LEGIONELLA INFECTIONS

Legionellosis is caused by the bacteria of the genus Legionella.

Two distinct clinic syndromes:

• Legionnaires' disease, is the designation for the pneumonia caused by  Legionella . It is the more severe form of the two
• Pontiac fever: is an acute, febrile self-limiting illness ( without pneumonia) linked to legionella.

History:

Legionnaires' disease acquired its name in 1976 when an outbreak of pneumonia occurred during a convention of the American Legion in Philadelphia, leading to 29 deaths.

Epidemiology -  Ecology - Transmission:

 An estimated 8,000 to 18,000 people get Legionnaires' disease in the United States each year. Legionella bacterium may lead to no illness at all or a mild to severe infection.

 

Incidence depends on the degree of contamination of water reservoirs.

The illness most often affects middle-aged and older persons, cigarette smoker or those with  chronic lung disease. Persons with  immune deficiency or immune suppression, cancer, kidney failure requiring dialysis, diabetes, AIDS, intubated surgical  patients, transplant recipients, are at increase risk.

Pontiac fever most commonly occurs in epidemics affecting persons who are otherwise healthy. It reflects airborne transmission.
It usually occurs as a single, isolated case not associated with an outbreak. When outbreaks do occur, they are usually recognized in the summer and early fall, but cases may occur year-round.
 

Legionaire’s occurs in outbreaks after persons have breathed mists that come from a water source (e.g., air conditioning cooling towers, whirlpools, spas, showers) contaminated with Legionella bacteria. Persons may be exposed to these mists in homes, workplaces, hospitals, public places, cruise ships, hotels.

Legionellosis is not passed from person to person, and there is no evidence of persons becoming infected from auto air conditioners or household window air-conditioning units.
Legionella organisms can be found in many types of natural water systems. However, the bacteria reproduce to high numbers in warm, stagnant water (90°-105° F), such as that found in certain plumbing systems and hot water tanks, cooling towers and evaporative condensers of large air-conditioning systems, and whirlpool spas. It is believed to occur worldwide.

Transmission mode remains controversial. It is probably multiple. Aspiration, aerosolization and direct instillation into the lung.

Clinical and laboratory features:

• Pontiac fever, is an acute, self-limiting, flu like illness with a 24 to 48 hour incubation period. Complete recovery occurs in a few days with or without  antibiotic therapy.
• Symptoms:  At first fever, chills, cough, commonly G.I. symptoms such as nausea vomiting. Also muscle aches,  headache, tiredness, loss of appetite are seen in patients .

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• Legionnaires' disease – Incubation of 2 to 10 days – present with the above symptoms that lead to severe  atypical pneumonia not responsive to standard antibiotic therapy
• Symptoms:  At first fever, chills, cough, G.I. symptoms such as abdominal pain, nausea,  vomiting , chest pain, shortness of breath, headache, tiredness, loss of appetite. Virtually all patients have an abnormal chest x-ray. The findings are non specific fro Legionaire’s.
• Mortality rate: 5% to 30% of people who have Legionnaires' disease die.

 Diagnosis:

1. Culture :The family legionellaceae comprises 41 species. Legionella pneumophila causes 80 to 90 % of human infections, It is an aerobic Gram-negative bacteria  that only grows in special culture media ( charcoal yeast extract agar) . 80% sensitive – 100% specific.
2. DFA Staining : Laboratory identification can also include microscopic examination in conjunction with a direct fluorescent antibody (DFA) test. 70% sensitive -100% specific.
3. Antibody detection: Acute and convalescent sera. Fourfold rise is diagnostic.   40-60 % sensitive – 99 % specific.
4. Urinary antigen: This test detects Legionella in the urine. It is rapid, inexpensive, and become positive within 3 days after onset of disease.
5. Molecular method: PCR with DNA probes may become the best method in the future.                   

Treatment:

• Erythromycin was  the antibiotic of choice following the 1976 outbreak. It can still be used but may not be well tolerated because of gastric irritation or ototoxicity.  Newer –better tolerated- macrolides such as azithromycin – clarithromycin are  very effective.
• Quinolones such as Ciprofloxacin are the agent of choice in transplant recipients on immunosuppressive drugs.
• In severe case a combination of rifampin and a macrolide or a quinolone is recommended.
  Pontiac fever requires only supportive treatment.

Prognosis for LD:  

If the patient is treated with appropriate antibiotics near the onset of pneumonia, the outcome is excellent, especially if the patient has no underlying illness that compromises his/her immune system. Complete recovery within one year is usually the rule.  Contracting Legionnaires' disease a second time is extremely rare.

 

Prevention:

Disinfection of the water supply is the ultimate preventive measure. Heat, copper and silver have proven to be effective measures of disinfection.

Improved design and maintenance of cooling towers and plumbing systems to limit the growth and spread of Legionella organisms should help in the reduction of cases.

 

Paul Assad MD traveldoc@shaw.ca

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