Legionnaires' Disease

 

LEGIONNAIRES' DISEASE

PKGhatak,MD

Legionnaires' disease and Pontiac fever.

In 1968 several workers of the health department of Pontiac, MI came down with a flu like illness. The cause remained unknown at the time. Several years later in 1976, a new disease called Legionnaires' pneumonia was named by the press following an outbreak of a cluster of 182 pneumonia cases in Philadelphia, PA. The cases happened suddenly in a hotel in Philadelphia during an American Legion convention. Pneumonia is caused by a gram negative, facultative intracellular bacteria called Legionnaires pneumophila. Pontiac fever was subsequently found to be caused by the same bacteria.

Characteristics of legionella bacteria.

Legionella bacteria live in nature in the soil and the surface of the water. The bacteria live intracellularly in symbiosis with amoebae. The bacterium is relatively resistant to heat and thrives in temperatures 77 to 113 degrees F. The bacteria are easily killed by chlorine, UV light and copper-silver ions. In nature, the bacteria can be found in water tanks, cooling towers, air conditioning units, water mists and sprays in supermarkets, tap water and hot water baths, swimming pools, streams and lakes, etc. Biofilms made by amoebae harboring legionella bacteria are resistant to elimination by usual disinfectants.

Pathogenic strains of Legionella that cause human illness are L pneumonia, L micdadei, L longbeachae, L feelcii and L anisa. In addition to pneumonia L pneumonia can cause pericarditis, gastroenteritis, hepatitis, systemic infection and death.

Wounds can be infected with Legionella.

The Legionella organism requires amino acid L-cysteine and Ferric iron for growth and is grown in special charcoal agar media. The bacteria derive energy from amino acids rather than carbohydrates. On the surface of the culture media, the bacteria take a filamentous motile form and use a single flagellum. The bacterium is gram negative but stains poorly with safranin.

Human diseases:

Legionella pneumonia.

The elderly, people having chronic obstructive lung disease (COPD), and those immunosuppressed are prone to infection. The mode of infection is by inhalation of water mist contaminated with these bacteria. The use of contaminated nebulizer machines, central air conditioners in closed spaces, polls, and showers are the main sources of infection. Person to person infection does not happen. In the USA 13,000 cases are seen yearly, more cases in summer due to the use of air conditioners.

The incubation period is 2 to 10 days. The initial symptoms are like any URI, followed by cough and high fever. Some develop nausea, vomiting, streaks of blood in sputum, headache, muscle aches, and neuromuscular symptoms.

Clinically two groups are recognized. One with mild symptoms. They usually recover in 2 to 5 days. The other more seriously ill patients develop shortness of breath, and chest tightness, have a high fever and require hospitalization and antibiotics and oxygen therapy.

Unusual characteristics of L.pneumonia.

The bacteria are recognized as invaders by macrophages of lung alveoli and the macrophages engulf the bacteria. But the bacteria not only neutralize the digestive enzymes of the macrophages but multiply rapidly and kill the macrophages, and infect more macrophages. The cytokines released by the dying macrophages attract monocytes and lymphocytes at the site and the inflammation and pneumonia begin. IL- 1 and TNF play a significant role in the genesis of inflammatory reactions. The Legionella spreads to the other areas in the lung by the movement of macrophages, by mucociliary escalator and lymphatics and in rare cases by blood.

There are no clinical or laboratory distinguishing features of L pneumonia. The initial chest x-rays show nodular infiltrates, in 5 days infiltrates spread to the other lung in spite of usual antibiotics. Lower lobes are usually involved in pneumonia but may spread to the middle lobes. Hyponatremia and abnormal liver enzymes, and renal functions may be present. The use of corticosteroids worsens the condition.

Diagnosis is made by the presence of the Legionella antigen in the urine, and L pneumophila growth by sputum culture. No antibody test is possible because of poor antibody response and for the same reason, no vaccine is available. Macrolides antibiotics are usually prescribed and quinolones and tetracyclines are also effective.

The mortality rate was  5 to 30% in earlier days. And inpatient mortality was as high as 28 %.

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