Contact investigation is iterative; the results of each contact investigation should be reviewed by the public health TB program as they become available, to guide decisions about expansion and/or additional outreach interventions. Recent transmission is considered to have occurred if a secondary case of active TB is identified in any contact, there are clear TST conversions among contacts, the prevalence rate of TST ≥10 mm among contacts is significantly higher than expected (for example, 60% among contacts when the expected prevalence rate is 40%) or a child contact less than 5 years is infected (without another probable source). See Appendix 3 for a table of LTBI prevalence in various Canadian population groups. A fundamental challenge is that transmission can be very difficult to evaluate when the background rate of positive TST results is unknown or high. This is often the case in Canada, where the majority of patients with TB — and many of their close contacts — are foreign-born, originally from high-TB-incidence countries; it is also the case in Indigenous communities with high rates of TB. Thus, Canadian-born contacts with no high-risk travel, particularly children, may have the clearest results for assessing transmission as they are less likely to have prior TB exposure (see Chapter 4: Diagnosis of Tuberculosis Infection).
When there is evidence of transmission, the investigation should first address any remaining unassessed high-priority contacts, and investigate medium-priority contacts if this has not already been done. Genotyping/whole genome sequencing to compare index and secondary cases should be requested, but further contact tracing should not be delayed pending results. Expansion to low-priority contacts with the next-most level of exposure or who are young children/immunocompromised, in a concentric circle manner, should only be considered if recent transmission is found in medium-priority contacts.
The probability of finding infected individuals among less-exposed contacts, and the likelihood that this group of contacts will follow up on screening and LTBI treatment recommendations, should also be considered in any decision to expand from the initial set of contacts.7,11,20,25 Contact participation rates tend to be lower in less-close contacts, contacts of less-infectious patients and in adults compared with children.78,79 For some TB patients, it may not be possible to identify a feasible group of next-most-exposed contacts. Individuals with only transient or occasional exposure (eg, attending the same school or workplace but not in the same classroom or area of the workplace; customers at a store with a staff case) rarely warrant investigation in the absence of a declared outbreak.80
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