Legionella pneumophila

In 1976, an outbreak of pneumonia occurred among people who attended the American Legion Convention in Philadelphia. Dubbed Legionnaire’s Disease, there were 182 documented cases with 29 mortalities. In January, 1977, within six months of the outbreak, Dr. Joseph McDade of the Centers for Disease Control, identified the etiology of the pneumonia as a gram negative, non-spore producing rod and classified it under a new family called Legionellaceae.

Legionella pneumophila

The family Legionellaceae is composed of approximately 50 species and sub-species with 71 serologic types of Legionella.  L. pneumophila is the primary human pathogen in this family. Other Legionella which cause human infection are the following:

  • Legionella micdadei 
  • Legionella bozemanii 
  • Legionella dumofii 
  • Legionella gormanii

The other species of Legionella are environmental pathogens.

Legionellae are thin, non-capsulated pleomorphic bacilli not easily stained with safrinin. They are obligate aerobes possessing specific nutritional requirements. They use proteins as their source of energy rather than carbohydrates and are biochemically inert. Legionella grow at temperatures ranging from 20 ° C to 42 ° C. L- cystine , an amino acid, is essential for their growth.  L. pneumophila has a minimum of 16 different serogroups. L. pneumophila serogroup 1 was identified as the causative organism for the Philadelphia outbreak in 1976.

Ecology

They are found in various aquatic environments such as lakes, ponds, swimming pools and so forth, and are capable of surviving temperatures ranging from 5° C to 42 ° C. Warm water (25° C to 40 ° C) supports the highest concentration of the organism. The bacilli are found to survive with free living ameba such as Naeglaria, Acanthamoeba and Hartmanella.

Spectrum of disease caused by Legionella pneumophila

Legionellosis includes all infections caused by Legionella including Legionnaire’s disease, Pontiac fever and focal extra pulmonary lesions.

  • Legionnaire’s disease: The predominant symptoms of this syndrome consist of fever with dissociation between temperature and pulse, myalgia, non-productive cough, diarrhea and confusion. A ‘flu-like’ prodromal illness characterized by headache, myalgia, asthenia and anorexia may be present for 3 to 4 days. Typical laboratory findings include hyponatremia, hypophosphatemia and increase in the liver specific enzymes. Myoglobinuria is a common finding in urine detected as a positive dipstick test for ‘blood’ with the absence of RBCs in the urine. The nonspecific nature of the symptoms makes diagnosis difficult therefore mandating empiric therapy for Legionella in community acquired pneumonias.
  • Extra pulmonary infections: These are extremely rare and are likely to occur as metastatic complications of pulmonary disease in immunocompromised patients. Disseminated abscesses in various organs such as the liver, spleen, bone marrow, lymph nodes, skeletal and cardiac muscle have been observed. Rarely, primary extrapulmonary lesions have been seen involving prosthetic heart valves or respiratory sinuses. 
  • Pontiac fever is a mild self-limiting febrile illness often diagnosed during an outbreak. Secondary attack rates are as high as 80% to 90%. The source is frequently contaminated aerosol from a water source such as a cooling tower at one’s workplace or in recreational parks. The symptoms persist for approximately 3 to 5 days and then subside.

Treatment

Since L. pneumophila is an intracellular organism, therapy must be directed so that the antimicrobial will achieve a high concentration intracellularly. Drugs such as betalactams, amino glycosides, monobactums and phenicols are not active against this pathogen due to their poor intracellular action. Conversely, macrolides, quinolones and tetracyline are effective intracellularly and therefore therapeutic. For mild pneumonia in an immunocompetent patient, oral therapy should be given. Macrolides and dosages are as follows:

  • Erythromycin 500 mg QID for 14 to 21 days
  • azithromycin 500mg OD X 3 to 5 days 
  • fluoroquinolones such as levofloxacin 500 mg OD for 7 to 10 days 
  • ciprofloxacin 500 mg BD for 7 to 10 
  • gaitifloxacin 400 mg BD for 7 to 10 days 
  • clarithromycin 500mg OD for 14 to 21 days

In hospitalized patients or those who are immunocompromised, a quinolone or long acting macrolide is the drug of choice administered intravenous for 14 to 21 days. Rifampin 200mg to 600mg BD administered for 3 to 5 days can be used in combination with erythromycin.

Prevention

  • Immunization: To date, no human vaccines are available for the prevention of Legionnaire’s disease. Also, with a large number of serotypes being present, prior infection does not prevent repeat infections. Nevertheless, animal studies have shown that a vaccine comprised of L. pneumophila antigens protects against lethal complications of the disease.
  • Chemoprophylaxis: A macrolide antibiotic can be used for chemoprophylaxis. This is especially useful in immunocompromised patients during nosocomial epidemics of the disease. Chemoprophylaxis can be administered to individuals in high risk groups prior to control of an epidemic.
  • Structural changes and modifications: Building and plumbing designs should be constructed so that water contamination is prevented. Hot water pipes placed next to cold water pipes must be properly insulated to maintain their respective temperatures. Pipe bends leading to stagnation of water should be avoided as well as the use of water holding tanks and plumbing materials which will support the growth of Legionellae. Hot water temperatures must be kept above 50° C while cold water temperatures must be kept below 20° C. To prevent scalding burns from occurring, thermostat-controlled mixing valves may be utilized. Instead of using chlorine, monochloramine can be used for treating public drinking water. Recreational spas must also take proper precautions such as complete emptying of water at regular intervals, hyper- chlorination, etc. to prevent colonization of the water.
  • Air conditioning systems: Colonization of air conditioning cooling towers can be prevented by placing towers away from the building and downwind of the air inlet. Drift eliminators can also be installed. Regular cleaning and maintenance of the cooling towers assists in prevention of colonization. Quantitative cultures must be performed at regular intervals to assess colonization of the cooling towers.
  • Environmental surveillance: There is no international consensus on environmental surveillance for Legionella. This is because a large majority of these organisms persist in our aquatic environment and do not cause human infections. Yet, a reasonable precaution would be to do monthly surveillance of the hospital potable water which supplying the immunocompromised wards. This must be done to specifically test for L.pneumophila which is the most critical pathogen in this group. Negative cultures do not rule out the possibility of Legionnaire’s disease, but make it less likely to occur. Surveillance also becomes important in settings where there is suspicion of an outbreak and to monitor the effectiveness of the treatment of cooling towers.