Legionnaires’ Disease – Significant Risk or Shiny Object?

This past summer, there was a cluster of Legionnaires’ disease (Legionellosis) cases in the Bronx. As most readers know, legionellosis is a disease that creates pneumonia-like symptoms caused by bacteria that belong to the species Legionella pneumophila. The species disease names both commemorate the sentinel outbreak that struck 221 attendees at the 1976 American Legion Convention in Philadelphia. Everything we know about Legionella today stems from the initial response to the 1976 outbreak.
Fast forward to the summer of 2015. In response to the cluster of cases in the Bronx, New York City issued emergency regulations and New York State is now drafting similar regulations.
Without diminishing the seriousness of the disease, it’s important to create a context. The Centers for Disease Control (CDC) reports incidence (morbidity) and death (mortality) statistics as incidence per 100,000 person-years (py) exposure (# of cases or # of deaths/100,000 people/year). The morbidity rate for Legionnaires’ disease 0.4 cases/100,000 py (all of the statistics in this post are approximations – epidemiology is an imprecise science). The mortality rate is 10 to 15% of the morbidity rate. By comparison, the morbidity rate for asthma 8,000 cases/100,000 py (mortality 8 deaths/100,000 py). Both diseases tend to occur in clusters and peak seasonally. Both are caused by inhalation. However, Legionnaires’ disease is an infectious disease (the symptoms are caused by a combination of direct and indirect effects of a microbe proliferating within the body) and asthma is typically allergenic (the body’s immune system reacts to inhaled substances). Now consider non-tuberculosis Mycobacteria (NTM). NTM are members of the genus Mycobacteria other than M. tuberculosis. Several NTM species are associated with pulmonary diseases (for example: hypersensitivity pneumonitis – extrinsic allergic aveolitis). The morbidity rate for NTM 2/100,000 py for the general population but can be as high as 20% among farm workers, pigeon breeders and other sectors of the population exposed to NTM sources.
All of these statistics can make your head spin. Morbidity and mortality rates mask the occurrence of disease clusters. Thus although L. pneumophila is ubiquitous, legionellosis typically occurs in clusters. The story for asthma and NTM infections is similar. So what is the shiny object? When a disease outbreak occurs, there is tremendous value in focusing on the immediate cause, but there’s also a need for maintaining a more global perspective. In her paper, presented at last month’s AWT Annual Convention, Diane Miskowski (EMSL Analytical, Inc.; www.EMSL.com) reported – almost parenthetically – that increased numbers of NTM had been recovered from cooling towers which had been treated to kill off L. pneumophila. Are our efforts to control Legionella spp. in order to decrease our legionellosis risk may be increasing our risk of contracting other respiratory diseases? More on this in another post. I’d be interested in reading your thoughts.