02 March 2015 | Research News | Commentary

It's quite common to find Legionella species other than Legionella pneumophila in cooling towers and water distribution systems. To date, fifty-eight Legionella species have been described in published articles. Of these approximately 25 are linked to disease.

Legionella pneumophila serogroup 1 is the most virulent strain causing the majority of infections. The remaining non-pneumophila species (found in water and soil) are considered nonpathogenic until shown to cause disease. Of the CDC reported cases less than 5% are attributed to these species. Since risk for infection is rare, adjusting your response to an adequate threat level is appropriate.

For example, there are Legionella species whose colonies exhibit blue-white fluorescence under long-wave UV light and some exhibit red fluorescence. These "blue-white" Legionella species include: L. anisa, L. dumoffii, L. gormanii, L. cherri, L. parisiensis and L. bozemanii. The red fluoresceing species include: L. erythra and L. rubrilucens. Such non-pneumophila species are commonly found in the environment, but rarely cause infection and when they do it is almost exclusively in very immunocompromised individuals.

In SPL's study, Role of Environmental Surveillance in Determining the Risk of Hospital-Acquired Legionellosis: A National Surveillance Study With Clinical Correlations (ICHE July 2007, Vol 28 No. 7. P 818 - 824) investigators found no infection from species like L. anisa even though it was present in the water systems.

Take the analogy of bacteria on skin. We know our skin is covered with many bacteria including Staphylococcus aureus, but we only get infections under extreme conditions. Most staph on our skin is inherently not pathogenic but if a patient is immuno-compromised or has a procedure that helps staph gain entry into the body, infection can occur.  This is sometimes referred to as an "opportunistic infection." Like staph other Legionella species are common in the environment but won't cause illness under normal circumstances.

To say that all Legionella species have the "potential" to cause illness is mere speculation. More than 30 years of research shows few or no reported illnesses from more than half of the known Legionella species. So it's safe to assume the risk of disease is so low as to not to be actionable.

When assessing risk rely on science rather than speculating about unknowns. When other or new Legionella species are found to contribute significantly to the disease threat, then risk assessments could be adjusted. For now it's appropriate to adjust your response to an adequate threat level as seen is the recent VA Directive that non-pneumophila species only require disinfection in limited circumstances.