Despite using substantial resources within BTBC and beyond, we did not clearly identify chains of transmission in this outbreak. Only 3 definite epidemiologic links were identified between patients, and only 1 was associated with the rapidly emerging or spreading isoniazid-resistant strain. The strongest link of this cluster is geographic; patients primarily spent time in the same neighborhoods. Although matching genotype does not always signify recent transmission, geospatial concentration and epidemiologic data indicate ongoing and recent transmission of this rare genotype in NYC. Contact investigation results showed evidence of possible transmission. However, no confirmed secondary TB cases were identified among >1,200 identified contacts, further demonstrating limitations of name-based contact investigation.
This outbreak was only identified through genotyping. PCR-based methods (spoligotyping and 12-loci MIRU-VNTR analysis) better defined this TB cluster. Supplementing contact investigation with laboratory tools to examine strain relatedness (e.g., real-time genotyping and DST) can help TB control program staff identify and investigate outbreaks. Although all patient specimens had a matching genotype, DST results showed 2 phenotypes, and therefore >2 distinct transmission chains within the cluster. Identifying separate transmission chains enabled cluster investigators to develop and test hypotheses specific to each chain of transmission. Common characteristics within each transmission chain implied discrete social networks, but these networks could not be confirmed by using routine cluster investigation methods.
Emergence of isoniazid resistance in this cluster cannot be clearly explained. None of the patients with drug-susceptible M. tuberculosis showed failure of treatment. Presumably, 1 person, identified by investigators as a shared contact between a patient with drug-susceptible M. tuberculosis and a patient with isoniazid-resistant M. tuberculosis, had a history of taking medications for TB and showed development of isoniazid-resistant M. tuberculosis that had not been reported to BTBC. This person died; therefore, cluster investigators were unable to confirm this hypothesis despite medical record review and pharmacy surveillance.
This investigation was limited by patients’ unwillingness to report their contacts, possibly because of fear of disclosing immigration status (not asked by BTBC staff), illegal drug use, or involvement in other illicit activities. Other possible explanations include forgetting or not knowing their contacts by name (2,23
). Certain patients used aliases (not tracked in the NYC TB registry) and claimed to only know their contacts by first names or aliases. Pervasiveness of aliases within patient social networks stymied contact investigation efforts and made establishing epidemiologic links between patients difficult.
High prevalence of illegal drug use within the cluster led investigators to explore how specific drug-use practices contribute to TB transmission. Studies reported that such specific drug-use practices as shotgunning (inhaling smoke from rock cocaine or marijuana and blowing the smoke directly into the mouth of another) and hotboxing (smoking drugs in a small, enclosed space to maximize narcotic effect through first-hand and second-hand smoke) were associated with TB transmission (24,25
). Although these practices were not specifically mentioned by patients or their contacts, specific questions were not asked until later in the investigation. After consulting with substance-use experts, BTBC revised their cluster-investigation questionnaire and provided investigators with additional training on patient-interview procedures and drug-use subculture. Understanding drug-use behavior helps TB control personnel elicit sensitive transmission information. BTBC also modified how substance-use information is collected and recorded in the TB registry.
Transmission through casual contact and increased virulence are possible explanations for extensive transmission of this strain and lack of recognition among patients. Although TB transmission from casual contact is considered rare, it has been documented (26–30
). If this strain, like other outbreak strains (29
), was highly virulent, extensive transmission among patients who did not recognize each other would have been possible. Moreover, geographic proximity of patients to one another might have increased opportunities for TB exposure and supported transmission through casual contact. In addition, positive results for acid-fast bacilli in smears of respiratory specimens among cluster-associated patients were substantial (70% overall, 93% among cocaine users) and considerably greater than recent past NYC TB patients (range 42%–46% during 2003–2008) (NYC DOHMH, unpub. data), thus increasing likelihood of transmission. Investigation findings were consistent with those of a London study that reported that pulmonary TB patients who used cocaine were more likely to be sputum smear positive at diagnosis (31
), perhaps related to delays in seeking medical care.
Photograph and name use yielded the strongest epidemiologic links between patients with isoniazid-resistant M. tuberculosis
. It was the only method that confirmed patient recognition within the cluster. All epidemiologic links established through photograph recognition were related to illegal drug activity. Other outbreak investigations have highlighted unwillingness of patients to share social contacts when these contacts are connected to illegal activities (4,5,13
Insights gained from using name and photograph data in an ongoing investigation will benefit TB control programs. This method would have been more successful if used earlier in the investigation. TB control personnel contemplating adopting this strategy should obtain legal guidance before an outbreak occurs because privacy laws vary from one locality to another.
This outbreak investigation highlights an array of challenges for US-based TB control programs. Understanding and preventing TB transmission among hard-to-reach populations requires considerable resources. Conventional contact investigation can be inadequate for identifying and curtailing TB transmission among difficult-to-reach- populations. New methods, including using name and photograph data, are needed for TB elimination.