Legionella

There is no room for complacency in dentistry.

Legionellae and Legionnaire’s Disease

Legionellae are waterborne bacteria that are found in all types of natural aquatic environments in low numbers.

They also thrive and grow in warm (20°C to 45°C), stagnant artificial water systems such as dental unit waterlines (DUWL), hot and cold water plumbing systems, cooling towers, evaporative condensers, and whirlpool spas.

Legionellae cause sporadic cases and large outbreaks of infection that have been associated with, amongst others, hotels, hospitals, schools, cruise liners, leisure centre cooling towers and humidifiers at garden shows.

Infections are more common in the summer months.

Risk Factors for Legionellae Infections

Legionellosis is the collective term for the range of respiratory infections caused by legionellae which encompasses a mild flu-like illness, Pontiac fever and the potentially fatal pneumonia Legionnaire’s disease.

Most cases of legionellosis are caused by Legionella pneumophila.

The mortality from legionnaire’s disease is 12% to 30% with the higher death rate occurring in patients over 70 years of age. Everyone is potentially susceptible to legionellosis.

Some people have specific risk factors that make them more vulnerable to Legionnaire’s disease:

  • People over 50 years of age.

  • Men > women 

  • Suffers from heart disease, diabetes, lung disease, renal disease & immunosuppression.

  • Smokers.

There is no person to person spread of legionellae and all cases are due to exposure to contaminated water aerosols.

The attack rate for legionellosis infection is about 5%, so for most healthy people the health risks associated with inhaling contaminated DUWL aerosols are thought to be low. 

Legionellosis Associated with Dentistry

There is no room for complacency in dentistry.

A dentist in the USA died from Legionnaires’ disease as a result of exposure to Legionellae in his surgery’s untreated dental unit waterlines.

In a dental school in Italy, 4% of the dental nurses were shown to have evidence of past legionella infection.

A recent death from Legionnaire’s disease occurred in an elderly female patient who, during the incubation period for legionellosis, only left her home on 2 occasions to visit her dentist. Investigations revealed high numbers of legionellae in the tap water and DUWL in the dental practice but none in the water supply in her home.  Molecular typing confirmed her lung isolates and the DUWL isolates to be identical. The legionella were then eradicated from the dental practice using alkaline peroxide and shock treatment with chlorine.

Do You Need to do a Risk Assessment for Legionellae?

All dental practices are required, under the guidance in HCAI Code of Practice and HTM 01-05, to undertake a written legionella risk assessment of their hot and cold water plumbing and DUWLs.

The aim of the risk assessment is to identify and assess sources of known risk for legionellae growth and aerosolisation such as excess storage capacity, low water usage, temperature distribution problems and use of inappropriate materials.  

HTM 01-05 recommends that the risk assessment is carried out by a Competent Person, such as a qualified water engineer who is a member of the Legionella Control Association.  

Control of Legionellae in the Dental Practice

The dental practice personnel should consult HTM 01-05 for full details of the mandatory engineering controls, maintenance and water testing requirements for compliance with essential quality standards.

In brief, controls include ensuring an adequate turnover of water to prevent stagnation occurring in hot and cold water storage tanks and pipework and DUWLs.

This can be achieved by:

  • Avoiding dead-legs on the system.

  • Eliminate rarely used taps.

  • Keep the volume of water stored in tanks to a minimum, for example just enough for one day’s usage.

  • Avoid build-up of sludge in tanks and pipework.

  • Thermal controls that are used to inhibit the growth of legionellae, which grow between 20°C and 45°C.

  • Ensure that incoming and stored water remains at a temperature of less than 20°C.

  • Site cold storage tanks in a cool place and provide insulation.

  • Hot water stored at 60°C. The water temperature should be at least 50°C within one minute of running the hot water tap.

Normally, routine microbiological testing is not required unless there is an apparent deterioration in the water supply or 'at risk populations for legionellosis' are identified during the risk assessment.

If cloudiness, taste and odour problems develop then a microbiological investigation by an accredited laboratory may be required as this could signal development of conditions that could promote growth of Legionellae.

Dip slides are not acceptable as they underestimate the bacterial count, which could result in a misleading false negative result.

Infrequently used taps and DUWL should be flushed weekly to prevent stagnation and build-up of biofilm.