By Betsy Todd, MPH, RN, CIC, AJN clinical editor
In the largest U.S. outbreak of Legionella infection since 1976, when there were 221 cases and 34 related deaths in an outbreak at a Philadelphia American Legion convention, more than 113 cases of the disease have been diagnosed in New York City since mid-July. Twelve people have died.
Legionnaires’ disease is neither rare nor exotic; it is a type of community-acquired pneumonia (it can also be hospital acquired). Symptoms include fever, cough, and progressive respiratory distress. Legionella can also cause a milder, flulike illness known as Pontiac fever that generally resolves without treatment. Because many cases of Legionnaires’ disease are never actually diagnosed, mortality rates are difficult to determine, but the rate currently is estimated at 5% to 30%.
The CDC estimates that 8,000 to 18,000 people are hospitalized with Legionnaires’ disease each year in the U.S., yet only about 3,000 cases are diagnosed and reported. Most cases of Legionnaires’ disease are sporadic, unlinked to any outbreak. The infections often are not recognized as Legionnaires’ disease, for several reasons.
- Legionella infection is easily treated empirically (that is, without confirmatory lab testing) with common antibiotics, with the patient usually recovering. This is a practical and cost-effective approach to community-acquired pneumonia, but many cases of Legionnaires’ disease are never diagnosed as anything more specific than “pneumonia.”
- When Legionnaires’ disease is suspected, the most common test ordered—Legionella urinary antigen—tests for only one of more than 46 Legionella species: pneumophila serotype 1. While a significant percentage of cases may be attributable to pneumophila serotype 1, a negative Legionella urinary antigen test does not rule out Legionnaires’ disease.
- Only a Legionella culture has the potential to identify any Legionella strain, and special culture media is needed. In most labs, a respiratory specimen sent for culture is not routinely tested for Legionella.
Legionella does not spread from person to person. It is transmitted by aerosolized water from sources such as whirlpools, hot tubs, hydrotherapy tubs, showers, indoor waterfalls or decorative fountains, grocery produce misters, or cooling towers on large buildings. Legionella prefers large, complex plumbing systems over natural bodies of water, because plumbing systems provide the temperature range, commensal organisms, and stasis that best support Legionella growth.
Who’s at risk. As with community-acquired pneumonia caused by other organisms, the people most likely to become infected are those with preexisting health problems such as COPD, diabetes, or immunosuppression; smokers; and people over 50. Children are usually not infected with Legionella unless they are immunosuppressed.
In the current New York City outbreak, the first cases appeared in four neighborhoods in the south Bronx, pointing to a probable common water source in the neighborhood. Several cooling towers have been tested, but final results are not yet available. (Only matching genotypes—”DNA fingerprints”—of Legionella isolates from the patient and water samples cultured can confirm the source of an outbreak.)
Socioeconomic factors. The size of this particular outbreak may turn out to suggest as much about socioeconomic conditions and health as it does about pathogenicity. A 2014 review of cases reported to the New York City Department of Health and Mental Hygiene found the following:
Overall, incidence of Legionnaires’ disease in the city of New York increased 230% from 2002 to 2009 and followed a socioeconomic gradient, with highest incidence occurring in the highest poverty areas.
The reasons for this aren’t clear, but may reflect some combination of poor health related to poverty, increased levels of risk factors in the population or in some types of work, a lack of insurance and/or access to health care, or even building construction and maintenance in the area.
For more information about Legionnaires’ disease, read AJN‘s short November 2005 article (free until September 15), which gives some essential information on the disease.