legionnaires Disease Case Studies

The World's Largest Outbreak of Legionnaires Diease

The world's largest outbreak of Legionnaires Disease took place in Murcia, Spain in July 2001. Patients started appearing at hospital on 07 July and by 22 July there were 449 confirmed cases with a total of 800 suspected cases reported. This meant that the number of people exposed to the bacterium was estimated at 16,000. The first confirmed case was diagnosed on 26 June but until 01 July only a small number of patients came forward. After this date the outbreak became explosive with 83% of cases onset being confirmed between 02 July and 09 July Allowing for an incubation period of between two and ten days this meant that the majority of patients were infected during late June and early July. Of the 449 confirmed cases there were 6 deaths making the mortality rate of this outbreak 1% of those infected.

In order to identify the source of the oubreak 662 questionnaires were completed by patients suspected of having the disease. These epidemiological questionnaires were used to elicit information on clinical aspects, predisposing factors, risk factors, place of residence and recent urban mobility within the city of Murcia pointed to an outdoor source in the northern part of the city.

A further case study involving 85 patients who lived outside the city and were reported cases between 08 July and 20 July questioned them on travel within the city, means of transport and frequency of visits.Again other identifying factors such as place of residence and employment, occupation, smoking habits and underlying health issues in the two weeks prior to infection were also considered.

Following these studies thirty zones within the northern part of the city were identified as at risk defined by the block around buildings with air conditioning cooling towers or areas containing large ornamental fountains. The results identified eight high risk zones which were then further investigated. Subsequently the air-conditioning system coolking towers at the city hospital, Morales Meseguer Hospital, provided and identical molecular pattern as the clinical isolates identified this is the most likely source of the outbreak, this was supported by the epidemiological survey.