Guide to the Application of Genotyping to Tuberculosis Prevention and Control

Applying Genotyping Results to Tuberculosis Control Practices

Outbreak Investigations

Goals

The goals of an outbreak investigation are to identify and treat all case-patients with active TB to stop transmission and to identify all case-patients with LTBI that would benefit from treatment, and assure that treatment is completed so the outbreak does not continue in the future. Another goal of an outbreak investigation is to a) understand the transmission dynamics that link TB patients and persons with LTBI involved in the outbreak so that high-risk populations can be identified and potentially targeted for future screening efforts and b) expand our knowledge of how outbreaks occur.

Steps

An outbreak investigation has similar components to those of a contact investigation with several important differences. One important difference is the need to expand the scope of the identification and evaluation of potential contacts. In many contact investigations, the focus is on contacts that were named by the TB patient. In contrast, in an outbreak investigation, the focus shifts to persons who spent time at any location or social gathering identified by the TB patients.

Since this expanded universe of potential contacts often identifies many more persons than can be evaluated with resources that are usually available, an important step of an outbreak investigation is to develop an algorithm that identifies the contacts who are at highest risk of TB transmission and progression to TB disease once infected. This involves defining the infectious period for each TB case-patient, defining the exposed cohort of persons at each location or gathering, determining the duration of exposure to the case-patient, and using this information to prioritize the various exposed cohorts for screening. For details about conducting an outbreak investigation, see Appendix B, Core Steps in an Outbreak Investigation.

In the previous discussion of the decision analysis depicted in Figure 6.1, RFLP analysis was described only for patients who had possible epidemiologic links. Most patients in an outbreak are connected by known epidemiologic links, and an RFLP analysis of their isolates usually will show a matching pattern. Nevertheless, it is usually wise to perform an RFLP analysis on isolates from all patients involved in an outbreak who have matching spoligotypes and MIRU types in order to obtain additional confirmation of these suspected transmission links.

Outcome

The expected outcome of an outbreak investigation is to stop transmission of TB. This involves the rapid identification and treatment of all infectious cases and the identification and treatment, if indicated, of cases of LTBI. Evaluation of the success of an outbreak investigation involves monitoring outbreak-related cases to verify the cessation of treatment, or when additional cases are diagnosed, to ensure they are rapidly detected and treated so they do not transmit TB to others.