Late last year, multiple news outlets covered an outbreak of a serious infectious disease traced to Disneyland. No, it wasn’t another cluster of measles cases, or even of pixie dust-induced lung cancer. This time it was an even deadlier infection, and thankfully one that is much more difficult to catch, known as Legionnaires’ disease.

A total of 15 cases of Legionnaires’ disease were diagnosed during the outbreak, with 11 of the 15 having visited Disneyland from late August through October. The other four cases occurred in people who had been in Anaheim during this window but who had not visited the park. Ten hospitalizations and two deaths occurred, both people who had not been to Disneyland. As would be expected with this condition, the infected skewed older, with the youngest being 52.

According to Disney Parks and Resorts Chief Medical Officer Pamela Hymel, Disneyland was notified of the cluster of cases on October 27th and promptly began investigating their facility. Disneyland has 18 cooling towers in use at the park, and because these types of systems are notorious breeding grounds for the bacteria that causes Legionnaires’ disease they were the focus of testing. Disney found that two of them were contaminated with the pathogenic bacteria.

The two large cooling towers, which could have showered up to a 3 mile area with the potentially deadly bacteria, were shut down immediately and treated. Repeat testing in early November was negative for the bacteria but health officials requested that the towers be kept out of use for a while longer. No follow-up information is available online regarding the outbreak, so it is probably safe to assume that no additional cases have occurred, at least not in Anaheim. Unfortunately, outbreaks are common and, as I’ll get to shortly, appear to be on the rise.

What is Legionnaires’ disease?

Legionnaires’ disease is primarily thought of as a form of community and hospital-acquired pneumonia, although it can present with significant gastrointestinal and neurologic abnormalities. It is caused by infection with Legionella bacteria, named so because it was first identified during a 1976 outbreak of disease at an American Legion convention. Since then, it has increasingly been recognized as one of the more common causes of pneumonia in adults, being diagnosed in about 4% of cases, particularly if they smoke, have chronic lung disease, have had a solid organ transplant, and/or have had significant exposure to contaminated water.

Infection with Legionella is very rare in children. This is because they tend to not smoke but also because healthcare providers don’t typically look for it in kids like they do in adults. In fact, there has been a trend over the past decade of increasing incidence in adults that is at least partially the result of improved diagnostic testing. Rather than having to wait on a culture for several days, there is now widespread availability of rapid urine antigen testing that is highly sensitive and specific. Many hospitals now have protocols where testing for Legionella is performed in all adults admitted for pneumonia

There are no such protocols in pediatric patients. We would only test for the bacteria if there was very high suspicion, such as when treating an immune compromised transplant patient with pneumonia and severe diarrhea. But empiric treatment of community and hospital-acquired pneumonia often includes antibiotics that treat Legionella as well as more common infectious agents, so many adult (and possibly some pediatric) patients have recovered despite a presumed but incorrect diagnosis of a more common type of pneumonia when they actually had Legionnaires’ disease.

One exception to the rarity of pediatric infection with Legionella is something I’ve actually written about on three separate occasions: water birth. Water birth is the absurd and dangerous practice of delivering a baby into water, often at home and without guidance from a trained medical professional. There have been at least three cases, including one death, involving a newborn infection with Legionella when born into contaminated water.

Legionnaires’ disease is considerably more deadly than other common causes of pneumonia, especially if not treated in a timely manner with appropriate antibiotics. When diagnosed quickly and treatment with the right drug initiated early in the course, still up to 5% of patients will die. Delay in appropriate care results in much higher mortality that starts at 10% and rises with the presence and severity of co-morbid conditions. On average, one out of every four patients who acquire the infection while in the hospital does not survive. To put this in perspective, less than one out of every 1,000 patients diagnosed with measles, and who have access to modern medical care, will die.

Legionella and cooling towers

The natural habitat for the bacteria to blame for Legionnaires’ disease is water. This can include lakes, streams, and other sources of “raw water“, but there is typically not a high enough concentration of the bacteria to cause illness. Much higher numbers of the bacteria can be found in contaminated manmade collections of water, such as hot tubs, grocery store mist machines, fountains, and cooling towers. The key growth factors are higher temperature and stagnation, and cooling towers are full of warm water. Unless properly maintained, large amounts of Legionella can leave these towers in the spray of fine mist emerging from the top of the structure and travel a large distance.

Once aerosolized, the bacteria can be inhaled by those nearby. Legionella has also been spread by contaminated drinking water, hospital water supplies used to flush nasogastric tubes, and even damp potting soil. Spikes in incidence have also been associated with significant increases in rain and humidity. But there has never been a proven case of one person spreading Legionella to another, which is a big part of why, thankfully, this more deadly disease is so much less contagious than measles.

Is Legionnaires’ disease on the rise?

Over the past two decades there has been a steady increase in the reported incidence of Legionnaires’ disease in the United States. From 2000 to 2009 the incidence tripled. More recently, there were increases of roughly 8% and 14% from 2015-2016 and 2016-2017 respectively. Many cities are seeing record numbers.

One reason for the increase is almost certainly testing that has improved in both ease and accuracy. This has led to more facilities having protocols where every adult patient diagnosed with pneumonia is tested for Legionella. This is also why the mortality of the disease is actually much lower now than in historical data because a larger number of milder cases are being picked up. Other potential factors are higher temperatures, more flooding, an aging population and infrastructure, and higher population density in many cities.

Conclusion: A nasty disease likely to become more common

Legionella is a nasty bug, and it’s a good thing that it isn’t very easy to catch. It is yet another reason to consider quitting smoking or having your baby on dry land. Although effective treatment is available, the disease it causes is still potentially very deadly when there are delays in appropriate care, which is one more reason not to waste time seeking advice from unqualified pseudomedical professionals.

Don’t be too hard on Disneyland. Contamination of water supplies, such as in cooling towers used to heat and air condition large buildings, is hard to prevent and outbreaks are increasingly going to happen. Protocols for frequent testing of high risk water, and quick action when outbreaks are first noticed, is key to preventing cases and reducing mortality. Unfortunately, unlike with measles there is no vaccine against Legionella.

Posted by Clay Jones

Clay Jones, M.D. is a pediatrician practicing at Newton-Wellesley Hospital in Newton, MA, and a regular contributor to the Science-Based Medicine blog. He primarily cares for healthy newborns and hospitalized children, and devotes his full time to educating pediatric residents and medical students. Dr. Jones first became aware of and interested in the incursion of pseudoscience into his chosen profession while completing his pediatric residency at Vanderbilt Children’s Hospital a decade ago. He has since focused his efforts on teaching the application of critical thinking and scientific skepticism to the practice of pediatric medicine. Dr. Jones has no conflicts of interest to disclose and no ties to the pharmaceutical industry. He can be found on Twitter as @skepticpedi and is the co-host of The Prism Podcast with fellow SBM contributor Grant Ritchey.