From 29 June to July 1998, four cases
of legionnaires disease in British citizens were reported to the Réseau National de
Santé Publique (RNSP) by the statutory notification system (déclaration obligatoire
(DO)) and by the European Surveillance Scheme for Travel Associated Legionnaires
disease coordinated by the European Working Group for Legionella Infections (EWGLI). One
of the cases was detected by the enhanced surveillance scheme set up during the Football
World Cup. All four cases had stayed in Paris in the ten days before developing symptoms.
A survey was conducted in order to confirm the existence of an outbreak, to identify a
common source of contamination, and to adapt control measures in response to the
A case was defined as any person who stayed in Paris in June 1998 in
the ten days before developing pneumonia associated with at least one of the following
laboratory criteria: isolation of legionella, positive direct immunofluorescence,
detection of legionella antigen in urine, fourfold rise in antibody titre up to a minimum
of 1:128 (confirmed case), single high antibody titre (³ 256)
(presumptive case). Cases confirmed to have been exposed to infection in hospital in the
ten days before becoming ill were excluded.
Active case finding was undertaken, using five sources of information:
- review of all notifications of legionnaires disease received in June and July
1998 at the RNSP
- search for unreported confirmed cases from the National Reference
Center for Legionella (NCR) and from the microbiology laboratories in the Paris
- the surveillance system set up for the World Football Cup was
extended to include all district health offices.
- all EWGLIs European collaborators were alerted and public
hospitals in Paris were informed.
Cases who did not live in France were interviewed about the places they
visited while in France. Data collection for French cases focused on places visited in
June in Paris and on potential exposures related to housing, workplace, and domestic and
Case control study
The exploratory survey led to a hypothesis that the frequentation of a
particular area of Paris could be linked to onset of disease. A case control study was
therefore undertaken among cases living in France in order to determine whether visits to
this area in June were associated with the occurrence of legionnaires disease. Cases
were defined as people resident in France infected with Legionella pneumophila
serogroup 1 (Lp1), confirmed or possible, who had stayed in Paris in June in the
ten days before becoming ill. For each case, three controls were identified using
telephone random digit dialling matched by sex, age category (30 to 60 years, >60
years), and district of residence. Controls had to have visited Paris at least once in
June and had to have presented no respiratory symptoms (cough, fever) since the beginning
of June. Strengths of association between factors analysed and disease were calculated in
terms of matched odds ratio (OR). The stability of the association was analysed with the
Mac Nemar test or the corrected test when the number of discordant pairs was <10. The
OR precision was given by its 95% confidence interval, calculated with the Mantel Haenszel
This was carried out by the Health-Environment Service in Paris and the
Laboratory for Hygiene. At first, a survey was conducted in a hotel where two cases had
stayed and water samples were processed. Subsequently, it appeared that most of the other
cases had visited the same geographical area but had not stayed in any common
accommodation. The hypothesis of an external source of contamination, such as a cooling
tower, was then raised. An environmental survey began using the official inventory of
listed installations, which comprised 39 cooling towers located in this geographical area.
A telephone survey identified 25 locations presenting potential risks of contamination,
seven of which were classified as suspect, as malfunction had been reported in the month
before cases become ill or because they were located next to the areas visited by the
cases. These seven sites were inspected and water samples were collected.
The genomic profiles of the strains isolated in patients and in water
samples were determined by pulsed field gel electrophoresis (PFGE) at the National
Reference Center for Legionella.
A total of 20 cases of legionnaires disease were identified in
people who had visited Paris in June 1998: 11 were French residents and nine European
tourists (seven British, one Danish, and one Swedish). Their mean age was 51 years (29 to
77 years) and 19 of them were men. Four patients died (20%). The cases arose between 6
June and 14 July, and 11 (55%) were clustered between 15 and 21 June (figure). Nineteen
cases were confirmed (6 by culture, 11 by detection of urinary antigen, 2 by
seroconversion) and one was a presumptive case. The six clinical isolates were L.
pneumophila serogroup 1 (Lp1) with identical PFGE patterns. Predisposing
factors for the disease were found in 17 patients: chronic renal disease and/or impaired
immunity (3), diabetes (2), smoking only (12).
The case interviews did not reveal a common
visited site apart from a hotel at which two cases stayed. Sixteen out of 19 cases (84%),
about whom information was available had visited four adjoining districts north of the
Seine (9th, 2nd, 8th, and/or 10th arrondissements (districts).
Among the 12 cases admitted to hospital in France, four were excluded
from the case control study (residence in Scotland, could not be traced, died with no
family, Lp5-6 infection). Eight cases and 24 controls were included.
Visits to the 2nd, 8th, 9th, or 10th districts of Paris were
significantly associated with the onset of disease (OR = 15 (95% confidence interval (CI):
1.3-166.9)). The matched OR could not be calculated for limited time spent at work or at
home but the association remained significant (p=0.002). When the suspect area was limited
to the 2nd and 9th districts or to the 9th district alone, the risk remained high but the
confidence interval reached the limit of the significance threshold (table).
Table: Comparison of the visits of the districts or
homes and workplaces between cases and controls