Legionella

Legionella

Legionella and pseudomonas aeruginosa are bacteria invading heating and air conditioning systems, cooling towers, evaporative condensers and water heaters, causing nosocomial infections.

Both Legionella and Pseudomonas aeruginosa are opportunistic pathogens, meaning that they exploit a break in the host defenses to initiate infections. You are probably very aware of Legionella but Pseudomonas although not new is having resurgence in air conditioner evaporator coils in the hospital environment.
                 

Diseases Caused by Legionella

Legionella Bacteria, or Legionellosis, causes two diseases: Legionnaires’ disease and Pontiac Fever.

Of the two, Legionnaire’s disease is the more serious condition, causing a multi-system disease, including pneumonia. When outbreaks occur, usually fewer than 5% of exposed individuals develop disease, usually 3-9 days after exposure. About 15% of cases are fatal.

Pontiac fever is a non-fatal flu-like disease of short duration, which does not cause pneumonia. Approximately 95% of exposed individuals develop the disease usually within 2 to 3 days. The number of cases of Legionnaires’ disease occurring in the United States each year has been estimated by the Centers for Disease Control and Prevention (CDC) at 10,000 to more than 100,000 cases per year.
                   

What is Pseudomonas aeruginosa?

Pseudomonas aeruginosa is an opportunistic pathogen, meaning that it exploits some break in the host defenses to initiate an infection.

In fact, Pseudomonas aeruginosa is the epitome of an opportunistic pathogen of humans. The bacterium almost never infects uncompromised tissues, yet there is hardly any tissue that it cannot infect if the tissue defenses are compromised in some manner. It causes urinary tract infections, respiratory system infections, dermatitis, soft tissue infections, bacteremia, bone and joint infections, gastrointestinal infections and a variety of systemic infections, particularly in patients with severe burns and in cancer and AIDS patients who are immunosuppressed. Pseudomonas aeruginosa infection is a serious problem in patients hospitalized with cancer, cystic fibrosis, and burns. The case fatality rate in these patients is near 50 percent.

Pseudomonas aeruginosa is primarily a nosocomial pathogen. According to the CDC, the overall incidence of P. aeruginosa infections in U.S. hospitals averages about 0.4 percent (4 per 1000 discharges), and the bacterium is the fourth most commonly-isolated nosocomial pathogen accounting for 10.1 percent of all hospital-acquired infections.
              

What are the risk factors for an infection?

To cause disease, several factors must occur: the organism must be virulent, it must be in sufficient number to cause disease, the water source must be aerosolized and distributed to the human host, the Legionella must be inhaled by the potential host deeply into the lungs, and the human host’s defenses must be unable to stop the infection.

The infectious dose has not been determined but the larger the dose, the more likely an infection will occur. The risk of infection will be greater if the dose of Legionella-containing waste is in direct close contact with the target person (as is the case with humidifiers and foggers) than if the water is distant from the target person (as with cooling towers and evaporative condensers. Potable water systems may represent an intermediate category.

The risk of infection is greater and a lower dose is required in those individuals who are older, smokers, heavy drinkers, immunocompromised with other disease or on immunosuppressive therapy.
               

What are the sources of Legionella bacteria? 

Water in many natural or man-made systems provides suitable conditions for ample growth of Legionella bacteria.

Potential sources include cooling towers evaporative condensers, humidifiers, potable water heaters and holding tanks, pipes containing stagnant warm water, shower heads, faucet aerators, decorative fountains, nebulizers, misters reservoirs, and whirlpool baths.

Legionella apparently survives in low numbers in routine water treatment used to treat potable water and can be carried in the treated drinking water into building, where the bacteria can colonize in the plumbing fixtures, especially in hot water systems.

Cooling towers and other systems may become contaminated through the make-up water. Well-maintained systems are less likely to be colonized with Legionella than systems that are poorly maintained. Continued vigilance in terms of excellent preventive maintenance and an excellent water treatment program are required to minimize the risk of Legionella.