Legionnaires’ Disease: Recent Updates and Insights (2025)

Legionnaires’ disease, a severe form of pneumonia caused by Legionella bacteria, continues to pose significant public health challenges worldwide. First identified in 1976 during an outbreak at an American Legion convention in Philadelphia, this disease is contracted by inhaling aerosolized water droplets containing Legionella bacteria, often found in warm water systems like cooling towers, hot tubs, and plumbing systems. Despite not being contagious among individuals, it disproportionately affects older adults, smokers, and those with weakened immune systems or chronic conditions. This article synthesizes recent updates on outbreaks, epidemiology, prevention, and treatment, drawing from global reports and scientific insights as of August 2025.

Recent Outbreaks

Legionnaires’ disease outbreaks have garnered significant attention due to their potential severity and public health implications. Below are key updates from 2023 to 2025:

• New York City, USA (July–August 2025): A significant outbreak in Central Harlem sickened 58 people and resulted in two deaths. Health officials linked the cases to cooling towers testing positive for Legionella pneumophila. The New York City Health Department emphasized that the disease is not spread through drinking water or person-to-person contact, and urged residents, particularly those over 50, smokers, or with chronic lung conditions, to seek medical attention for flu-like symptoms. Building owners were directed to disinfect cooling towers within 24 hours of positive tests. This outbreak underscores the ongoing challenge of maintaining water systems in urban environments.
• Melbourne, Australia (July–August 2024): Victoria reported its worst outbreak in over two decades, with 71 confirmed cases, seven suspected cases, and one death—a woman in her 90s. Most cases involved adults over 40, with many requiring hospitalization and intensive care. Authorities identified a cooling tower in Derrimut and Laverton North as the likely source, which was disinfected promptly. The outbreak was attributed to Legionella pneumophila serogroup 1, detectable via urinary antigen testing. Recent weather patterns may have contributed to the wider spread of the bacteria.
• Poland (August–September 2023): A community outbreak in the Rzeszow area affected 166 individuals, resulting in 23 deaths. This significant event highlighted the risks in community settings and the importance of rapid identification and control measures.
• New Jersey, USA (January–August 2023): Two separate outbreaks occurred in Passaic and Bergen Counties (9 cases) and Middlesex and Union Counties (41 cases), with no reported deaths. These incidents emphasize the recurring nature of Legionella contamination in community water systems.
• Minnesota, USA (April 2023–February 2024): Fourteen cases were linked to the municipal water supply in Grand Rapids, Minnesota, highlighting the risks posed by aging water infrastructure.
• Sydney, Australia (January 2024): NSW Health issued an alert for Legionnaires’ disease in Sydney’s CBD, reflecting ongoing vigilance for travel-associated and urban cases.

These outbreaks illustrate that Legionella remains a persistent threat, particularly in urban areas with complex water systems. Cooling towers are frequently implicated, as they aerosolize water, facilitating bacterial transmission.

Epidemiology and Trends

The incidence of Legionnaires’ disease has been rising globally, with no single clear cause. In the United States, reported cases of Legionnaires’ disease have increased nearly 900% from 2000 to 2018, with an estimated 6,000 to 18,000 cases occurring annually; however, underdiagnosis is likely due to the disease’s similarity to other types of pneumonia. In 2021, the European Union/European Economic Area (EU/EEA) reported over 10,700 cases and 704 deaths, a significant surge attributed to an aging population, improved diagnostics (e.g., urine antigen testing and PCR), and changes in water system maintenance. Travel-associated cases also rose by 38% in 2021 compared to 2020, likely due to the lifting of COVID-19 restrictions.

The disease primarily affects individuals over 50, with 75–80% of cases in this age group, and men are disproportionately impacted (60–70% of cases). Other risk factors include smoking, chronic lung disease, immunosuppression, and chronic conditions like diabetes or kidney disease. Approximately 2–9% of community-acquired pneumonia cases are attributed to Legionella, and it ranks among the top three causes of pneumonia requiring ICU admission.

Symptoms and Diagnosis

Legionnaires’ disease typically presents with flu-like symptoms 2–10 days after exposure, including:

• High fever (often ≥104°F/40°C)
• Chills
• Dry cough or cough with sputum (sometimes bloody)
• Muscle aches
• Headaches
• Fatigue
• Gastrointestinal symptoms (nausea, vomiting, diarrhea in up to 50% of cases)
• Neurological symptoms (confusion, impaired cognition in nearly half of cases)

Severe cases may lead to respiratory failure, septic shock, or multi-organ failure, with mortality rates ranging from 4–40%, depending on the setting (community-acquired vs. nosocomial) and patient health status. Diagnosis relies on clinical findings (e.g., chest X-rays showing pneumonia) and laboratory tests, including urine antigen testing for Legionella pneumophila serogroup 1, culture, or PCR. However, non-serogroup 1 strains may be missed by standard urine tests, potentially underestimating incidence.

Pontiac fever, a milder illness caused by Legionella, presents with flu-like symptoms without pneumonia and resolves within 2–5 days without treatment.

Prevention and Control

Preventing Legionnaires’ disease hinges on maintaining water systems to minimize Legionella growth. Key measures include:

• Regular maintenance: Cleaning and disinfecting cooling towers, hot tubs, and plumbing systems. Hot water systems should be kept above 50°C, and cold water below 20°C to inhibit bacterial growth.
• Chlorination: Disinfecting water systems with high chlorine levels (e.g., 50 mg/L for 2–4 hours) after maintenance or during outbreaks.
• Water flow management: Ensuring proper circulation to prevent stagnant water, a breeding ground for Legionella.
• Building regulations: In New York City, cooling tower owners must register and routinely test for Legionella. Similar regulations exist in other regions.

Individuals can reduce risk by draining garden hoses, maintaining water filters, and flushing hot water heaters biannually. There is no vaccine for Legionnaires’ disease, making environmental controls critical.

Treatment

Early antibiotic treatment is essential for Legionnaires’ disease. Commonly prescribed antibiotics include macrolides (e.g., azithromycin) or fluoroquinolones (e.g., levofloxacin). Most patients recover with treatment, though severe cases may require hospitalization and oxygen therapy. About 1 in 10 patients may die, particularly those with comorbidities or delayed treatment. Recovery can take weeks to months, with some experiencing lingering fatigue.

Challenges and Future Directions

Multiple factors likely contribute to the rising incidence of Legionnaires’ disease.

• Aging infrastructure: Older water systems are more susceptible to Legionella growth.
• Climate change: Warmer water temperatures and changing weather patterns may enhance bacterial proliferation.
• Improved diagnostics: Greater use of urine antigen testing and PCR has increased case detection.
• Population aging: An older population with more comorbidities increases susceptibility.

Challenges remain in diagnosing milder cases and non-serogroup 1 infections, as current testing may miss these. Research into novel diagnostic tools and standardized surveillance systems is ongoing. Additionally, public health efforts focus on stricter regulations for water system maintenance and raising awareness among high-risk groups.

Conclusion

Legionnaires’ disease remains a critical public health concern, with recent outbreaks in New York City, Melbourne, and other regions suggesting that there must be vigilance. While treatable with antibiotics, its high mortality rate in severe cases and increasing incidence points to the need for early diagnosis, robust water system maintenance, and public awareness. High-risk individuals—those over 50, smokers, or with chronic conditions—should promptly seek medical care for flu-like symptoms, especially in outbreak areas. As urban environments and climate conditions evolve, ongoing research and proactive prevention strategies will be essential to curb this preventable yet potentially deadly disease.