Have you ever walked past a large building and felt droplets of water falling on you? This mist is from a cooling tower; or large air-conditioning unit atop the building. What you may not be aware of is that this mist may contain a bacteria known as Legionella; the agent responsible for causing Legionnaires’ Disease.

Recently, there has been an outbreak of Legionnaires’ Disease in New York city and the source of the outbreaks is being linked to 5 cooling towers located in the South Bronx region. Although, cooling towers are one of the main sites where the bacteria can be found; Legionella is ubiquitous in fresh water environments. That is it is found everywhere in the environment — in warm water.

Where Legionella is Commonly Found

Fresh water environments: lakes, ponds, wet soil

Artificial water systems: cooling towers, whirlpool spas, decorative fountains

In lakes and ponds, Legionella are found as parasites of protozoa (single-cell organisms, like the brain-eating amoeba we discussed a few weeks ago); its natural host. Ironically this parasitic characteristics relates to how it causes illness in humans, and why it’s not spread from person-to-person.

The History and Background of Legionella and Legionnaire’s Disease

In 1976 during the American Legion Convention in Philadelphia, there was a sudden outbreak of pneumonia. Now, an outbreak is defined as 2 or more people coming down with the same condition, in the same place, and at the same time. In this case reportedly 182 or more people were affected. At the time, it was unknown what the causative agent or the source of infection was. It was suspected and speculated to be a list of things: a virus, parrot fever, toxic gas, metallic poisoning, swine flu, and even terrorism. At least 29 people died as a result.

The culprit was later discovered to be a previously unrecognized bacterium, later named for the convention in which it caused an outbreak. Legionella, is a gram-negative opportunistic pathogen. It causes two illnesses: Legionnaire’s Disease and Pontiac Fever.

Legionnaires’ Disease is a serious, sometimes fatal, type of pneumonia (lung infection) that like other pneumonia’s causes, high fevers, chills, cough, and shortness of breathe. These symptoms are sometimes accompanied by headaches and muscle aches.

Pontiac Fever is a flu-like, self-limiting illness. It is less severe than Legionnaire’s Disease, causing a mild illness ranging from 2-5 days, and requires no antibiotic treatment.

How Legionella Causes Legionnaire’s Disease

The bacterium is present in mist and when inhaled it travels through the air-ways (nasal cavity and bronchial tree) into the air-space (the lungs). The functional unit of the lungs are known as alveoli and this is where Legionella take up occupancy. Phagocytic immune cells known as macrophages engulf the bacteria, but instead of dying they replicate protected inside of the cell (similar to how they parasitize protozoa). Ultimately, they multiple causing tissue damage and inflammation.

Susceptibility Factors for Legionnaire’s Disease

• Old Age
• Compromised Immune system: Chemotherapy, Organ transplant, HIV
• Smoking
• Pre-existing health Condition

It takes 2 – 10 days  for symptoms to appear, but it could take longer. This accounts for the lag in time of reported illness and realization of cases of outbreaks. There is an 8 – 12% chance of death after infection in people with susceptibility factors. Treatment is with antibiotics. Usually macrolides, fluroquinolones, and cyclins; antibiotics that can enter the hosts cell.

Summary

Although the Legionella bacteria has always been around and is found throughout the environment; Legionnaires’ Disease emerged in the second half of the 20th Century, partly due to the development of artificial water systems. Normally Legionella colonizes protozoa in bodies of fresh water. However, cooling towers, showers, and other devices that aerosolize water have allowed Legionella to gain direct access to the humans. While most healthy people who come in contact with Legionella won’t develop Legionnaires’ Disease or Pontiac Fever, susceptible people who have pre-existing diseases, weakened immune systems, and the elderly are at increased risk. In fact during  the New York City 2015 Outbreak, of the 7 deaths reported, all were amongst the already infirm. Reportedly, in total 86 people have been infected and 64 hospitalized since July 10th.

Legionnaires’ Disease is a relatively uncommon respiratory infection, but it is a significant Public Health disease due to its substantial ability to cause disease and death. According to the Centers for Disease Control (CDC), every year between 8,000 to 18,000 people are hospitalized due to Legionnaire’s Disease in the United States. However, many cases are never properly diagnosed or even reported, and individual incidents fail to make the news. Only in times of serious outbreak does this illness receive media coverage.