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  • Writer's pictureP.K. Peterson

Legionnaires’ Disease: An Unusual Return to Philadelphia

“Nature has the greatest imagination of all.”

- Richard Feynman, American theoretical physicist


“For many of us, water simply flows from a faucet, and we think little about it beyond this point of contact. We have lost a sense of respect for the wild river, for the complex workings of a wetland, for the intricate web of life that water supports.”

- Sandra Postel, director and founder of the Global Water Policy Project




In her September 15, 2023, MedPage Today article titled “Legionnaires’ Disease in Lung Transplant Recipients Likely From Donor,” journalist Michele Sullivan reported on two lung transplant recipients who developed Legionnaires’ disease directly from their transplanted organs. The organ donor had died after drowning in a river near the Philadelphia hospital where the patients received their transplants.

I thought these cases worthy of a Germ Gems post not only because it’s the first time Legionella pneumophila, the bacterium that causes Legionnaires’ disease, has been transmitted by this unusual waterborne route, but also because of the geographic connection these cases have to Philadelphia, the city where Legionnaires’ disease first emerged almost a half century ago.


History of Legionnaire’s disease. In July 1976, members of the Pennsylvania State American Legion, an organization of World War II veterans, gathered in Philadelphia to celebrate our nation’s bicentennial of its separation from Great Britain. A day after the convention began some of the Legionnaires started to become ill with pneumonia-like symptoms. By mid-August, over 200 people had contracted the disease and 34 of them had died.

Medical investigators were at first baffled by the disease. But by December 1976, Dr. Joseph McDade, a laboratory scientist at the Centers for Disease Control and Prevention (CDC), had isolated the bacterium that caused the disease and identified it as Legionella pneumophila—a pathogen now recognized as one of the most common causes of community acquired pneumonia (CAP) in the developed world. (L. pneumophila is responsible for 8,000-18,000 hospitalizations per year in the U.S. alone, with a mortality of about 10%.)


What is Legionnaires’ disease? Legionnaires’ disease is a bacterial infection of the lungs that afflicts and kills more people in the U.S than any other reportable waterborne disease. (I reviewed “waterborne infections” in my March 24, 2021, Germ Gems post, “Water, Water Everywhere, but Is It Safe to Drink?”) L. pneumophila loves fresh water. But unlike most other waterborne pathogens, it is not acquired by drinking contaminated water. Rather the bacterium is inhaled in aerosols that upon reaching the lungs cause pneumonia.

L. pneumophila thrives inside protozoa, such as free-living amoebae associated with microbial biofilms that coat wet surfaces, including pipes of drinking water distribution systems, building plumbing systems, faucets, showerheads, cooling towers, hot tubs, and fountains. And these biofilms are extraordinarily difficult to remove.


Symptoms and Treatment. As Legionnaires’ disease can be severe, prompt diagnosis of the disease and appropriate antibiotic therapy are important. Common symptoms of Legionnaires’ pneumonia include fever, cough, shortness of breath, headaches, muscle aches, loss of appetite, confusion, diarrhea, and nausea. None of these symptoms, however, is specific for Legionnaires’ disease. Fortunately, there is a diagnostic test for Legionnaire’s. The most commonly used test is the urinary antigen test which detects a molecule of L. pneumophila in the urine.


When aerosolized L. pneumophila bacilli enter the lungs and seek an intracellular environment, akin to the amoeba they reside in in nature. Alveolar (lung) macrophages, cells of the immune system that are evolutionarily related to amoeba, fit the bill. So, when selecting appropriate treatment, antibiotics that penetrate alveolar macrophages are given. After almost five decades of experience with L. pneumophilia, such antibiotics are now routinely ordered in the treatment of most cases of CAP.


Scope of the problem and risk factors. The CDC reports that since 2000, the number of cases of Legionnaires’ disease has been on the rise. (On July 19, 2023, the Minnesota Department of Health reported it was investigating five confirmed cases among people who live or spent time in Grand Rapids, Minnesota. As of this writing, the source of this outbreak remains a mystery.) But the U.S. isn’t the only country seeing an increase in cases. According to a July 3, 2023, report from the European Centre for Disease Prevention and Control, increasing rates of Legionnaires’ disease have occurred throughout the EU. Improved diagnostic capabilities are thought to be a factor in detecting infections – thus we are not sure if there are increased rates of infection or increased rates of detection of infection.


Most people exposed to L. pneumophila don’t develop Legionnaires’ disease. People at risk of severe illness are those age 50 years and older and current or former smokers. Other risk factors include chronic health conditions, such as lung, kidney, or liver disease, diabetes, cancer, and conditions and medications that suppress the immune system, such as those given to prevent rejection of their transplanted lung in the two recent transplant recipients in Philadelphia.

What’s next? Stay tuned. When Legionnaires’ disease erupted in 1976, I was finishing my fellowship training in infectious diseases and had no idea then how common such “emerging infections” would become. By some counts, roughly 140 infections have emerged over the course of the following several decades with COVID-19 and monkeypox being the most recent.


All public health authorities agree it’s crucial to be prepared for the next pandemic. From what Legionnaires’ disease taught us, this means be prepared to be surprised. None of the new (emerging) infections was predicted. Also, be prepared to be humbled. None of the emerging pathogens has been eradicated.


Even though the microbes seem to be winning most of the battles, our knowledge of infectious diseases has increased remarkably. And given the highly dedicated and expert staff at public health institutions like the CDC, we have a fighting chance to contain the enemies. But to do so reliably, we must be prepared to increase supportfor these public health institutions.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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