Legionnaires Disease | FAQs


How common is Legionnaires’ disease?

More common than you probably realize.

According to the CDC, “The number of people with Legionnaires’ disease grew by nearly 4 times from 2000–2014. …About 6,100 cases of Legionnaires’ disease were reported in the United States in 2016. However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate the true incidence.”

Dr. Paul Edelstein, a Legionnaires’ disease researcher, has estimated that more than 100,000 cases of Legionnaires’ disease could be occurring each year in the U.S.

One reason Legionnaires’ disease is falsely perceived as rare is that even when cases are detected, the public rarely hears about them. Only a small percentage of cases occur as part of the multi-case outbreaks that sometimes make the news. Cases of the disease are seldom publicized even when lawsuits are involved because most Legionnaires’ lawsuits are settled under terms of confidentiality.

Why Legionnaires Goes Undetected?

A case of Legionnaires’ disease will go undetected unless special laboratory tests are performed, which often does not happen. It is reasonable to assume that undetected cases of Legionnaires’ are occurring because experience has shown that increased suspicion of the disease among physicians, when combined with increased patient testing, leads to more diagnoses. Some hospitals have recognized cases of Legionnaires’ disease only after increased testing of patients with pneumonia. Likewise, in hospitals where only one to three cases of Legionnaires’ were identified over several months, numerous additional cases were recognized after surveillance was intensified.

Studies of community-acquired pneumonia (cases acquired outside hospitals) have also indicated that increased surveillance leads to more diagnoses. A large-scale study in Ohio (U.S.A.) suggested that only 3 percent of sporadic cases of Legionnaires’ disease were correctly diagnosed.  By comparison, in studies in which diagnostic tests have been consistently used, Legionella has been recognized among the top three or four microbial causes of community-acquired pneumonia.

Because the symptoms of Legionnaires’ are similar to those of other types of pneumonia, undetected cases of Legionnaires’ disease end up being classified merely as pneumonia with no apparent cause. Based on CDC estimates, this means that 8 to 39 pneumonia deaths occur each week in the United States without anyone knowing that the cause was Legionella. What’s worse is that many of these deaths could be prevented because, unlike most cases of pneumonia, the source (e.g., a hot-water system) of Legionnaires’ cases can be identified. But if Legionella is not recognized as the cause, no investigation ensues to pinpoint and disinfect the source, so the same source remains a threat.

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