Legionella

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Nomenclature
  • Legionnaires' disease a pneumonia caused by any serogroup or sub-group of Legionella pneumophila

  • Legionella pneumonia a pneumonia caused by any species of Legionella

  • Pontiac Fever an acute, self-limiting, “flu-like” illness caused by certain species of Legionella

  • Legionellosis any human disease caused by a species of Legionella

Legionnaires' disease – an introduction

Legionnaires' disease is a pneumonia caused by Legionella pneumophila. Infection occurs by inhalation of contaminated aerosols produced by cooling towers, showers, nebulizers etc. The incubation period of the disease is two to ten days. Symptoms include malaise and headache followed by fever, chills and a consolidating pneumonia, which primarily involves the alveoli and terminal bronchioles; intra-alveolar extudate is characteristic of the disease.

Other common symptoms include chest and abdominal pains, vomiting, diarrhoea and mental confusion. Mortality may be high, particularly in susceptible individuals.

Most cases of Legionnaires' disease are sporadic; the rest arise from outbreaks.

Legionnaires disease is a relatively rare illness but important to those responsible for the operation of water systems which can from time to time act as a source of the disease and to workers and the general public who may be exposed to increased risk.

It is not precisely known what number of aerosolised legionellae need to be inhaled by a susceptible individual to cause an infection. It is considered that amoeboid vesicles containing large numbers of viable legionellae might be the infective agents because the disease usually starts in one lung a few days before the other.

Susceptible individuals include the elderly; smokers; people with a history of bronchial disease and patients on immunosuppressive drugs.

Treatment for Legionnaires' disease

Legionella pneumophila is sensitive to a number of antibiotics with erythromycin, rifampicin and genamycin showing significant anti-Legionella potency. Antibiotic therapy is therefore an important tool in managing Legionella pneumonia.

General support measures are also employed to assist the Legionnaires' disease patient including intensive therapy, assisted ventilation, relief of fever with aspirin, chest physiotherapy/drainage and prevention of dehydration.

Pontiac Fever

Pontiac fever is an acute, non-pneumonic, self limiting, “flu-like” illness which has been reported to have been caused by Legionella pneumophila and other Legionella species.

The incubation period is approximately 36 hours. The illness spontaneously recedes within 2-5 days.

The attack rate is usually high in that most people exposed to a virulent Pontiac Fever Legionella will develop symptoms of he illness which include fever, chills, headache and diarrhoea.

Morphology

Legionella bacteria are typically rod shaped, of width 0.3 to 0.9 micrometers and length 2 to 20 micrometers.

The cells are usually weak motile with only one or two polar flagella, non-spore forming and show a Gram-negative staining reaction.

Nutrition and Physiology

Legionella bacteria unusually use amino acids such as L-cystine as an energy source. They are aerobic, neutrophilic in culture, require a pH value of 6.9 and show enhanced growth in the presence of ferric ions.

Legionella bacteria prefer temperatures in the region of 20 to 45°C for growth although they can be found present in water samples outside this range.

Occurrence and the effect of temperature

Legionella bacteria have been isolated from many natural resources including rivers, streams, lakes, underground waters and moist soils, as such Legionella bacteria are likely to be ubiquitous in the environment.

Man-made water systems that have found to harbour Legionella bacteria include cooling water circuits, domestic water systems incorporating calorifiers, showers and water storage tanks; Spa baths and Jacuzzia; Industrial vehicle washers and industrial coolants.

  • Temperature is an important factor influencing the growth of Legionella bacteria in water systems.

  • At temperatures of 20°C or less Legionella bacteria will remain dormant

  • Temperatures above 20°C and 45°C favour the growth of Legionella bacteria

  • At temperatures of 45°C to 70°C Legionella bacteria will not readily multiply and the higher temperatures will kill Legionella bacteria in time.

  • Above 70°C no viable Legionella bacteria will exist.

Relationships with other mirco organisms and habitats

Legionella bacteria are not considered to be entirely free-living and are dependent upon other microbes for nutritional and reproductive purposes.

The ability of Legionella bacteria to survive is very likely due to their capacity to invade and multiply in amoebae and other protozoa. Legionella bacteria have been associated with Acanthamoeba, Naegleria, Tetrahymena, Hartmanella and Cyclidium.

The predominant bacterial form in water systems exist in biofilms which are communities of bacteria and other micro-organisms embedded in a matrix with trapped organic and inorganic debris attached to a surface as a slime layer.

Legionella bacteria can be protected within biofilms and amoeboid hosts and it is important that this aspect is considered when applying control procedures.

Testing for Legionella bacteria

The requirement and process for legionella testing varies throughout the world and is dependent upon local country regulations.

In many countries the international standard ISO 11731:1998 however the approved test procedure and follow up actions must be agreed for each international location.

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