Rare and Deadly Diseases: Legionnaires' Disease

Legionnaires' Disease

In the bicentennial summer of 1976, the American Legion symbolically held its fifty-eighth annual convention in Philadelphia at the landmark Bellevue-Stratford Hotel. The eagerly anticipated event began with its usual fanfare and high spirits. But the convention mood turned dour as one veteran after another became deathly ill.

Within a short period, 221 people were stricken with an unknown respiratory syndrome characterized by high fevers, rigors, extreme exhaustion, and poor response to antibiotic therapy. At its conclusion, the epidemic resulted in 34 deaths due to rapidly progressive pneumonia and related complications. The cause of the epidemic baffled the medical and public health community since it did not appear bacterial in origin. Some suggested it was the beginning of an influenza pandemic, since “Swine Flu” had already started infecting large populations in Asia that year. Other speculation ranged from chemical agents to covert conspiracies directed against the American veterans.

Federal and State epidemiologists were investigating the hotel as the epidemic's source because the outbreak was confined, while scientists looked for clues in diseased tissue and bodily fluids. For months, questions of origin swirled in the media. The puzzle was ultimately solved by CDC scientist Joseph McDade. Surprisingly, it was a bacterium, Legionella pneumophila, aptly named in tribute to the afflicted veterans and as a reflection of the nature of the disease. Once identified, the organism was traced to an air conditioning/cooling system that served the convention hall. Not long after the identification of Legionella, it was determined that a similar unexplained episode some eight years earlier in Pontiac, Michigan was caused by the same organism. Cases have now been traced back to 1947.

The Cause

Legionella pneumophila is one of many species of Legionella, but only half cause disease in people. The organisms are intracellular pathogens, so cell-mediated immunity plays the most important role in host defense.

Legionella continues to cause minor epidemics throughout the world, and legionellosis is a worldwide problem. Cases appear in sporadic, endemic, and epidemic fashions. Epidemics usually result from exposure to aerosolized bacteria in the hospital or in the workplace. It accounts for one to three percent of community-acquired pneumonias and 13 percent of pneumonias acquired in the hospital.

Other Epidemics and Outbreaks

The second largest outbreak was at the Stafford hospital in England in 1985, where a total of 101 people contracted the disease and 28 died. In late February 1999, an epidemic emerged at a large flower show, the Westfriese Flora (WF) in Bovenkarspel, The Netherlands. Some 181 people developed Legionella pneumonia and 21 died. In this case, a whirlpool in one of the main halls was found to be the likely source of infection.

Symptoms of Legionnaires' Disease

Symptoms usually begin 2 to 10 days after a person is infected with the bacterium, but in most cases, symptoms begin after five to six days. Patients develop flu-like symptoms with muscle aches, headache, loss of appetite, and dry cough with persistent high fever. A chest X-ray usually shows pneumonia. A related illness caused by Legionella, called Pontiac Fever, consists of fever, headache, weakness, and muscle ache. It is generally less severe, lasts for two to five days, and there is no pneumonia.

How You Can Get It

Legionnaires' disease is acquired by inhaling the Legionella bacteria, usually carried as an aerosol mist. It is not spread person to person. It can affect all adults but is most common among the elderly and those with impaired immune systems. Men appear more susceptible than women, and diabetics, smokers, or heavy consumers of alcohol are also at increased risk of getting Legionnaires' disease.

Clinical symptoms are confirmed by growing the bacterium from lung secretions, testing the blood for an immune reaction, or analyzing urine for the presence of the organism. The urine test is most effective three days after symptoms appear.

Treatment and Prevention

Erythromycin is the antibiotic of choice, although doxycycline has also been used effectively. Exposure can be minimized by inspections of large air conditioning systems with cooling towers and evaporative condensers where the organisms proliferate. Window air conditioners are not a risk for disease transmission.

Where Legionella Likes to Live

Water reservoirs are the natural habitat of Legionella pneumophila, although the bacteria are found in many environments including soil and dust. The bacteria are also found in hot and cold water taps, showers, whirlpool baths, creeks, ponds, and wet soil. Warm water helps Legionella multiply and cases increase in warm weather. Initially, air conditioning cooling towers were implicated as the primary source of infections through aerosols. However, it is generally recognized that contaminated supplies of drinking water are also a common source of spread to patients.

Trojan Horse Theory

It was recently found that amoebae also proliferate in warm waters and provide Legionella shelter from unfavorable environmental factors. The exact role of amoebic hosts in bacterial survival, multiplication, and transmission is just emerging. But it has been suggested that once amoeba harboring Legionella are ingested, the bacterium quietly emerges from its shelter, where it can infect and cause disease. Thus, the free-living amoebae are the “Trojan horses” of the microbial world.