- First Identified as a New Pathogen 40 Years Ago, Legionella Persists
- Legionella’s Life Cycle Involves “Biological Sanctuaries”
- qPCR Proven to Outperform Antibody-Based Detection of Legionella
When I read about an outbreak of Legionnaires’ disease (LD) in New York City, baseball legend Yogi Berra’s famous quote, “It’s déjà vu all over again” immediately came to mind, along with the irony of Berra playing for the New York Yankees. So, if you’re much younger than me, you’ll likely not know why “It’s déjà vu all over again” and you may wonder who Berra was. You can read about him later elsewhere, but for now you should read on to learn about Legionnaires disease and why déjà vu is apropos.
History of LD
Notable positive events during 1976 in the United States included our Bicentennial Celebration, unveiling by NASA of the first space shuttle (the Enterprise), establishment of Apple Computer Company by Steve Jobs and Steve Wozniak, and Silly Love Songs by Paul McCartney and Wings ascending to #1 on the charts. While many of these events were the beginning of fabulous things to come, one proved to be the beginning of something catastrophic. American Legionnaires who gathered in Philadelphia, Pennsylvania for the Bicentennial were struck with a mysterious epidemic of fatal respiratory disease.
Sadly, 182 members of the Pennsylvania American Legion were affected, and 29 individuals died after they returned from the convention in Philadelphia. The epidemiological and microbiological studies continued for months before scientists began to understand what had happened. Much of the basic framework of our knowledge of Legionnaires disease, as the epidemic came to be known, was developed by a team from the CDC and the Pennsylvania Department of Health, as detailed elsewhere.
The cause of the disease remained a mystery until 1977 when an investigative team led by J. E. McDade and C. C. Shepard (of the Leprosy and Rickettsia Branch, Virology Division, Bureau of Laboratories, CDC) reported on the isolation of a Gram-negative bacillus found in patient samples. As often done for naming pathogens after sources, the genus of this rod-shaped bacterium was aptly named Legionella. Legionella includes the species L. pneumophila, which caused the pneumonia-like illness medically named legionellosis, but commonly referred to as LD.
2017 LD Outbreak Hits New York City—Again
In June of this year, forty years after the first characterization of Legionella, it’s lethal infectivity reoccurred in an outbreak in the Upper East Side of the Manhattan borough of New York City, leaving one person dead and six other people sickened. According to a newspaper account, this outbreak occurred within 11 days, and may have been triggered by contacting contaminated water as has happened in other cases.
While this incident affected relatively few people compared to other previous outbreaks, including one in the Bronx borough of New York City in 2015 that killed 15 people and sickened more than 70, it’s a scary reminder of the persistence of Legionella in the environment. In this regard, it has been reported that 200 to 400 cases of the illness are recorded each year in New York, despite the monitoring of 6,000 water systems wherein Legionella can flourish in warm conditions. This environmental factor provides a segue into what genomic sequencing has revealed about Legionella.
Genomics-Based Insights on Legionella
The bacterial pathogen L. pneumophila is found ubiquitously in fresh water environments where it replicates within protozoan hosts. When inhaled by humans it can replicate within alveolar macrophages and cause severe pneumonia associated with Legionnaires disease. As detailed elsewhere, recent advances in genome sequencing has had a major impact on understanding of the pathogenesis, evolution and genomic diversity of Legionella.
A lipopolysaccharide cell wall and several outer membrane proteins are essential virulence factors. Central to the pathogenesis of L. pneumophila is its Type IV secretion system, which translocates over 270 effector proteins into the host cell, thus allowing this bacterium to manipulate host cell functions to its advantage and assures intracellular survival and replication.
Within aquatic media, as depicted below, Legionella exist as part of biofilms, which provide a protective environment—or biological sanctuary, if you will—wherein the bacteria exhibit marked increase in resistance to biocidal compounds and chlorination. Aside from the resultant difficulty of purging water systems to be free of Legionella, these bacteria can invade and multiply within protozoa (which are ubiquitous and include amoeba), thus providing yet another biological sanctuary. Protozoa are present in all aquatic or moist environments, and can be found in even the most inhospitable parts of the biosphere, thus providing further protection to Legionella.
The actual infectious particle is not known but may include excreted legionellae-filled vesicles, intact legionellae-filled amoebae or free legionellae that have lysed their host cell. Transmission to humans occurs via mechanical means, such as air-conditioning units, taps and showerheads, as well as others listed by the World Health Organization (WHO).
Infection in humans occurs by inhalation of the infectious particle and establishment of infection in the lungs. After ingestion by macrophages, L. pneumophila have been found to inhibit acidification and maturation of its phagosome. Following a 6–10 hour lag period, the bacteria replicate for 10–14 hours until macrophage lysis releases dozens of L. pneumophila progeny.
It’s worth noting that, according to WHO, there is no direct human-to-human transmission of Legionella, which in my opinion is why incidence of LD remains relatively low.
Gardening Can Be Bad for Your Health—No Joke.
Unfortunately, there are other ways of contacting LD besides ingestion of tainted water. At the risk of sounding flippant, gardening can be seriously bad for your health because of contracting LD by breathing in aerosolized Legionella from contaminated soil. This is especially true in New Zealand, which has the highest incidence of LD in the world, according to a recent publication, with L. longbeachae being the most clinically relevant species. This infectious agent is predominantly found in soil and composted plant material. Most cases occur over spring and summer, and the people at greatest risk are those involved in gardening activities.
Some agricultural experts advocate smelling soil to assess its quality, stating that “[t]he smell of a soil can often reveal its state of health, sweet or offensive or plain bland,” and adding “the smell does not actually come from the dirt itself, but from soil microbes that inhabit a healthy soil environment. Sweet smelling soil has good levels of organic carbon which is vital to supporting the world of billions of beneficial bacteria and fungi in every cup of healthy soil.”
These soil sniffing experts, however, fail to consider the presence of pathogenic organisms including L. longbeachae. I, for one, will carefully avoid purposefully smelling any soil when gardening, and will instead be sure to wear a good mask capable of filtering out aerosolized Legionella, as you should too!
Nucleic Acid-Based Detection of Legionella
Rapid and effective diagnosis of LD is extremely important so that timely and appropriate therapy can be provided, thereby lowering the morbidity and mortality rates and reducing the health and economic costs associated with this disease. Surprisingly, diagnosis is reportedly established solely by time-consuming microbiological tests. Luckily, it looks like testing procedures could soon change for the better, thanks to PCR and NGS.
Earlier this year, Christovam et al. assessed the accuracy of various detection tests in patients suspected of being infected with Legionella and in patients with laboratory-confirmed LD. Investigators analyzed urinary Legionella antigen detection, direct fluorescent antibody (DFA) staining, serological testing and PCR vs. culture analysis (the reference standard). The sensitivity and specificity for PCR were 83 % and 90 %, respectively, whereas DFA sensitivity and specificity were 67 % and 100 %, respectively. Moreover, PCR had high sensitivity and specificity for early diagnosis of LD.
While the study results reported by Christovan seem promising, less definitive results have been reported. Krøjgaard et al., who compared culture and qPCR assays for the detection of Legionella in 84 samples from shower hoses and taps in a residential area before and after two decontaminations. Detection by qPCR was suitable for monitoring changes in the concentration of Legionella but the precise determination of bacteria is difficult. Risk assessment by qPCR only on samples without any background information regarding treatment, timing, etc. was said to be “dubious.” However, the rapid detection of high concentrations of Legionella by qPCR was said to be valuable as an indicator of risk, although it may be false positive compared to culture results. Detection of a low number of bacteria by qPCR was said to be a strong indication for the absence of risk.
Not surprisingly, the advent of powerful next-generation sequencing (NGS) is emerging as a better method for genus-specific, sensitive and quantitative determination of Legionella. In 2017, Pereira et al. reported findings from a study using NGS to differentiate 20 pathogenic strains of Legionella in fresh water systems. A genome standard and a mock community consisting of six different Legionella species demonstrated that the reported NGS approach was quantitative and specific at the level of individual species, including L. pneumophila. Comparison of quantification by real-time PCR showed consistency with the NGS data, thus indicating that NGS “provides a new molecular surveillance tool to monitor all Legionella species in qualitative and quantitative terms if a spiked-in genome standard is used to calibrate the method.”
Aside from providing a brief introduction and update on LD, my additional intent was to alert readers—without undue alarm—to the myriad circumstances in which Legionella can infect humans. According to the aforementioned list provided by WHO, the most common form of transmission of Legionella is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients.
Researching transmission of Legionella in Google Scholar led me to find additional information (see links below) that you may find useful or interesting.
- Commercial potting soils as an infection source of Legionella pneumophila and other Legionella species in Switzerland
- Evaluation of Legionella for risk assessment in hospital water distribution systems
- Legionella risk assessment in cruise ships and ferries
- Occurrence of Legionella in showers at recreational facilities
- Legionella contamination of dental-unit waters
- Outbreak of Legionnaire’s disease linked to a decorative fountain by molecular epidemiology
- An outbreak of Legionnaires disease associated with a decorative water wall fountain in a hospital
Thankfully, as I’ve said before, Legionella is not transmitted human-to-human. The scary aspect of Legionella, however, is that it’s continually mutating, which raises the specter of emergence of a strain that can spread within a human population. Let’s hope that this doesn’t happen and/or that modified mRNA vaccines can be quickly produced to combat that possibility.
As usual, your comments are welcomed.
After finishing this blog, there was a Reuters news report on October 9, 2017 that Michigan’s top medical official, Dr. Eden Wells, will be charged with involuntary manslaughter for her role in the city of Flint’s water crisis, which was linked to an outbreak of LD that caused at least 12 deaths. Dr. Eden Wells would become the sixth current or former official to face involuntary manslaughter charges related to this crisis, which principally involved lead contamination in the city’s water supply.