Today’s blog post is about the Harvard Airborne Infection Control Course and the high profile Legionella outbreak in the Bronx. At the Airborne course yesterday, we were in the laboratory at Northeastern University doing experiments like measuring velocities in ducts and testing the efficiency of HEPA filters. It was fun and gave me a new understanding of the complexity of designing safe and effective ventilation systems in healthcare facilities. Although the majority of the Airborne course has focused on TB, we also had a great lecture by Dr. Al Demaria from the Massachusetts Department of Public Health about the airborne transmission of influenza and other pathogens (measles, smallpox, anthrax, etc). I have a personal as well as professional interest in the transmission of influenza, as I think I caught the flu in the hospital last winter.
What I enjoyed the most about Dr. Demaria was his historical perspective on influenza transmission. Dr. Demaria lectured us about two important studies, the historic 1961 VA Hospital Livermore Study and a 1979 Alaska airplane study. It’s worth knowing that influenza is usually thought to be transmitted by large droplets which will typically fall to the ground within 3 meters of the infected person and be expected to infect individuals in direct contact, or via surfaces. Airborne transmission (small particle aerosols, < 10 µm in mass diameter) also could play a role. The extent of airborne transmission is uncertain, however, and would be important in the event of novel pandemic influenza virus with high pathogenicity and sustained human-to-human transmission.
In the Livermore study, UVGI was installed in one VA building and not in another (see figure below), and the influenza attack rate was much lower where UVGI had been installed. This raised the possibility of airborne transmission. The Alaska study was of a commercial airliner that was delayed on the tarmac in Homer, Alaska for 4.5 hours with a poorly functioning ventilation system. The risk for transmission of flu was related to the amount of time passengers spent on the plane and not on their proximity to the index case, and airborne transmission was thought to be likely. However, some people later rationalized that the Livermore and Alaska studies were isolated events and had not been replicated. There were also doubts that the passengers on the Alaska airplane hadn’t walked around and touched each other, thereby spreading flu by droplet/contact instead of the airborne route. The lack of attention to the Livermore/Alaska studies affected the influenza infection control guidelines released by the government many years later. As we learn more about the architectural/ engineering approaches to reducing TB transmission in healthcare facilities, we also must contemplate other pathogens such as influenza, measles, and even norovirus which may be transmitted by the airborne route.
I also wanted to write a bit about the Legionella outbreak in the Bronx. I am certainly no expert, as I’ve only taken care of two patients with Legionella in my career and have never done any research in the field. But I do know that Legionnaires’ disease is a cause of severe pneumonia and Legionnaires’ cannot be differentiated from typical bacterial pneumonia (i.e. Pneumococcal) by history, exam, or chest x-ray. Therefore, the Legionella urine antigen test must be sent, and it is an imperfect test. It only detects serotype 1 (thankfully the Bronx outbreak was serotype 1) and it has low sensitivity (although it has high specificity, so it’s a good “rule-in” test).
Focusing on the transmission of Legionella, today’s New York Times article described it as an “airborne illness” that should be unpacked a bit. First, the bacteria itself. Legionella lives in natural aqueous environments (i.e. lakes, streams, and oceans), and especially loves warm water. Think about that the next time you are taking a hot shower in that fancy hotel. Interestingly, amebas in the water support the growth and survival of legionella. Legionella can multiply thousands of times within the amebas and these organisms often live together in a slimy biofilm. When the biofilm is disrupted, there can be a sudden and massive release of Legionella bacteria into the water. If this water is then aerosolized, people can get sick.
Many people remember that the 1976 Legionnaires’ disease outbreak at the Bellevue-Stratford Hotel in Philadelphia was due to a cooling tower. After inhalation of an infectious aerosol +/- microaspiration of Legionella into the lungs, a fraction of people get sick. Usually, the people who get sick are in closer proximity to that cooling tower, water spa, water fountain, or water mister. The duration of exposure and presence in an area downstream of the contaminated device are also risk factors. (Importantly, Legionella is not spread person-to-person, so there will never be an explosive pandemic of Legionella around the world like there can be with influenza).
What about the description of Legionella as “airborne?” It’s different than TB or measles because legionella depends on the interaction of air and water whereas TB/measles are spread in the air alone. Lets take the example of a water cooling tower (see below). The tower has a pipe providing incoming water from the building’s condenser, air intakes for fans, and demisters. Aerosolized water enters the building through fresh air intakes. In one legionella outbreak in Ohio, the biocide system of the cooling tower reservoir likely malfunctioned, leading to legionella overgrowth and transmission of aerosols. We have created this problem ourselves with modern plumbing.
Finally, a provocative idea from Prof David Fisman, an ID doctor from Toronto. Fisman argued that the ongoing rise in legionellosis in the US could be at least partly due to climate change. That could be the case. A recent article from Portugal described a large Legionnaire’s outbreak. Meteorological data indicated that winds, humidity, and a cloud of sand and dust lifted by a storm in the Sahara desert covered Portugal and increased airborne concentrations of small particulate matter and may have contributed to Legionella’s spread.