1) In my opinion, sporadic cases of Legionnaires’ disease [LD] probably come from exposure to airborne droplets containing Legionella pneumophila from cooling tower drift during warm days when the relative humidity is equal to or greater than 80% – particularly during periods of heavy rainfall.
2) If we were going to design a piece of mechanical equipment that would perfectly simulate the aerobic, natural freshwater environment for Legionella growth, cooling towers would be the result.
3) Legionella bacteria are aquatic freshwater, obligate aerobes that live at the air-water interface in freshwater lakes, streams, ponds, and in water-containing mechanical equipment open to the environment, such as cooling towers, evaporative condensers, and fountains.
4) Legionella bacteria die in stagnent water plumbing lines because of low oxygen levels. Legionella bacteria can only survive these low oxygen conditions by colonizing and living within aquatic amoeba and other related protozoan microorganisms. Legionella bacteria are facultative intracellular parasites. That allows them to live inside some cells [similar to the way that Coxiella bacteria grow inside cells and cause the disease Q-Fever].
5) The reason that Legionella bacteria can multiply in human lung air sac alveolar macrophages is because they provide the growth conditions that approximate the natural freshwater amoeba hosts for Legionella [Ines G. Goncalves et al. September 01, 2021. Microbe of the Month. Vol.29. Issue 9. Pages 860-861].
6) When Legionella grow inside lung air sac alveolar macrophages, they cause tissue damage. The body mounts a response to Legionella presence with an inflammatory, thrombotic, and fibrinolytic response resulting in pneumonia with the formation of large quantities of fluid in the lungs, similar to drowning [2001, New England Journal of Medicine, Vol. 344:700].
7) The U.S. Department of Commerce, National Atmospheric Administration, National Environmental Satellite Data and Information Service provides local climatological data monthly and daily summaries for most areas of the country. National Centers for Environmental Information data provided for a particular city or area will show when warm temperatures, high humidity, and heavy rainfall occur. This information can be used to correlate with community-acquired Legionnaires’ disease cases in that area. We used this information to correlate weather and Legionella counts during an investigation of a cooling tower that was published in 1985.
8) There are more community-acquired Legionnaires’ disease cases during warm, humid, heavy rainy weather: “LD is more likely to occur in warm (60°–80°F) and very humid (>80.0%) months. For example, the odds of LD being diagnosed in a pneumonia patient during a month when the rainfall is <18 mm and the temperature is 60°–80°F was 3.1 times higher when the relative humidity was >80.0% than when it was <50.0%. When rainfall amounts were greater, the risk also increased; however, regardless of rainfall, warm and humid weather was a major risk factor. Also, we found a dose-response relationship between relative humidity and the odds of an LD diagnosis during periods of warm weather. In contrast, hot, cool, or dry weather patterns produce no meaningful increase in LD.” [“Weather-Dependent Risk for Legionnaires’ Disease, United States.” Jacob E. Simmering, et al., Emerging Infectious Diseases, November 2017, Vol. 23, Number 11, Pages 1843–1851].
9) Cooling towers transmit Legionella greater than 1-mile. Cooling towers and evaporative condensers produce large volumes of droplet aerosols. Cooling towers have been found to be the sources of community outbreaks in a number of published investigations. For example, “Community Outbreak of Legionnaires’ Disease: An Investigation Confirming the Potential for Cooling Towers to Transmit Legionella Species.” David W. Keller, et al. Clinical Infectious Diseases, 1996, Vol. 22, Pages 257-61 © 1996 by The University of Chicago.
10) Google Scholar search words |cooling tower legionella transmission distances| produces several peer-reviewed papers documenting large distance transmission. Such as, “Community-acquired Legionnaires’ disease associated with a cooling tower: Evidence for longer-distance transport of Legionella pneumophila.” David G. Addiss et al. September 1989, American Journal of Epidemiology, Vol. 130, Issue 3, Pages 557–568.
11) Why are today’s Legionella testing methods often unable to detect Legionella in cooling tower water? Today’s Legionella testing methods are based on research with potable water supplies. We need environmental microbiology researchers, not engineers, to develop useful Legionella tests of cooling tower water that contains heavy metals, debris, etc.
12) Forty-five-years after the 1976 Philadelphia cooling tower outbreak, we still do not have adequate testing methods to locate sources of airborne droplets containing Legionella bacteria emerging from water-containing mechanical equipment such as cooling towers.
13) The current agar culture and genetic marker qPCR testing methods usually find Legionella in potable water samples, but these methods become inadequate when testing water samples from water-containing mechanical equipment exposed to the environment. The agar tests often produce false negative Legionella test results because many species of microorganisms in the water will either inhibit the growth of Legionella colonies, or the other microorganisms will overgrow the slower-growing Legionella colonies. The qPCR genetic tests are inadequate because the organic and inorganic debris found in cooling tower samples interferes with PCR gene testing by producing high background noise levels so great that the Legionella genetic markers sometimes cannot be detected.