Legionnaires’ disease outbreak: a quick look at the disease and it prevention

Legionnaires’ disease outbreak: a quick look at the disease and it prevention

A recent outbreak of Legionnaires’ disease in Edinburgh has claimed one man, 15 critical and another 15 suspected cases. The local NHS incident management expect those numbers to rise as the incubation period of the disease is 2-10 days, usually around 5-6 days where the disease is expected to peak.

It isn’t known at this time what the source of the infection is, but estimates indicate a source to be around the south-west of Edinburgh and the home of 16 industrial cooling towers. As a precaution, those towers were subjected to shock treatment in an effort to flush out the infection.

Legionnaires’ Disease

Legionnaires’ disease is caused by infection with legionella spp, collectively called Legionellosis. 90% of cases of infection are from Legionella pneumophila and in all there are 35 species of legionellaLegionellosis is an acute bacterial pneumonia associated with a rapidly rising fever with chills, a persistent cough, abdominal pain and diarrhoea are also common. In some cases the same bacteria produces a mild respiratory illness in reaction to the inhalation of the antigens, rather than a bacterial invasion, this syndrome is called Pontiac Fever and patients recover spontaneously in 2-5 days without treatment.

Sources of Infection

As it looks like in this story, one of the common sources of legionnaires’ disease is contaminated water in microscopic droplets, generated by a number of different equipment, typically showers, air-conditioning systems, ventilation ducts (wet), cooling towers, spas, whirlpools and water fountain systems.

Airborne transmission in water aerosols is believed to be the sole means of infection. Person to person transmission is not known to occur


legionella thrive in warm (25-42oC) and humid conditions, ideally in stagnate water and with an inert base on which to colonise, typically scale or sediment. Therefore it is essential that:

  • All potable water be either boiled or the water system is chlorinated (2 ppm of free chlorine) with either chlorine dioxide or hypochlorites
  • All ventilation ducts are completely dry and pools of condensated water are not allowed to form
  • AC units and ventilation ducts are carefully cleaned and dewetted
  • Tap water be either chlorinated (including time needed for disinfection) or passed through a UV light filter


Muraca, P et al, Comparative assessment of chlorine, heat, ozone, and UV light for killing Legionella pneumophila within a model plumbing system, Appl Environ Microbiol, Feb 1987

Damani, N, Manual of Infection Control Procedures, 2nd Ed, Cambridge University Press, 2004