US-born persons who reported substance abuse most strongly characterized the TB outbreaks in this review: 24 of the 27 total outbreaks involved primarily US-born patients, and 19 of these outbreaks involved >40% of patients with reported drug or alcohol abuse. This predominance of substance abuse suggests that it remains one of the greatest challenges to controlling TB transmission in the United States.
Because this descriptive review of TB outbreaks in the United States was restricted to investigations that prompted public health jurisdictions to request CDC assistance, it might lack generalizability to all TB outbreaks in the United States. Outbreaks involving hard-to-reach populations, such as those involving substances abuse or homelessness, with a tendency to overwhelm local public health resources, might be overrepresented. On the other hand, social risk factors such as substance abuse that are based on self-reported behavior might have been underdisclosed because of associated social stigma. Data on key medical risk factors such as HIV and diabetes might have been pending or missing during an investigation and therefore not systematically included. Because genotyping might not have been conducted on every culture-positive case in the jurisdictions affected by these outbreaks, especially during the first 2 years of this review, some cases could have been missed, underestimating the scope of these outbreaks. Despite these limitations, characteristics found to be associated with intense TB transmission are consistent with findings in the previous literature.
Although the case rate is 10× higher among foreign-born than among US-born persons (21
), this disparity was markedly lacking in our review; 91% of outbreak patients were US born. Prior studies have demonstrated that recent transmission occurs mainly among US-born persons, with foreign-born persons more likely to develop reactivation of latent TB infection acquired before immigration (24–26
). Similar to other studies (25–27
), our few examples of TB outbreaks among immigrants were all associated with crowded living conditions and lack of access to medical care, whereas outbreaks that involved mainly US-born persons were associated with substance abuse and other risk factors, such as homelessness and incarceration.
Among nationally reported TB cases, substance abuse has been estimated to be the most prevalent modifiable TB risk factor, reported by 29% of US-born vs. 8.3% of foreign-born patients (28
). In our overview, 58% of outbreak patients self-reported substance abuse. Consistent with national TB surveillance regarding substance abuse, alcohol was the most commonly reported substance. Alcohol has been documented to increase the risk for TB exposure, susceptibility to infection, and progression to active disease (29,30
). Contact investigations among bar patrons have yielded latent TB infection rates of 40%–50% (29
), highlighting the transmission risks in this population. Failure by contacts who abuse alcohol to be treated for latent TB infection can prolong outbreaks if active TB subsequently develops in these persons and they then serve as additional sources of transmission (4
Substance abuse is a long-established risk factor for TB infection and disease (32,33
), but in recent years its role in fueling TB transmission has also been recognized (10–12,14,15,18,19,28,29,31,32
). Persons who report substance abuse are associated with increased TB transmission because of sociobehavioral and clinical TB risk factors. First, persons who report substance abuse are more likely to have smear-positive disease and experience treatment failure (28,34
), e.g, because of nonadherence, both of which can increase infectiousness (20
). The higher prevalence of smear positivity might be attributed to delayed diagnosis, or, in cases of crack cocaine use, pulmonary damage that leads to alveolar macrophage impairment and cytokine dysfunction (34
). Second, persons who report substance abuse are likely to experience a prolonged infectious period because of delays in seeking medical care and, once they are medically evaluated, receiving a TB diagnosis (18,31,32
). Third, sharing of drugs or alcohol often occurs in confined and poorly ventilated settings such as drug houses (4,10–12,15,18,19
), bars (15,29,31
), homes (7,10
), and vehicles (18
)—all of which facilitate close and prolonged contact. The most common hotspots in this review were settings in which drug use occurred; poverty, unstable housing, and overcrowded conditions exacerbated TB transmission (4,7–9,11,15,18,19
). Fourth, contacts of TB patients are often difficult to identify because patients want to protect the names of contacts with whom they engage in illicit or other activities perceived to have social stigma (10,28,31,32
). Our finding of an overall 21% latent TB infection rate among contacts, lower than the expected 30% (20
), might reflect evaluation of relatively lower risk contacts whose names were easier to elicit. Finally, contacts with substance abuse can be difficult to locate, be less likely to accept and adhere to treatment, and have a greater risk for adverse reactions from medication, e.g., related to interaction of alcohol with isoniazid (28,29,32
Given the predominance of patients with substance abuse in our review, it is not surprising that prolonged infectious period was the most common outbreak contributing factor. Delayed diagnosis was the most common cause (14/27 outbreaks) and has been cited as a major contributor to TB outbreaks (3,7,11–15,22,23,25,27,35,36
). In 1 outbreak, during a 1-year infectious period, the source patient lived in 4 locations, all crowded settings, and shared illicit drugs with household members, facilitating TB transmission to 3 adults and 3 children (11
). In another outbreak, during the 9 months that the source patient’s diagnosis was delayed, the patient was in and out of jail and had multiple moves to new residences, resulting in 37 additional cases (including 10 children) across 3 counties (9
). These examples of intense transmission occurring before a correct diagnosis was made highlight the need for educating health care providers to suspect TB when encountering either persons born abroad or domestically with social risk factors for TB, such as substance abuse, homelessness, and incarceration history (6,7,9,11,12,15,35
). Failure to do so can lead to outbreaks that overwhelm public health resources.
Additionally, raising general awareness about TB so that patients seek early medical care and know the value of completing treatment are critical to ending transmission (22,36
This review found that incomplete contact investigation was the second most common contributing factor to TB outbreaks. When contact investigations are incomplete, a pool of latent TB infection remains, threatening to generate additional cases and cause ongoing transmission (10,15,24
). Compounding these risks, persons who report substance abuse are more likely to be poor, homeless, and have an incarceration history—all documented TB risk factors (5,11,13–15,28,37,38
). When contact investigations involve a hard-to-reach population, conventional methods of contact tracing may need to be expanded to include other approaches (2,5,10,13–15,27,39
). To optimize the yield of contact investigation, the 2 interventions most frequently used in these outbreaks were prioritizing screening of contacts on the basis of TB risk (3,5,6,8–11,18
) and offering location-based TB screening at specific venues associated with each outbreak, including bars, shelters, and drug houses (5,6,10,13,14
). Although this intervention is resource-intensive, its benefits have been recognized in several investigations involving hard-to-reach populations (10,14,31
). In 1 outbreak, unnamed contacts encountered at a drug house frequented by numerous TB patients were offered screening and were found to be 8× more likely to have a positive tuberculin skin test result than were named contacts (10
Although this review was limited to outbreaks in which CDC was invited to assist and might not represent all TB outbreaks in the United States, it provides an opportunity to identify common themes among outbreaks which, when present, tend to challenge local public health capacity. These outbreaks featured US birth and substance abuse—factors shown to be independently associated with genotype clustering, a marker for recent TB transmission (39,40
). Although TB incidence has been decreasing in the United States, its elimination will not be achieved without more effective strategies to prevent, detect, and treat TB among persons who are known to abuse substances.