Two TB outbreaks identified in British Columbia

Gardy JL. N Engl J Med. 2011;364:730-739.

Issue: March 2011

Whole-genome data suggest that there were two concomitant outbreaks of Mycobacterium tuberculosis in a community in British Columbia, Canada, according to a report in The New England Journal of Medicine.

The two genetically distinct lineages had identical mycobacterial interspersed repetitive unit–variable number tandem repeat (MIRU-VNTR) genotypes, according to investigators from the British Columbia Center for Disease Control. Both lineages descended from a common ancestor. They also were detected in the community before the outbreak, which suggested that the outbreak had a social trigger, rather than a genetic one.

The British Columbia Center for Disease Control initiated the epidemiologic investigation when an outbreak of TB occurred in a medium-sized community in British Columbia. The first case was identified in May 2006. The investigators initially used traditional contact-tracing approaches to identify cases.

However, a single source case could not be identified. A social network questionnaire was also created to identify possible settings of transmission. The questionnaire was focused on drug and alcohol use, residential and travel history, places of social aggregation and identification of contacts in the context of high-risk behaviors and locations.

Through the social network approach, the investigators identified the most likely source case. The adult had cavitary, smear-positive pulmonary TB and had been symptomatic and untreated for 8 months before the first identified case. This source was connected to all but two of the cases.

“Use of the social-network questionnaire improved subsequent case-finding efforts by revealing previously unreported social interactions and identifying several locations frequented by infectious patients, including two hotels, a meal center, two community centers and a series of crack houses,” the investigators wrote.

By December 2008, 41 cases of TB were identified. Of the 41 cases, 37 were laboratory-confirmed and the other four were clinical cases, in which M. tuberculosis could not be cultured but was characterized by a radiologic, pathologic or therapeutic response indicative of TB. All of the cases were susceptible to first-line antibiotics.

Most patients with TB were adults and presented with pulmonary TB (68%) or pleural TB (24%). There were two pediatric cases. Outcomes were recorded through 2009, which resulted in a minimum of 12 months of follow-up for each patient. Almost all patients completed treatment and were cured.

For the genomic investigation, the M. tuberculosis isolates were cultured, and MIRU-VNTR analysis was performed on the 37 laboratory-confirmed isolates. Thirty-two of the outbreak isolates and four historical isolates were sequenced. All of the cultured M. tuberculosis isolates had an identical pattern of MIRU-VNTRs. The researchers identified two distinct lineages of M. tuberculosis that were circulating.

After additional epidemiologic follow-up, the investigators found that 61% of the 41 patients with TB had a self-reported history of crack cocaine use.

“Cocaine-related police files in the outbreak community mirrored the epidemic curve, peaking in 2006 before declining over the subsequent years, and a number of crack houses proliferated in the region during the same period, suggesting that the use of crack cocaine may have played a role in triggering and sustaining the outbreak,” the researchers wrote.

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