Legionnaires' Disease on the Rise in the United States


Over the last few years there have been several high-profile outbreaks of a particular infectious disease: Legionnaires.’

Although the disease was first described over 40 years ago, in a recent ID Week 2017 presentation, Laura Cooley, MD, MPH, from the Respiratory Diseases Branch of the Centers for Disease Control and Prevention (CDC), referred to it as an emerging disease in the sense that the number of recorded cases of Legionnaires’ in the United States continues to increase.

Why the increase? Dr. Cooley admitted that a solid answer remains elusive, but she postulated that it’s probably multifactorial. “There is definitely an increase in the susceptibility of the population in the United States. The population is aging, and there are more and more people on immunosuppressive medications,” Dr. Cooley shared.

She also pointed out that there could be more Legionella in the environment—with warmer temperatures making a great habitat for bacterial growth. Aging infrastructure could also encourage biofilm and Legionella growth. Dr. Cooley pointed out that improved diagnostic capabilities with the introduction of the urinary antigen test (UAT) for respiratory infections could also be a contributing factor; however, she feels it’s more so that “more clinicians are aware that this is a problem” thanks to the large outbreaks that have sprung up over the past years.

“There’s been political attention to the disease [as well as] media attention, and so, I think it’s a combination of this increase in cases and really big outbreaks that shifted our priorities and really encouraged our engagement,” Dr. Cooley said.

A recent analysis of 27 outbreaks that occurred between 2000 and 2014, found that the most common settings for Legionnaires’ outbreaks were hotels, long-term care facilities, and hospitals. The most common sources were potable water sources, such as showers and sinks, followed by cooling towers, and then hot tubs and decorative fountains. Dr. Cooley said that outbreak size differences were noted by source and by setting. For example, cooling tower outbreaks “were typically larger,” with a median of 22 cases per outbreak, versus potable water outbreaks, which were half as big—a median of 10 cases per outbreak.

Dr. Cooley highlighted the surveillance systems that are used to collect data on Legionellosis. The first was the National Notifiable Disease Surveillance System (NNDSS), through which local, state, territorial, federal, and international agencies all share health information on notifiable diseases; and the Supplemental Legionnaires’ Disease Surveillance System (SLDSS). The CDC get their annual case counts from the NNDSS, as well as basic demographics on the cases and case-level data. The NNDSS collects information on clinical disease from Legionella; however, it does not differentiate between Legionnaires’ disease and Pontiac fever. Similarly, the SLDSS collects case-level data; however, it also collects epidemiologic risk factors (travel history, health care exposures, disease presentation), as well as laboratory testing information and case status. The SLDSS does differentiate between Legionnaires’ and Pontiac fever.

In 2015, 6079 cases of Legionellosis were reported through the NNDSS. Some of those cases were also reported through the SLDSS, with more information. “We got almost 3700 cases of Legionellosis reported [from the SLDSS] and 98% of those were Legionnaires’ disease. So, at least of [the cases reported to the SLDSS], Legionnaires’ disease accounts for the vast majority of Legionellosis,” Dr. Cooley said. To illustrate how these numbers have grown over time, she showed conference attendees a heat map of the United States, with the colors darkening as rates of Legionellosis increased over time. “I wanted you to notice that as I go from 2005 to 2010 to 2015, the country as a whole is getting darker,” stated Dr. Cooley, driving home the grave fact that Legionnaires’ disease incidence in the United States is increasing.

Additional key takeaways from Dr. Cooley’s presentation include the following:

  • More cases/outbreaks of Legionellosis are seen in the warmer months: summer and early fall
  • Individuals over 50 are at highest risk
  • Regardless of age, 90% of individuals who contract Legionnaires’ disease are hospitalized for treatment

The UAT and a lower respiratory culture are the preferred diagnostic methods for Legionnaires’ disease, according to Dr. Cooley. “UAT is great and it’s easy; it detects Lp1, which is a common cause of reported cases in the United States, but it has some gaps,” she explained. “[The test] isn’t completely sensitive [and] that’s why we also recommend the culture of respiratory secretions on selective media be performed at the same time. Isolation of Legionella in culture is important to pick up those gaps that are missed by the UAT.” Respiratory culture is also used in outbreak settings “to compare environmental and clinical isolates for source attribution purposes,” Dr. Cooley shared.

Many of the cases of Legionnaires’ disease in the United States are being diagnosed by UAT; however, Dr. Cooley explained that further research on the test’s limitations would be a good idea to determine how many cases are being missed because of these limitations.

Although many advancements have been made when it comes to response to the disease, Dr. Cooley stressed the importance of Legionellosis prevention efforts. The CDC, for example, has been promoting tools for effective water management. In addition, the United States Environmental Protection Agency released a review of different technologies that could be used to control Legionella in plumbing systems, while the Veteran’s Health Administration has required prevention activities in their facilities for some time now. The Centers for Medicare & Medicaid Services has also issued a requirement for water management programs in some health care facilities.

Looking at the subset of outbreaks that occurred last year (2016), Dr. Cooley said, “These outbreaks could be prevented with more effective water management. [Thus,] we spent some time talking about how this requires a multidisciplinary response. It’s public health, it’s clinicians, it’s health care facility leaders, it’s the environmental health team, it’s everyone working together to prevent Legionellosis in health care facilities.”

Legionnaires’ disease is on the rise in the United States. Improved surveillance and testing are needed to improve understanding of disease and outbreak burden,” Dr. Cooley concluded. “It’s important to encourage clinicians to order tests (urinary antigen test and lower respiratory culture), for patients with severe pneumonia or healthcare-associated pneumonia. Improved uptake of water management is critical for controlling disease and healthcare-associated cases are often deadly and prevention in healthcare facilities is key.”

The CDC has several tools available on their website for water system maintenance, which is key to preventing the diseases.

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