Legionnaires is a form of pneumonia caused by a bug that occurs naturally in water. The latest work, combined with earlier studies, suggests the bacteria often grow in the slimy gunk lining residential hot water pipes, and home water may be responsible for about 20 percent of cases.
"The evidence suggests that the residential water system is an underappreciated source of Legionnaires disease," said Janet Stout, a microbiologist who heads that special pathogens lab at the Veterans Administration Medical Center in Pittsburgh.
Stout presented her latest findings Sunday at a conference in Chicago of the American Society for Microbiology.
Stout estimates that between 2 percent and 5 percent of the 600,000 pneumonia cases requiring hospitalization in the United States each year are causes by Legionella pneumophilia bacteria. The diagnosis is often missed because finding it requires both a bacterial culture and a special urine test.
Her team set out to track the sources of Legionnaires infections reported to the health departments in Allegheny County in Pennsylvania and Cuyahoga County in Ohio. The families of 21 victims agreed to allow testing of their home water, and the Legionnaires bug was found in 24 percent of them. Two of the patients studied died of their infections.
The bacteria flourish at temperatures between 90 and 105 degrees. People catch the germs by inhaling drops of water. This can occur while showering, washing or even drinking, especially if people have swallowing difficulties and accidentally aspirate water.
Most people exposed to the bacteria never get sick. Those who are susceptible may include the elderly as well as people with diabetes or diseases that weaken the immune defenses.
"The overall perception we have that drinking water in the home is free of bacteria is a misconception," said Stout. "Although Legionnaires is a naturally occurring organism in water, people should be aware this is a potential source of disease."
People often keep the temperature in their hot water tanks set low to save electricity and prevent scalding. To kill off the Legionnaires bacteria, Stout recommends temporarily turning up the temperature to above 140 and running the hot water outlets for a half hour. Since the bug quickly returns, this should be done every two or three months, especially if people prone to the infection are using the water. If the temperature is kept high, the bacteria return much more slowly or not at all.
Another strategy is to let the shower run on hot for a few minutes before jumping in. This flushes out some of the bacteria that have built up in the pipes.
Typically, Legionnaires is blamed on air conditioning systems and cooling towers in large buildings, such as hotels and hospitals, where outbreaks can be especially serious.
"Everybody has been so focused on hospitals," said Richard Miller, a microbiologist at the University of Louisville. "Homes have always been in the background, but they are clearly a risk factor."
Legionnaires can be treated with the antibiotic erythromycin. The disease is found worldwide and does not spread from person to person. It was first recognized after an outbreak at an American Legion convention in 1976 in Philadelphia, where it made 182 people sick.
By Daniel Q. Haney