Seasonality can have a significant effect on the proliferation and virulence of Legionella bacteria. Warmer temperatures, humidity and increased rainfall – which are common during Spring - have all been associated with an increase in the growth and spread of Legionella bacteria, and an increase in Legionnaires’ disease risk.
A study carried out in the Netherlands recently confirmed that a record number of cases of Legionnaires’ disease, which occurred in 2010, was directly linked to seasonality. The two factors which caused the spike were increase in temperature and intense rainfall1. The investigative team proposed that there had been a period of amplified Legionella growth, supported by the warm weather, followed by a period of Legionella spread over the wet weeks1. Another study investigating a spike in cases of Legionnaires’ disease in the Mid-Atlantic region of the US in 2003 had similar results. The study found that the increased number of cases was associated with both increased rainfall and temperature2.At 40 degrees Celsius, Legionella are at peak replication but temperatures ranging anywhere between 20 and 45 degrees Celsius can provide Legionella with optimal conditions for rapid growth3. CDC states that in warm climates, cold water pipes can be compromised and may reach temperatures that encourage Legionella to grow4. Increase in temperature also has an effect on Legionella virulence (virulence is a pathogen's or microbe's ability to infect or damage a host). In other words, increased temperature has been found to increase Legionella’s ability to infect human lung cells5. A study investigating the effects of temperature on virulence in Legionella found that Legionella grown at 37°C was significantly more virulent than those grown at 24°C5. The study concluded that the presence of heat, due to hot weather, warm water or evaporation processes (as used in cooling towers) could actually prime Legionella for infection in humans5.
When temperatures rise, air in the atmosphere holds more water vapor, which can lead to more drizzle, rain, sleet, snow and hail7. This can encourage the growth and spread of Legionella bacteria in soil, puddles and surface water in the natural environment, as well as outdoor structures such as cooling towers7. Researches argue that rainfall might affect exposure to Legionella, via a range of potential mechanisms. Contamination of recirculated drinking water is one possibility8. Another suggestion is that vehicles might produce aerosols containing Legionella, as they drive on wet road surfaces9. The general public can then get infected by inhaling these aerosols.
Humidity and dew have also been associated with increased risk of Legionellosis and Legionnaires’ disease. Moisture in the air may allow Legionella to remain suspended in aerosolized droplets for longer time periods, possibly increasing exposure to bacteria via inhalation7.
In sum, seasonal changes that start in Spring can encourage the growth and spread of Legionella bacteria and increase the risk of contracting Legionellosis or Legionnaires’ disease. This is an important finding for water management professionals and responsible persons, who should be aware that controls could be compromised by increased heat and rainfall, and that additional measures should be taken to protect vulnerable people. Individuals who manage systems in high-risk industries, such as cooling towers and hospitals, should consider testing their systems more frequently over Spring and Summer. Rapid on-site testing methods are a great tool to mitigate seasonal risk and can provide immediate results with little to no training.
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