria 1,3,4,5
Foodborne or Waterborne Outbreaks 1,2,3,4
* Haemophilus influenzae invasive disease, (including epiglottitis) 1,2,3,5
Hantavirus 1,2,4,5
* Hepatitis A 1,2,3,4,5
* Measles 1,2,3,4,5
Meningococcal disease 1,2,3,4,5
Pertussis (“Whooping cough”) 1,2,3,4,5
Plague 1,4,5
* Poliomyelitis 1,4,5
Rabies (Human) 1,4,5
Ricin toxin 4,5
* Rubella 1,2,4,5
Rubella (Congenital syndrome) 1,2,5
Smallpox 1,4,5
Tuberculosis 1,2,3,4,5
Yellow Fever 1,4

2. Wisconsin Disease Surveillance Category II: The following diseases shall be reported to the local health officer on an Acute and Communicable Disease Case Report (DPH 4151) or other means within 72 hours of the identification of a case or suspect case. Public health intervention is expected. See Wisconsin Administrative Code HSS 145.04 (3) (b).

Amebiasis 1,3,4
Arboviral infection (encephalitis / meningitis) 1,2,4
Babesiosis 2,4,5
Blastomycosis 2,4
Brucellosis 1,4
Campylobacter 3,4
Cat Scratch Disease (Bartonella species) 2,5
Cryptosporidiosis 1,2,3,4
Cyclosporiasis 1,4,5
E. coli 0157:H7 1,2,3,4
and other enterohemorhagic E. coli,
enteropathogenic E. coli,
enteroinvasive E. coli,
enterotoxigenic E. coli 1,2,3,4
Encephalitis, viral (other than arboviral)
Ehrlichiosis 1,2,5
Giardiasis 3,4
Hemolytic Uremic Syndrome 1,2,4
* Hepatitis B 1,2,3,4,5
Hepatitis C 1,2
Hepatitis D 2,3,4,5
Hepatitis E 3,4
Histoplasmosis 5
Kawasaki disease 2
Legionellosis 1,2,4
Leprosy1,2,3,4,5
Leptospirosis 4
Listeriosis 2,4
Lyme disease 1,2
Malaria 1,2,4
Meningitis, bacterial (other than Haemophilus influenzae or
meningococcal disease) 2,5
Meningitis, viral (other than arboviral)
* Mumps 1,2, 4,5
Mycobacterial disease (nontuberculous)
Psittacosis 1,2,4
Q fever 2,4,5
Reye syndrome 2
Rheumatic fever (newly diagnosed and meeting the Jones criteria) 5
Rocky Mountain Spotted Fever 1,2,4,5
Salmonellosis 1,3,4
Sexually transmitted diseases:
Chancroid 1,2,4,5
Chlamydia trachomatis infection 1,2,4,5
Genital herpes infection (1st episode identified by health provider) 2
Gonorrhea 1,2,4,5
Pelvic inflammatory disease 1,2,5
Syphilis 1,2,5
Trichomoniasis2
Shigellosis 1,3,4
Streptococcus group A invasive disease 1,5
Streptococcus group B invasive disease 1,5
Streptococcus pneumoniae (pneumococcus) invasive disease 2
* Tetanus 1,2
Toxic shock syndrome 1,2
Toxic substance related diseases:
Infant methemoglobinemia
Lead intoxication (specify Pb levels)
Other metal and pesticide poisonings
Toxoplasmosis
Trichinosis 1,2,4
Tularemia 1,4
Typhoid fever 1,2,3,4
Typhus fever 4,5
Varicella (chickenpox) - reported by number of cases only
Yersiniosis 3,4

* For diseases proceeded by an asterisk (*) in Categories I and II give vaccination history.

3. Wisconsin Disease Surveillance Category III : The following diseases shall be reported to the state epidemiologist on an AIDS case report (DPH 4264) or a Wisconsin Human Immunodeficiency Virus (HIV) Infection Confidential Case Report (DPH 4338) or by other means within 72 hours after identification of a case or suspected case. See s.252.15 (7) (b), Stats., and s. HFS 145.04 (3) (b).

Acquired Immune Deficiency Syndrome (AIDS) 1,2,4
Human Immunodeficiency Virus (HIV) infection 2,4
CD4+ T-lymphocyte count <200 / m L, or CD4+ T-lymphocyte percentage of total lymphocytes of <14

Key:

1 Infectious diseases designated as notifiable at the national level.
2 Wisconsin or CDC follow-up form is required. LHDs have templates of these forms in the Epinet Manual and on the HAN.
3 High-risk assessment by LHD is needed to determine if patient or member of patient's household is employed as a food worker, day care or health care.
4 Source investigation by LHD is needed.
5 Immediate treatment is recommended, i.e., antibiotic or biologic for the patient or contact or both.