Legionnaires Disease | FAQ


How common is Legionnaires disease?

Legionnaires’ is not rare. It is perceived as rare only because most cases are never detected, and not all detected cases are reported to public health authorities. Because under-diagnosis and under-reporting make incidence of the disease difficult to estimate, figures have varied widely. The (U.S.) Centers for Disease Control and Prevention (CDC), Atlanta, has estimated that the disease infects 10,000 to 15,000 persons annually in the United States, but others have estimated as many as 100,000 annual U.S. cases.

Another reason that Legionnaires’ is falsely perceived as rare is that when cases are detected, the public rarely hears about them. Most cases—at least 65 to 80 percent in the United States and the United Kingdom —occur sporadically (one or two at a time). Thus, only a small percentage of cases occur as part of the multicase outbreaks that sometimes make the news. Cases of the disease are seldom publicized even when lawsuits are involved, because most Legionnaires’ lawsuits are settled quickly and under terms of confidentiality.

A case of Legionnaires’ disease will go undetected unless special laboratory tests are performed.  Unfortunately, most U.S. hospitals still have not made these tests routinely available.  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 pneumonias, 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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