Testing for Legionnaires’ disease
Legionnaires’ disease is a severe form of pneumonia. In New Zealand, it is most commonly associated with gardening activities. Christchurch Hospital now tests every pneumonia patient for Legionnaires’ disease.
Listen to this RNZ audio from May 2014 Testing for Legionnaires’ disease.
Duration: 12:54
A potentially fatal disease
One third of those suffering from Legionnaires’ disease end up in intensive care, and for 10% of the patients, it is fatal. Getting the correct diagnosis for Legionnaires’ disease is essential. Ruth Beran speaks with clinical microbiologist David Murdoch, infectious disease clinician Stephen Chambers and Ros Podmore about their collaborative efforts to quickly screen for this disease.
Using PCR – polymerase chain reaction tests and traditional cultures
PCR: A scientist explains
Clare Eagleton, a scientist at Genesis R&D, explains how multiple copies of DNA are made during a PCR reaction. The reaction involves a repeating cycle of denaturation (the two strands in the DNA double helix are separated), annealing (DNA primers bind) and extension (new DNA is made). The new DNA is complementary to the DNA that is being sequenced. Each time the cycle is repeated, the number of DNA copies is doubled. In the first cycle, one DNA double helix is copied to form two double helices. These two helices are then copied, resulting in four copies. The result at the end of the third cycle is eight copies of the DNA. Sixteen copies after four cycles, and so on.
PCR: A scientist explains
Clare Eagleton, a scientist at Genesis R&D, explains how multiple copies of DNA are made during a PCR reaction. The reaction involves a repeating cycle of denaturation (the two strands in the DNA double helix are separated), annealing (DNA primers bind) and extension (new DNA is made). The new DNA is complementary to the DNA that is being sequenced. Each time the cycle is repeated, the number of DNA copies is doubled. In the first cycle, one DNA double helix is copied to form two double helices. These two helices are then copied, resulting in four copies. The result at the end of the third cycle is eight copies of the DNA. Sixteen copies after four cycles, and so on.
Every patient presenting with pneumonia is routinely tested for Legionnaires’ disease. Each sputum sample has a polymerase chain reaction (PCR) test to detect Legionella bacteria DNA. The PCR test is very sensitive so it can identify the bacteria much more quickly than culturing it in an agar plate. The clinicians still culture the bacteria for their reference lab. This allows them to look at the bacteria strains in detail and test to see if antibiotics are working.
Find out more about Using PCR in medicine.
Identifying trends to prevent future outbreaks
David and Stephen found that, in Canterbury, the disease peaks every year in November. The disease cannot be transmitted from person to person – it has to come from an environmental source like soil or compost. They are working to identify the risk factors in people’s gardening behaviours with the hope of introducing prevention strategies.
Useful links
Legionella longbeachae is the most common Legionella species found in New Zealand. This fact sheet from the Ministry of Business, Innovation and Employment provides suggestions on safety tips for those who work with soil.
Programme details: Our Changing World.
