On Wednesday, the United Nations hosted the first-ever high-level meeting on tuberculosis (TB), with the theme of “United to end TB: an urgent global response to a global epidemic.” This was only the fifth time that the UN had called for a high-level meeting devoted to a specific health issue. Previous meetings have been held on HIV/AIDs, non-communicable diseases, antimicrobial resistance, and Ebola. The goal of the meeting was to adopt a Political Declaration on TB, endorsed by Heads of State, that will “secure commitment from Heads of State and government for a coordinated global response, agree to substantially increase financial resources for TB, and lead to millions of lives saved from premature deaths from TB.” This meeting was a “historic and unprecedented step” towards fighting a disease that infects a quarter of the world’s population, killed over 1.7 million people in 2016 alone, and is a leading killer of HIV-infected individuals.

While much of the attention at the high-level meeting was appropriately focused on reducing TB transmission in high-burden settings, we thought for this week’s blog post, we would examine progress in a setting where TB incidence has been greatly reduced over the years—the United States. Specifically, we review progress towards elimination in the United States with the goal of understanding some of the barriers that may still exist in low incidence settings.

Current status of TB in the US

TB is a bacterial disease caused by Mycobacterium tuberculosis that typically affects the lungs. In the United States, TB cases have been on the decline--in 2016, there were 2.9 cases per 100,000 people, a 3.6% decrease from 2015. Preliminary data from 2017 suggests that this progress has continued, with a reported incidence rate of 2.8 cases per 100,000 people, a decrease of 1.8% from 2016, and the lowest case count on record. However, even at the current rate of annual decline, the country will not achieve TB elimination this century, highlighting a need to intensify efforts to test and treat those populations most at risk for disease.

Only some of those exposed to TB will develop active TB disease, while others will develop what is known as latent TB infection (LTBI), an inactive, asymptomatic form of the disease that can last for a lifetime. While those with LTBI are not contagious, they can develop active TB if their immune system is weakened, and will then be able to transmit the disease. A recent study published in the journal EID estimated that 3.1% of the US population (8.9 million people) were latently infected with TB during 2011-2015. Since 80% of all TB cases are due to reactivation of LTBI (and not recent TB transmission), identifying those who are latently infected will be an important step in preventing TB outbreaks and reaching elimination targets.

Outbreaks of TB in the US

Identifying populations at highest risk for TB outbreaks can help prioritize resources to the places in which they will have the highest impact. For example, a TB resurgence in the late 1980’s/early 1990’s was attributed to an increasing number of HIV-infected individuals, nosocomial transmission, drug-resistant TB, and immigrants coming from countries with high TB incidence. Targeted interventions such as enhanced infection control within hospitals led to a 44% decline in TB cases from 1993-2003. However, outbreaks of TB continue to persist within the United States, emphasizing the need for continued understanding of what populations are being impacted and what factors exist that might be undermining control efforts.

One recent study of 21 TB outbreaks that occurred between 2009 and 2015 found that most of the patients were US born (79%) and used alcohol excessively or illicit substances (83%). A previous study that looked at 27 TB outbreaks that occurred between 2002 and 2008 found similar results: 91% of patients were US born and 58% reported some sort of substance abuse. However, there were some notable differences: in the earlier study, only 20% of patients reported a history of homelessness, a number that grew to 45% in the later study. Additionally, in the later study, mental illness was also noted to be a risk factor. Since most of the outbreaks in the later study involved congregate settings (eg, homeless and correctional facilities), the authors highlight the need to target infection control and TB control strategies towards the vulnerable populations that reside within these locations.

Both studies demonstrated that US born individuals were primarily impacted by the outbreaks, which is interesting, given that the majority of TB cases (68.5% in 2016) typically occur in patients born outside of the US. However, other prior studies have also indicated that recent TB transmission tends to occur in US-born individuals, and reactivation of LTBI tends to occur in immigrants who acquired the infection abroad. This indicates that the epidemiologic pattern of TB within the US differs depending on the population impacted, necessitating different control strategies, including prompt contact tracing once a case is identified in a high-congregate setting, and screening for LTBI in populations at high risk.

The authors of the later study also found delayed diagnosis to be a contributing factor in nearly all of the outbreaks studied (95%), a large and concerning increase from the earlier study (44%). This not only included delays in patients seeking care for symptoms, but also delays in diagnosis once under medical care. This is likely a result of lack of access to routine health care due to poverty and other social circumstances.

These findings tell us that providers who work with these patient populations must be made aware of their increased risk for TB, particularly because TB does not have a high index of suspicion in the US due to its low incidence, and because it is more often diagnosed in foreign-born individuals. Ensuring equal access to care for all individuals is of the utmost importance, to ensure timely diagnosis and access to treatment that will not only decrease morbidity and mortality, but also decrease community TB transmission. These efforts will be necessary to eliminate TB in the United States this century.

Photo: Produced by the National Institute of Allergy and Infectious Diseases (NIAID), this digitally colorized scanning electron microscopic (SEM) image depicts a grouping of red colored, rod shaped, Mycobacterium tuberculosis bacteria, which cause tuberculosis (TB) in human beings. Please see the Flickr link below, for additional NIAID photomicrographs of various bacteria.

Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly to allow others to learn from these experiences in order to improve global outbreak response capabilities.