The hospital culture of infection

Legionella - an important cause of hospital-acquired pneumonia

Historically associated with cooling towers and air conditioning systems, outbreaks of Legionnaires’ disease continue to appear around the world ^{1 –6}. The increasing number of outbreaks in hospitals is a major concern to health professionals and the public alike, and it is emerging that the water supplies in such institutions are the main source of infection ^7,8. Alison Smith and Ian Hart discuss recommendations for the prevention of hospital-acquired Legionnaires’ Disease and some of the methods available for the detection of pathogenic Legionella species in clinical and environmental samples.

Legionnaires’ Disease Legionnaires’ Disease is the pneumonic form of legionellosis caused by pathogenic strains of Legionella bacteria. The most commonly isolated cause of Legionnaires’ disease is Legionella pneumophila serogroup 1, responsible for around 90% of cases. However this strain is thought to be responsible for only 80% of hospital-acquired cases ⁸. Other pathogenic legionellae include L. pneumophila serogroups 2-14, L. longbeachae serogroups 1 & 2, L. bozemanii serogroups 1 & 2, L. dumoffii, L. gormanii, L. jordanis, L. micdadei and L. anisa.

Symptoms of Legionnaires’ Disease are non-specific, including fever >39°C, chills, muscle aches, cough and diarrhoea, and consequently hospital-acquired disease is thought to be under diagnosed ⁹. Legionella species are a common inhabitant of man-made water distribution systems, however, and it has been suggested that a single case of hospital-acquired disease may be an important indicator of additional undiscovered cases9. Risk factors in a hospital setting include underlying disease, chronic lung disease, surgery under general anaesthetic, organ transplant and corticosteroid administration ⁹.

Improved detection methods and increased awareness are perhaps contributing to the increasing number of outbreaks reported in healthcare facilities around the world.

Preventative measures Cases of hospital-acquired Legionnaires’ disease can be prevented by disinfection of the water supply. Several methods for disinfection have been suggested and used, including thermal irradication (heat and flush), hyperchlorination and copper-silver ionisation ⁸. Subsequent maintenance of the temperature in hot water tanks at 50-60°C is also recommended ⁹.

Routine sampling and testing of water supplies will confirm that such preventative measures are effective; however such screening is only mandatory in a few countries, including some parts of the USA and Spain, France and Denmark ⁹. In other countries, sampling of the water supply is only required when cases of hospital-acquired Legionnaires’ Disease have been confirmed. Yet this method is clearly not preventative and it has been suggested that proactive routine screening is the responsible approach ^5,9.

Prompt diagnosis of Legionnaires’ Disease allows patients to receive appropriate treatment at the earliest opportunity, thus improving outcome, and allows investigations into the source of infection to be initiated without delay. To do this, hospitals and public health laboratories must be prepared and equipped to perform microbiological testing for pathogenic Legionella species.

A recent example of how prompt action can save lives occurred during the UK’s largest ever outbreak of Legionnaires’ Disease in Cumbria in 2002 ^10-13. Despite the size of this community outbreak, there was a surprisingly low case-fatality rate. This was attributed to the vigilance and prompt action of local health workers ^14,15.

Rapid detection When Legionnaires’ Disease is suspected, culture of a respiratory specimen is used to provide a definitive diagnosis. In addition, a urinary antigen test is widely used, however this method will only detect soluble antigen of L. pneumophila serogroup 1. As discussed previously, this strain is only responsible for 80% of hospital-acquired cases of Legionnaires’ Disease and so some cases may not be identified using this method.

Culture, although taking a day or two longer, offers several advantages. It allows microbiological classification and subtyping, and it can be critical in subsequent epidemiological investigations.

[caption id="" align="alignleft" width="200" caption="Figure 1: Legionella colonies growing on BCYE agar with L-cysteine, but not on BCYE agar without L-cysteine."][/caption]

Both clinical and environmental specimens can be cultured on selective BCYE agar, such as Oxoid Legionella CYE Agar Base (CM0655) supplemented with Legionella BCYE Growth Supplement (SR0110) with or without L-cysteine. Colonies growing on BCYE agar with L-cysteine but not on BCYE agar without L-cysteine are regarded as presumptive Legionella spp (Figure 1).

Confirmation of presumptive positive colonies can be performed using a rapid latex agglutination screening method, such as the Oxoid Legionalla Latex Agglutination Test (DR0800M). Previous methods for confirmation were lengthy and labour intensive, and were usually only performed by larger reference laboratories. The latex agglutination method, by comparison, is quick and simple to perform. It is ideal for use by any size laboratory, enabling pathogenic Legionella species to be confirmed in less than a minute.

[caption id="" align="alignright" width="300" caption="Figure 2: A positive and a negative result using the latex agglutination method."][/caption]

The Oxoid Legionella Latex Agglutination Test detects all the predominant pathogenic Legionella species mentioned above using just three blue latex reagents for the separate confirmation of Legionella pneumophila serogroup 1 (reagent 1), Legionella pneumophila serogroups 2-14 (reagent 2) and the seven other afore-mentioned pathogenic Legionella species (reagent 3). Results are clearly visible and easily interpreted (Figure 2).

The Oxoid Legionella Latex test is also available in the convenient Dryspot format (DR0200M, DR0210M, DR0220M). In each Dryspot kit, test and control latex reagents are supplied dried onto the reaction cards, eliminating the need to store and dispense latex reagents. Positive and negative controls are also supplied in a dried format on specially designed mixing sticks. This significantly extends the shelf-life of the kits and enables them to be stored conveniently at room temperature.

The Oxoid Latex agglutination tests have demonstrated high sensitivity and specificity compared to other tests ^16,17.

Importance of culture Culture of Legionella is extremely important in pinpointing the source of infection, whether it is hospital or community-acquired. However, fewer samples are being cultured from patients due to the wide use of the urinary antigen test. The rapid results from this method allow patients to receive antibiotics promptly and no further specimens are taken from them. Dr John Lee from the Public Health Laboratory Service at Colindale, UK stresses, however “From an epidemiological point of view, it is vitally important to obtain specimens that can be cultured. Once antibiotic treatment has been started it is difficult to isolate the organism for use in investigative work.”¹².

Microbiology laboratories, therefore, have an important role to play in educating health professionals about the need for clinical isolates in pinpointing and eradicating sources of Legionella infection.

By Ian Hart, Marketing Manager, Oxoid