Legionellosis: What You Need to Know About the Disease Caused by Legionella Bacteria

Legionellosis is a term that refers to any infection caused by Legionella bacteria, which can cause a range of respiratory illnesses, from mild flu-like symptoms to severe pneumonia and death. Legionella bacteria are found naturally in freshwater environments, such as lakes, rivers, and streams, but they can also grow and multiply in man-made water systems, such as cooling towers, air conditioners, hot tubs, and showers. Legionellosis can be transmitted by inhaling or aspirating contaminated water droplets or aerosols, or by contact with infected animals or their products.

In this article, we will explain what Legionellosis is, how it is transmitted, how it is diagnosed and treated, and how to prevent and control it.

What is Legionellosis and what causes it?

Legionellosis is an infectious disease caused by Legionella bacteria, which belong to the family Legionellaceae. There are more than 60 species and 70 serogroups of Legionella bacteria, but the most common and virulent one is Legionella pneumophila, which causes 90% of human cases of Legionellosis. Legionella pneumophila has 16 subtypes, but the most prevalent one is subtype 1, which accounts for 84% of human cases.

Legionella bacteria were first identified in 1976, after an outbreak of pneumonia among people who attended an American Legion convention in Philadelphia, USA. The disease was named Legionnaires’ disease, and the bacteria were named Legionella. Since then, Legionella bacteria have been associated with sporadic cases and outbreaks of respiratory illnesses worldwide, especially in industrialized countries.

Legionellosis can manifest in two different forms: Legionnaires’ disease and Pontiac fever. Legionnaires’ disease is a severe form of pneumonia that can affect the lungs, the heart, the kidneys, and the brain. It can cause fever, chills, cough, shortness of breath, chest pain, headache, muscle ache, and gastrointestinal symptoms, such as nausea, vomiting, and diarrhea. It can also cause complications, such as respiratory failure, septic shock, acute kidney injury, and endocarditis. Legionnaires’ disease has a mortality rate of 10% in the general population, and up to 25% in high-risk groups, such as the elderly, smokers, immunocompromised people, and people with chronic diseases.

Pontiac fever is a milder form of Legionellosis that does not affect the lungs. It causes flu-like symptoms, such as fever, headache, muscle ache, and fatigue, but no pneumonia. It usually resolves within a week without treatment. Pontiac fever has a low mortality rate, but it can affect a large number of people during outbreaks.

How is Legionellosis transmitted?

Legionellosis can be transmitted in several ways:

• Through inhalation of contaminated water droplets or aerosols. This is the main mode of transmission of Legionellosis, as Legionella bacteria can survive and multiply in warm water (20-50°C) and form biofilms in water systems. When these water systems are disturbed or aerosolized, such as by showering, flushing, or spraying, they can release water droplets or aerosols that contain Legionella bacteria. These droplets or aerosols can be inhaled by people who are nearby, and reach their lungs, where they can cause infection.
• Through aspiration of contaminated water or ice. This can happen when people choke or gag on water or ice that is contaminated with Legionella bacteria, and accidentally inhale it into their lungs. This can occur in hospitals, nursing homes, or other settings where people have difficulty swallowing or have medical devices, such as tubes or ventilators, that bypass their natural defenses.
• Through contact with infected animals or their products. This can happen when people handle, slaughter, or consume animals that are infected with Legionella bacteria, such as cattle, sheep, pigs, or poultry. This can also happen when people are bitten or scratched by animals that are infected with Legionella bacteria, such as dogs, cats, or rodents. This mode of transmission is rare, but it has been reported in some countries, such as China, Thailand, and Australia.

The risk of transmission of Legionellosis depends on several factors, such as the concentration and virulence of Legionella bacteria in the water source, the production and dissemination of water droplets or aerosols, the exposure time and distance, the environmental conditions and season, and the host factors, such as age, health status, and immune response.

The incubation period of Legionellosis, which is the time between exposure and onset of symptoms, ranges from 2 to 14 days, with an average of 6 days for Legionnaires’ disease, and 1 to 2 days for Pontiac fever. People are infectious from the onset of symptoms until they are treated with antibiotics, but they cannot transmit the disease from person to person, unless they share contaminated water or ice.

How is Legionellosis diagnosed and treated?

Legionellosis can be diagnosed by clinical examination and laboratory testing. The clinical signs of Legionellosis include:

• Fever, chills, or sweating
• Cough, sore throat, or runny nose
• Shortness of breath or difficulty breathing
• Loss of appetite or weight
• Fatigue, headache, or muscle ache
• Nausea, vomiting, or diarrhea
• Confusion, altered mental status, or seizures

The symptoms of Legionellosis can vary from mild to severe, and some people may have no symptoms at all. The symptoms can also overlap with other common illnesses, such as the flu, the common cold, or COVID-19.

To confirm the diagnosis of Legionellosis, laboratory testing is needed. The preferred method is to detect the Legionella antigen in a urine sample, using a rapid immunochromatographic test. This test is simple, fast, and sensitive, but it can only detect Legionella pneumophila subtype 1, which is the most common cause of Legionellosis. Other methods, such as culture, PCR, or serology, can detect other types and subtypes of Legionella bacteria, but they are more complex, time-consuming, or less specific.

There is no specific treatment for Legionellosis, and most people recover without complications. The aim of treatment is to manage the symptoms and prevent secondary infections or complications, such as sepsis, organ failure, or long-term sequelae. The treatment options include:

• Antibiotics, such as macrolides (e.g., azithromycin), fluoroquinolones (e.g., levofloxacin), or tetracyclines (e.g., doxycycline), to kill or inhibit the growth of Legionella bacteria. Antibiotics should be started as soon as possible after diagnosis, and continued for 7 to 14 days, depending on the severity of the disease and the response to treatment.
• Supportive care, such as fluids, oxygen, painkillers, or anti-inflammatories, to relieve the symptoms and improve the general condition of the patient. Supportive care may also include intensive care, such as mechanical ventilation or dialysis, in case of respiratory or renal failure.

No vaccine is available to prevent Legionellosis, and immunity after infection is not complete or long-lasting. Therefore, prevention and control depend on good maintenance and disinfection of water systems, and prompt identification and management of cases and outbreaks.

How to prevent and control Legionellosis?

The best way to prevent and control Legionellosis is to follow the public health measures recommended by the WHO and the local authorities. These include:

• Maintaining and disinfecting water systems regularly, to prevent the growth and spread of Legionella bacteria. This involves keeping the water temperature below 20°C or above 60°C, cleaning and replacing filters and pipes, adding chlorine or other biocides, and monitoring the water quality and microbiology.
• Avoiding or limiting exposure to water sources that may be contaminated with Legionella bacteria, such as showers, hot tubs, fountains, or humidifiers. This involves using low-pressure water flow, avoiding aerosol generation, wearing protective equipment, and following hygiene and safety rules.
• Reporting any suspected cases or outbreaks of Legionellosis to the local health authorities, and cooperating with the investigation and response activities. This involves collecting and testing clinical and environmental samples, tracing and testing contacts, and implementing control measures, such as isolation, treatment, or closure.

Conclusion

Legionellosis is a serious and potentially fatal disease that can affect anyone who is exposed to Legionella bacteria, which are found in freshwater and man-made water systems. Legionellosis can cause a range of respiratory illnesses, from mild flu-like symptoms to severe pneumonia and death. Legionellosis can be transmitted by inhaling or aspirating contaminated water droplets or aerosols, or by contact with infected animals or their products. Legionellosis can be diagnosed by clinical examination and laboratory testing, and treated by antibiotics and supportive care. Legionellosis can be prevented and controlled by maintaining and disinfecting water systems, avoiding or limiting exposure to water sources, and reporting any suspected cases or outbreaks of Legionellosis to the local health authorities, and cooperating with the investigation and response activities. This involves collecting and testing clinical and environmental samples, tracing and testing contacts, and implementing control measures, such as isolation, treatment, or closure.

Summary

Legionellosis is a term that refers to any infection caused by Legionella bacteria, which can cause a range of respiratory illnesses, from mild flu-like symptoms to severe pneumonia and death. Legionella bacteria are found naturally in freshwater environments, but they can also grow and multiply in man-made water systems, such as cooling towers, air conditioners, hot tubs, and showers. Legionellosis can be transmitted by inhaling or aspirating contaminated water droplets or aerosols, or by contact with infected animals or their products. Legionellosis can be diagnosed by clinical examination and laboratory testing, and treated by antibiotics and supportive care. Legionellosis can be prevented and controlled by following the public health measures, such as maintaining and disinfecting water systems, avoiding or limiting exposure to water sources, and reporting any suspected cases or outbreaks of Legionellosis to the local health authorities, and cooperating with the investigation and response activities.