11 Jun 2019

Legionnaires' disease more prevalent than previously thought - study

4:21 pm on 11 June 2019

By Besa Chembo

University of Otago researchers have found the number of Legionnaires' disease cases is three times higher than was previously thought.

Inadequate labels on bags of potting mix may be leading to an increased number of cases of Legionnaires' disease in New Zealand.

Legionella longbeachae is commonly found in soil and composted plant material. Photo: 123rf.com

The burden of Legionnaires' disease in New Zealand (LegiNZ) study funded by the Health Research Council of New Zealand found that many cases were undiagnosed and under-reported because patients with Legionnaires' don't present distintive symptoms to other forms of pnuemonia.

Legionnaires' is a severe form of pneumonia caused by bacteria and in 63 percent of the cases it is caused bacteria called Legionella longbeachae which is commonly found in soil and composted plant material.

From May 2015 to May 2016, researchers arranged for people hospitalised with pneumonia all over New Zealand to have a specialised test that detects legionella bacteria and three times the number of cases were diagnosed compared with the average number of cases confirmed three years prior.

More than 20 hospitals participated and tested 4862 pnuemonia patients using PCR testing, detecting 197 cases through direct testing and another 41 cases through mandatory reporting. Overall 238 cases or 5.4 cases per 100,000 New Zealanders required hospitalisation.

The study was conducted to improve the identification of cases of legionnaires' in patients with pneumonia. Many cases of the 'potientialy deadly and preventable' bacteria go undetected because of incosistent use of diagnostic testing and the uncertainty about whom to test.

New Zealand has the highest reported incidence of Legionnaires' disease in the world which clinical microbiologist and Otago University dean David Murdoch said it was due to New Zealand's testing being more extensive than elsewhere in the world.

"The results are relevant to other countries as the bacteria causing Legionnaires' disease is present internationally but few countries have routine testing in place."

Prof Murdoch said the polymerase chain reaction (PCR) test is the best way to diagnose Legionnaires' because other tests cannot differentiate it from other forms of pnuemonia.

Hawke's Bay, Bay of Plenty and Waitematā had the highest rates of the disease, with eight cases per 100,000 people, with cases mostly underreported in previous years.

Canterbury also had a high rates of between six and eight cases per 100,000 people, but its reporting was more accurate because it adapted the PCR testing in 2010 and as a result has the most thorough testing for the disease in New Zealand.

As a result, it has the highest reported incidence of legionnaires' in the country, and accounts for a third of national case notifications despite being less than 10 percent of the country's total population.

Professor Murdoch said PCR testing was crucial to and supported the routine use of the test to detect and ensure access to appropriate treatments.

"The sooner the infection is treated with bacteria-specific medication, the better the prognosis."

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