Borella et al identified risk factors for isolating Legionella from a domestic hot water source. These can help identify water heaters to screen and interventions to consider. The authors are from multiple universities in Italy.
Domestic sites ranged from private homes to large apartment houses.
Normally a domestic water source is colonized only by a single Legionella type. The species that colonizes the water system is influenced by the water temperature, level of free chlorine and the heater type (electric vs fossil fuel).
Factors associated with increased risk of colonization on multivariate analysis:
(1) central water heater (shared between several apartments)
(2) heating system > 10 years old
(3) distance of sample site > 10 meters from the water distribution point (this makes me think of a system with a lot of pipes)
Factors associated with a decreased risk:
(1) zinc concentration < 100 µg/L
(2) copper concentration > 50 µg/L
Recommendations to reduce colonization:
(1) maintain high cleaning standards
(2) periodically replace components of the system that favor the harboring of bacteria
(3) maintain a water copper concentrations > 50 µg/L
(4) use independent domestic heaters when possible
