People who developed illness did so between 2 and 10 days after they had attended meetings in the convention center’s hotel. High fever, chills, headaches, and muscle aches and pains were the initial symptoms. Some people had dry coughs, chest pains, shortness of breath, mental confusion, vomiting, and diarrhea. All eventually developed pneumonia. Ultimately, 34 of the 221 people who became ill died. No person-to-person spread was documented in this outbreak or any others. Careful epidemiologic investigation eliminated a multitude of possible sources. Air was implicated as the probable pathway of infection. It was shown statistically that patients had spent a significantly longer period of time in the lobby of the convention center’s hotel than those who remained healthy.

Examination of sera and tissue specimens from some previously unsolved outbreaks that had been stored at the Centers for Disease Control (CDC) in Atlanta, Georgia, and elsewhere revealed that outbreaks of this disease had occurred earlier. The earliest case so far documented occurred in a soldier who had an unidentified pneumonia in 1947 when he was at Fort Bragg, North Carolina. The earliest outbreak (three or more cases from the same source) of legionellosis appears to have occurred in a meat packing plant in 1957.

Since the Philadelphia outbreak, additional outbreaks and numerous sporadic cases of legionellosis have been reported. About 1,400 cases are officially reported annually to the CDC, and about 500 are confirmed. A study of people hospitalized with pneumonia calculated that 11,000 cases of legionellosis occur annually in the United States. This calculation is thought to be an underestimate. Estimates as high as 100,000 cases annually in the U.S. have been suggested, although the scientific basis for these estimates is not clear. Serologic surveys indicate that many people in the general population have antibodies to legionellae in their blood. This suggests that large numbers of people may have been exposed and possibly infected with legionellae. While most of the earlier outbreaks were traced to aerosols contaminated with these organisms from either cooling towers or evaporative condensers, other outbreaks have been traced to potable water services and components, such as water heaters, showers, faucets, whirlpool baths, respiratory therapy equipment, decorative fountains, and an ultrasonic mister sometimes used in grocery stores.

Legionellosis is an illness that has two distinct clinical forms. The most serious form is a pneumonia illness that is commonly referred to as Legionnaires' disease. Generally, in epidemic settings, the proportion of people who contract the disease is 5% or less; however, risk factors such as age, smoking, underlying disease, and compromised immunity play a major role in susceptibility. The incubation period is 2 to 10 days, and treatment with erythromycin with or without rifampin is usually effective. Fatality rates in outbreaks have averaged about 15%. Early diagnosis and treatment (possibly including hospitalization) will reduce the chance of fatality, although the chance of succumbing to the disease may be higher among particularly vulnerable people.

The other clinical illness, Pontiac fever, is a flu-like illness without pneumonia. In contrast to Legionnaires' disease, most of those who are exposed develop illness within 48 to 72 hours and most recover without antibiotic therapy.

Q. How do people contract Legionella?

A. The most popular theory is that the organism is aerosolized in water and people inhale the droplets containing Legionella. However, new evidence suggests that another way of contracting Legionella is more common. "Aspiration" is the most common way that bacteria enter into the lungs to cause pneumonia. Aspiration means choking such that secretions in the mouth get past the choking reflexes and instead of going into the esophagus and stomach, mistakenly, enter the lung. The protective mechanisms to prevent aspiration is defective in patients who smoke or have lung disease. Aspiration now appears to be the most common mode of transmission.

Q. What are the usual symptoms?

A. Symptoms commonly include headaches, fatigue, weight loss, body aches, and fever. Non-productive cough, abdominal pains, and diarrhea occur in many patients. In patients with Legionnaires' disease, pneumonia develops which can cause death. With Pontiac fever there is no pneumonia, but the patient may have a high fever for 2-5 days and then recover without treatment. Legionellosis commonly occurs in patients who are elderly, immunocompromised, or otherwise in poor health.

Q. Is the illness always severe?

A. Studies have shown that about 5% to 15% of known cases of Legionnaires' disease have been fatal. However, many people may be infected with the bacterium causing Legionnaires' disease without developing any symptoms. Others may be treated without having to be hospitalized. Recent information suggests that many cases of Legionnaires' disease go undiagnosed.

Q. How is the disease diagnosed?

A. Your physician can order tests to confirm the diagnosis of Legionnaires' disease. The most useful tests include culturing the bacteria from sputum, detecting the presence of bacteria in a urine sample, and comparing antibody levels in two blood samples collected three to six weeks apart.

Q. Is there effective treatment for Legionnaires' disease?

A. The currently recommended drug of choice is erythromycin, which appears to be effective in treating the disease. Other drugs are available for patients unable to tolerate erythromycin.

Q. How is Legionnaires' disease spread?

A. There is no evidence for person-to-person spread of the disease. Most people contract Legionnaires' disease by inhaling mist that comes from a water source (e.g. showers, cooling towers, whirlpool baths) contaminated with Legionella bacteria. In some cases, the disease may be transmitted by aspirating contaminated water.

Q. Who gets Legionnaires' disease?

A. People of any age may get Legionnaires' disease, but the illness most often affects middle-aged and older persons, particularly those who smoke cigarettes or have chronic lung disease. Also at increased risk are persons whose immune system is suppressed by diseases such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those that take drugs that suppress the immune system are also at higher risk.

Pontiac fever most commonly occurs in persons who are otherwise healthy.

Q. What have been the water sources for Legionnaires' disease?

A. The major source is water distribution systems of large buildings including hotels and hospitals. Cooling towers have long been thought to be a major source for Legionella, but new data suggest that this is an overemphasized mode of transmission. Other sources include mist machines, humidifiers, whirlpool spas, and hot springs. Air conditioners are not a source for Legionnaires' disease. They were suspected to be the source in the original American Legion outbreak in a Philadelphia hotel, but new data now suggests that the water in the hotel was the actual culprit.

Q. Where is the disease found?

A. Cases have been identified throughout the United States and in several foreign countries. It likely occurs worldwide.

Q. How common is Legionnaires' disease in the United States?

A. It is estimated that around 10,000 people develop Legionnaires' disease each year in the United States. An additional unknown number are infected with Legionella bacteria but have only a minor illness or no illness at all. The disease can occur in outbreaks or as single cases.

Other Links