Strength-of-Evidence Classification for Waterborne Disease & Outbreaks

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Waterborne disease outbreak reports are reviewed and classified according to the strength of

  1. Traditional and molecular epidemiologic data and
  2. Environmental health data implicating water as the vehicle of transmission (see table below).

Outbreaks and subsequent investigations occur under different circumstances, and not all outbreaks can be investigated rigorously. Outbreak strength-of-evidence levels do not necessarily indicate how adequate or complete the investigation was because multiple factors (e.g., timeliness of outbreak detection) contribute to the ability to collect optimal epidemiologic and environmental health data. This classification was updated for the 2013-2014 outbreak data report to provide more detailed definitions of categories and terms used in prior versions.

Strength-of-Evidence Classification of Investigations of Waterborne Disease Outbreaks — United States

2013-2014

Class

Traditional and molecular epidemiologic data

Environmental health data1

I

Provided and adequate (PaA)

Provided and adequate (PaA)

TRADITIONAL: Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05, that links outbreak cases to the same water exposure 2;

OR MOLECULAR 3: Molecular characterization of pathogens from clinical specimens matched for ≥2 cases who had the same water exposure 2

Laboratory data 4 (e.g. water quality, environmental, or microbiological) or historic information 5 (i.e. contextual to outbreak);

OR Molecular characterization of pathogens isolated from water matched at least one clinical specimen 6

II

Provided and adequate (PaA)

Not provided or inadequate (NPol)

TRADITIONAL: Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05, that links outbreak cases to the same water exposure 2 ;

OR
MOLECULAR 3: Molecular characterization of pathogens from clinical specimens matched for ≥2 cases who had the same water exposure 2

No or inadequate 7 laboratory data or historic information to implicate water.

III

Provided but limited (PbL)

Provided and adequate (PaA)

Traditional or molecular epidemiologic data provided that did not meet the criteria for Class I or II;

OR
No data provided to support that ill persons had water as their only common exposure 8

Laboratory data 4 (e.g. water quality, environmental, or microbiological) or historic information 5 (i.e. contextual to outbreak);

OR
Molecular characterization of pathogens isolated from water matched at least one clinical specimen 6

IV

Provided but limited (PbL)

Not provided or inadequate (NPol)

Traditional or molecular epidemiologic data provided that did not meet the criteria for Class I or II;

OR
No data provided to support that ill persons had water as their only common exposure 8

No or inadequate 7 laboratory data or historic information to implicate water.

1 Evaluated based on environmental health data in the outbreak report. Adequate environmental health data must implicate the vehicle of transmission (i.e., a recreational water venue, drinking water system, or other water source [e.g., cooling tower]). Information is contextual to the outbreak and might take into consideration aspects such as water exposure type and etiology. Environmental health data might be adequate even when pathogens are not detected, provided other contextual information.

2 A shared recreational water venue, drinking water system, or other water source, but not necessarily an identical activity or identical time of exposure.

3 Adequate molecular epidemiologic data means matching molecular characterization (e.g., subtyping), results of which must be provided

4 Providing a statement that testing occurred is inadequate environmental health evidence; if laboratory testing was conducted, the test results must be provided.

5 Examples include a history of a chlorinator or acid feed pump malfunction, no detectable free-chlorine residual, a breakdown in circulation system, a water source contaminated during temporary connection with a contaminated surface water source, a disinfection system that was not operational or not being monitored properly at the time of the outbreak.

6 Adequate molecular data means matching molecular characterization (e.g., subtyping), results of which must be provided.

7 For laboratory testing, inadequate data refers to results that neither identify the pathogen nor describe water conditions conducive to an outbreak occurring. For historic information, the provided data does not implicate water.

8 Submission of NORS report to CDC is a claim that ill persons had a common water source as their exposure.

2009-2012

Class

Epidemiologic and clinical laboratory data

Environmental health data

I

Provided and adequate

Provided and adequate

Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05;

OR

Molecular characterization of pathogens linked multiple persons who had a single identical exposure

Laboratory data or historic information (e.g., history of a chlorinator or acid feed pump malfunction, no detectable free-chlorine residual, or a breakdown in circulation system);

OR

Molecular characteristics of pathogens isolated from water and at least one clinical specimen were identical

II

Provided and adequate

Not provided or inadequate

Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05;

OR

Molecular characterization of pathogens linked multiple persons who had a single identical exposure

E.g., laboratory testing of water not conducted and no historic information available

III

Provided but limited

Provided and adequate

Epidemiologic data provided that did not meet the criteria for Class I or II or claim made that ill persons had no exposures in common, besides water, but no data provided

Laboratory data or historic information(e.g., history of a chlorinator or acid feed pump malfunction, no detectable free-chlorine residual, or a breakdown in circulation system);

OR

Molecular characteristics of pathogens isolated from water and at least one clinical specimen were identical

IV

Provided but limited

Not provided or inadequate

Epidemiologic data provided that did not meet the criteria for Class I or II or claim made that ill persons had no exposures in common, besides water, but no data provided

E.g., laboratory testing of water not conducted and no historic information available

2008 and Earlier

For the strength-of-evidence classifications of investigations of waterborne disease outbreaks prior to this time period, refer to the MMWR surveillance summary reports available online.

Page last reviewed: April 25, 2019
Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Foodborne, Waterborne, and Environmental Diseases (DFWED)