A. Infuso^1,2, B. Hubert¹, D. Dumas³, M. Reyrolle⁴, S. De Mateo⁵, C. Pelaz⁶, C. Hemery⁷, I. Pérez⁸
1. Réseau National de Santé Publique (RNSP), Saint Maurice, France
2. European Programme for Intervention Epidemiology Training (EPIET)
3. Service de Médecine Interne, Centre Hospitalier, Millau, France
4. Centre National de Référence des Legionella, Lyon, France
5. Centro Nacional de Epidemiología, Instituto de Salud Carlos III, Madrid, Spain
6. Centro Nacional de Microbiología, Instituto de Salud Carlos III, Madrid, Spain
7. Cellule Inter-régionale d'Epidemiologie Ouest, Toulouse, France
8. Dirección de Salud, Huesca, Aragón, Spain.

Introduction

On 11 June 1996, three suspected cases of legionnaires' disease in a group of 42 Dutch tourists were reported to the local public health authority by Millau hospital in south west France. The group (group 1) had been touring with caravans and staying at different camp sites in France and Spain since 15 May. On 15 June, two people in a second group of 52 Dutch tourists following the same organised tour with a delay of one week (group 2), were admitted to hospital in Millau with pneumonia. The present report summarises the epidemiological and environmental investigations undertaken to identify the source of infection. Materials and methods

A descriptive and retrospective cohort study of the two tourist groups was conducted using a self administered questionnaire to identify further cases and potential risk factors in the four camp sites visited during the incubation period.

A case was defined as a tourist of group 1 or group 2 with evidence of fever >38.0 °C and cough during or within 10 days after the trip. Cases were classified as confirmed (culture of legionella or fourfold rise in antibody titre to Legionella pneumophila or presence of soluble urinary antigen or positive direct immunofluorescence), probable (single antibody titre > 256 for L. pneumophila), or possible (no laboratory confirmation).

The incubation period of legionellosis and dates of stay in the camp sites allowed us to suspect one specific camp site to be the possible source of infection.

The list of tourists who stayed at the camp site where exposure was likely to have occurred was obtained from the camp site manager and sent to the health authorities of the regions of residence of Spanish tourists exposed. Microbiologically diagnosed cases were reported to the European surveillance scheme for travel associated legionnaires' disease (EWGLI).

Clinical descriptions were obtained from hospital records and from the general practitioner of one case diagnosed in the Netherlands. Serological tests (IFA) for anti-Legionella antibodies (Lp1 to Lp10) and urinary antigen tests (EIA, BinaxTM, specific to Lp1) were performed at the National Reference Centre for Legionella in Lyon (France) for all but one case, which was diagnosed in the Netherlands.

Local public health authorities visited camp sites B and C and sampled water. Water samples were processed with standard methods for culture of legionella.

Data were entered and analysed using Epi Info version 6.04. Attack rates (AR) were compared by calculating relative risks (RR). Confidence intervals at 95% were computed for each RR. Statistical significance (p <0.05) was evaluated by Fisher's exact test.

Results

Thirty-five out of 42 people in group 1 and all 52 members of group 2 responded to the questionnaire (age: mean: 64 years, range: 56-75). Fifty per cent of respondents were men. Six people described a febrile lower respiratory illness consistent with the clinical case definition, an overall AR of 7% (4 cases in group 1, AR: 11% and 2 in group 2, AR: 4%).

Four of the cases were men and two were women, aged 60 to 74 years. Two cases (a couple) were confirmed (2 seroconversions to L. pneumophila sg1, one of whom had positive urinary antigen), one case was probable (titre of 512 to L. pneumophila pooled antigens sg1 to 6), and three cases were possible (3 with negative urinary antigen test and 2 no seroconversions). Five cases were admitted to hospital. Among these, chest X-ray showed uni (n=3) or bilateral (n=2) infiltrates. The case treated as an outpatient had fever, unproductive cough, and mild dyspnoea. All cases recovered.

Symptoms were reported to have begun between 2 and 7 June for group 1, and on 15 June for group 2 (figure 1). Figure 1 shows the length of stay at each camp site during the incubation period.

Case 1 left the tour on 31 May and had thus stayed only in camp sites A and B. Exposure during stay in camp site B was compatible with the date of onset of symptoms for all cases. The incubation period associated with exposure in camp site B ranged from 3 to 10 days for cases in group 1, and from 9 to 12 days for cases in group 2.

No air conditioning system was present in or around camp sites B, and no spas or other recreational waters were attended by the cases. The only exposures at risk identified among cases in camp site B were showers taken in the sanitary buildings. Cases and non-cases had taken a similar mean number of showers in camp site B. Two sanitary buildings were available and each person always took showers in the same building. The tourists (five cases and 46 non-cases) who took showers in one building were 2.9 times (95% CI: 0.36-24) more likely to be ill than tourists who showered elsewhere. The three cases who provided information about the time of showers had taken showers in the morning.

Between 25 May and 4 July about 405 people registered in camp site B, 192 of whom were Spanish (47%), residing in five different regions. Neither regional health authorities nor the European surveillance scheme for travel associated legionnaires' disease reported other cases associated with camp sites A, B, C, and D.

Environmental investigations

In camp site B, water is taken from a well to a 800 m3 tank where it is chlorinated before distribution. Residual chlorine concentration is recorded daily on a logbook. Two electric boilers (2 m3 each) provide warm water to the whole camp site. Samples were taken at different points of the distribution system but not from the boilers. L. pneumophila sg1 was found in six out of 16 samples taken at taps and shower heads in both sanitary buildings; the concentration was not reported.

In camp site C, water samples were taken from the two sanitary buildings used by both groups and included swabbing of the shower heads. All samples were negative.

Control measures

A hyperchloration of the water system was performed in camp site B on 4 July. The existing procedure of chlorination was not modified and water samples taken thereafter were negative for legionella.

Discussion

Travel is increasingly recognised as a common risk factor for legionnaires' disease. Fifty-six per cent of cases reported in England in 1995 were associated with travel, particularly travel abroad (1). One hundred and seventy-two travel associated cases were reported to EWGLI in 1995 (2).

Climate, type of accommodation, and standards for disinfecting and maintenance of water and air conditioning systems can influence the incidence of legionnaires' disease in tourists travelling to a specific country. Surveillance of legionnaires' disease differs among European countries and reported information on exposures to risk are difficult to compare.

The sensitivity of the notification system for legionnaires' disease has been estimated to be 9% in France (unpublished data) and small travel-related outbreaks in which cases are diagnosed in different places can easily go undetected. The proportion of diagnosed/incident cases can vary markedly according to the laboratory techniques used in each country (3). Urinary antigen detection, which accounts for 21% of diagnoses in countries that participate in EWGLI (2), was used in France for the first time during this outbreak. Only one case out of five tested was positive, a lower than expected sensitivity possibly due to very early testing in two cases.

Far fewer travel related outbreaks have been reported in association with camp sites than with hotels. One outbreak associated with a camp site was reported in France in 1989, in which hot water showers were heavily contaminated with L. pneumophila (4). The only common exposure identified in the present outbreak was the hot water used for showers in the camp sites. The three cases who provided information took showers in the morning and the concentration of legionella in hot water can be enhanced after stagnation overnight.

The dates of onset of symptoms are compatible with the hypothesis of a common exposure in camp site B, even though the incubation period is slightly longer than the 5 to 6 days usually reported.

The presence of L. pneumophila sg1 in the hot water in showers of camp site B supported the epidemiological findings, although no clinical isolate was available for comparison with the environmental strains. Water samples analysed in camp site C were negative and no investigation was carried out in camp site D where only cases 2 to 6 had stayed.

The fact that no other cases associated with the four camp sites were reported in Spain or through EWGLI may reflect underdiagnosis and/or a low sensitivity of the Spanish and the European surveillance systems.

According to European legislation (5), tour operators may under some circumstances be considered responsible for preventable health hazards of their clients and the issue of legionnaires' disease is becoming sensitive. Informed of the present outbreak, the tour operator removed camp site B from the tour of a third group scheduled in August 1996.

To improve prevention and control of legionnaires' disease outbreaks and avoid unnecessary economic consequences:

- rapid diagnostic techniques should be available, to allow diagnoses to be confirmed quickly;

- a more structured coordination between reference laboratories and public health institutions would ensure thorough outbreak investigation and improve the identification of specific exposures to risk during travel;

- standard recommendations to prevent multiplication of legionella in water could be made available to hotels, camp sites, and other facilities open to the public, and their implementation verified if two or more cases were associated with a single facility.

References

1. Newton LH, Joseph CA, Hutchinson EJ, Harrison TG, Watson JM, Bartlett CLR. Legionnaires' disease surveillance: England and Wales 1995. Commun Dis Rep CDR Rev 1996; 6: R151-5.
2. Hutchinson EJ, Joseph CA, Bartlett CLR. EWGLI: A European surveillance scheme for travel associated legionnaires'disease. Eurosurveillance 1996; 1(5): 37-9.
3. Legionellosis 1995, Denmark. Wkly Epidemiol Rec 1997; 72(9): 62-4 based on EPI-News Denmark 1996; (24).
4. Pelletier A, Hubert B. Cas groupés de Légionellose de 1988 à 1990. Bulletin Épidémiologique Hebdomadaire 1991; 38: 164-5.
5. Directive 90/314/CEE du 13/06/90 concernant les voyages, vacances et circuits à forfait; Journal Officiel des Communautés Européennes, 23/6/90.