Evaluating When an Outbreak Is Over

Key points

  • Public health officials performing an investigation need to make decisions about the end of an outbreak on a case-by-case basis.
  • Follow-up environmental sampling and testing helps confirm the effectiveness of short- and long-term activities.
  • An effective water management program should be in place prior to declaring the end to any outbreak.

Considerations supporting the outbreak's end

Possible considerations in favor of declaring an outbreak over include:

  • Lack of new cases
  • No detection of Legionella

Lack of new cases

Identifying no new cases of Legionnaires' disease during a period of careful monitoring is encouraging. So is a lack of cases following implementation of long-term Legionella control strategies as part of a water management program.

No detection of Legionella

If Legionella isn't detected in post-remediation environmental samples after a pre-determined timeframe, the facility can return to routine monitoring activities.

Legionella can regrow in a building water system following remediation events if the conditions are right. This is particularly true during the period before a water management program is fully implemented.

Efforts to monitor the building water system(s) can focus on the outbreak strain if it's confirmed. However, identification of other Legionella species or serogroups, indicates conditions support the growth of Legionella. All Legionella species or serogroups are potentially pathogenic. Therefore, consider needed adjustments to the water management program if other strains are detected.

Considerations against the outbreak's end

Public health officials can extend the timeframe for enhanced environmental and clinical surveillance if there's concern for ongoing transmission. These concerns can be based on factors such as

  • Legionella-positive environmental sample results
  • New cases of Legionnaires' disease
  • Suboptimal performance of the water management program
  • Identification of areas where Legionella can grow and spread

Approaches to post-remediation sampling

There are two objectives with follow-up sampling after an outbreak:

  • Confirm remediation strategies were effective
  • Validate that the long-term prevention strategies are effective

Post-remediation samples should be collected at least 48 hours after restoring the water system or device to normal operating conditions.

HICPAC approach

One common approach, described in HICPAC guidance, consists of collecting environmental samples for culture at 2-week intervals for three months. If Legionella isn't detected during that time, then collect cultures monthly for another 3 months. Adjust the sampling plan over time based on trend data.

If Legionella is detected in one or more cultures, then:

  • Review and modify the water management program
  • Perform additional remediation, if indicated
  • Implement a new 6-month period for post-remediation follow-up sampling

Other approaches

Other approaches that meet the testing objectives above may be considered by the public health agency with jurisdiction.

For healthcare settings, it would be reasonable for follow-up sampling to occur for several months post-remediation. Sampling frequency could range from biweekly to monthly in settings that serve a vulnerable population.

Settings that don't serve populations at increased risk for Legionnaires’ disease may find one or both of the following reasonable:

  • A shorter follow-up timeframe
  • Less frequent testing