11 June 2009
Enhanced influenza surveillance on Réunion Island (southern hemisphere) in the context of the emergence of influenza A(H1N1)v
With the winter season on the southern hemisphere that starts in Réunion Island in June seasonal influenza activity usually increases shortly afterwards. The new influenza A(H1N1)v virus is rapidly spreading worldwide and may reach the island during the coming winter season. We have therefore enhanced influenza surveillance to detect the introduction of influenza A(H1N1)v, monitor its spread and impact on public health and characterise potential viral changes, particularly if seasonal influenza A(H1N1), resistant to oseltamivir, co-circulates with A(H1N1)v.
Influenza virus type A is associated with annual epidemics and occasional large-scale global pandemics. Both are characterised by increased morbidity and mortality . In temperate regions, a clear seasonality exists in the influenza activity with a marked peak in cold winter months. In tropical regions however, where there is less fluctuation in seasonal temperature this is not noticeable to the same extent .
Réunion Island, a French overseas administrated territory with 800,000 inhabitants, is located in the southern hemisphere in the south-western Indian Ocean, 700 km east of Madagascar and 200 km south-west of Mauritius, at a longitude of 55°3 east and latitude of 21°5 south, above the Tropic of Capricorn. In Réunion Island, influenza activity has been monitored since 1996 , but influenza virus circulation remains poorly documented. Results of past monitoring suggest that annual influenza activity increases in June-July  and the last reported seasonal influenza epidemic occurred in August-October 2007 . The island is presumed to have a double exposure to seasonal influenza, one from the southern hemisphere and the other one from the intense link with metropolitan France [4,6] (Figure 1).
Figure 1. Seasonal influenza activity on Réunion Island and in continental France, 2007-2009
In April 2009, a new strain of human influenza A(H1N1) virus, the influenza A(H1N1)v virus, was identified in USA and Mexico . As of 10 June 2009, a total of 74 countries reported 27,737 cases and 141 associated deaths to the World Health Organization (WHO) demonstrating the pandemic potential of the virus . Anticipating the start of the influenza season in Réunion Island sometime in June (Figure 1), the Regional epidemiology unit of Réunion-Mayotte (Cellule interrégionale d’épidémiologie, Cire) of the French Institute for Public Health Surveillance (Institut de Veille Sanitaire, InVS) is implementing an enhanced surveillance system to face the likely introduction and spread of influenza A(H1N1)v during the coming winter months in Réunion. The aim of this system is to detect the introduction of influenza A(H1N1)v timely on the island, monitor its spread and impact on public health and characterise potential viral changes, particularly if seasonal A(H1N1) resistant to oseltamivir co-circulates with A(H1N1)v. Furthermore, the surveillance we describe here is an attempt to include the specific surveillance of influenza A(H1N1)v virus into the global influenza surveillance system. It could be an example for other countries in the tropics and results will provide useful data about the effectiveness and limits of such system. Our experience might guide northern hemisphere countries in how to adapt their surveillance system before the upcoming influenza season in the winter.
Organisation of the influenza surveillance on Réunion Island, 2009
Figure 2 shows the organisation of the enhanced surveillance for imported cases of influenza A(H1N1)v. Timely detection of the introduction of cases by travellers coming or returning from affected areas is crucial to implement control measures around each case and limit the indigenous spread of the virus. Our enhanced surveillance is based on the national protocol set up by InVS  and the management of patients follows recommendations of the French pandemic plan . Case definitions of possible, probable, confirmed, excluded and close contacts of cases are shown in the Table.
Figure 2. Organisation of the influenza surveillance on Réunion Island, 2009 *
Table. Case definition and classification, influenza A(H1N1)v infection, France, 10 June, 2009
Sentinel practitioners network
A sentinel network, consisting of 40 general practitioners (GP) and two paediatricians, scattered across the island conducts prospective influenza surveillance on Réunion Island [3,4]. On a weekly basis, they report the percentage of consultations for influenza-like illness (ILI) using the following case definition: sudden onset of fever > 38°C AND cough OR breathing difficulty. Every physician is expected to perform a nasal swab for each first patient of the week presenting with ILI symptoms that started within less than 48 hours.
Hospital emergency departments
Data are collected daily directly from patients’ computerised medical files that are filled in during medical consultations regardless of the diagnosis. All data are extracted automatically and transmitted electronically in real-time to the InVS. Items collected include diagnosis, coded according to ICD-10, with a severity score ranked from 1 to 5 after medical examination, date of admission to emergency department, orientation (hospitalisation or discharge), age, sex, postal code, and main symptoms. Each patient corresponds to a single record, including all variables . Moreover, data concerning ILI patients will be extracted using influenza associated ICD-10 diagnosis codes (codes for influenza and more acute respiratory tract infections). On Réunion Island three out of the four existing hospitals participate in the network. The forth one is being integrated and should participate starting in July 2009. Nasal swabs will be performed daily for every first adult and paediatric patient seen in emergency departments.
General practitioner house calls network (SOS Médecins)
In the western coast of Réunion Island, SOS Médecins is composed of eight GPs that are involved in more than 100 interventions per day for a population of about 100,000 inhabitants (one eighth of the population). Telephone calls are handled by a call center and logged in a local database. This database is linked via internet to electronic notebooks held by GPs who can update the database with additional information following the visit of a patient. The data collected include: date of the visit, postal code, age, sex, symptoms of the patient and the medical diagnosis. Each morning, data for all visits logged during the previous 24-hour period (midnight to midnight) are downloaded .
Local hospital based Mobile Emergency Unit (Samu Centre 15)
On Réunion Island, a single ‘Samu Centre 15’ operates for the entire island. This mobile unit receives emergency calls and provides emergency healthcare and medical transport of patients. Total phone calls (regardless of diagnosis), phone calls for ILI and for advice on influenza will be analysed weekly.
To monitor and describe severity, cases hospitalised for ILI will have a nasal swab for viral testing. Clinical and epidemiological information will be collected by Cire in collaboration with a clinical research project for hospitalised cases currently under preparation.
The National Institute for Statistics (Institut National de la Statistique et des Etudes Économiques, Insee) conducts the administrative recording of deaths from all causes in France. For several years, Insee has been monitoring and centralising daily mortality in France including Réunion Island. In case of an influenza epidemic on the island, we will analyse this total number and excess of deaths from all causes. This system will be completed by analysis of all death certificates received by the regional public health authority that mention ‘influenza’. These certificates will be recorded as influenza-associated deaths. Electronic death certification which is being implemented in France will be used by the Intensive Care Department of Saint-Denis Hospital, and be analysed in real-time by the Cire.
Despite a well functioning surveillance system, imported cases of influenza A(H1N1)v might be missed and result in outbreaks of ILI in closed communities (schools, children, workers, elderly). In order to prevent this from happening, reporting of outbreaks in such communities, particularly in the early phase of the influenza season, has been fostered and will lead to prompt investigation including virological testing. Furthermore, to improve self-notification of clusters, heathcare professionals have been informed on the relevance of such measures.
An enhanced virological surveillance will be implemented in order to identify and characterise circulating influenza viruses during the coming winter season in Réunion Island. Specimens will be collected by members of the sentinel network and hospitalised patients with ILI symptoms will also be tested. We estimate an average of 80 specimens to be tested weekly at the Laboratory of Virology of Saint-Denis Hospital, one of the 24 laboratories approved by the French Ministry of Health. Specimens will be tested for influenza A and B virus by RT-PCR. For positive influenza A specimens, specific RT-PCR for influenza A(H1N1)v will be performed. All positive influenza specimens (A(H1N1)v and others) will be sent for further viral isolation and complementary analysis, including oseltamivir resistance monitoring, to one of the two French National Reference Centres (NRC) for influenza.
The beginning of the winter season in Réunion Island in June is usually followed by an increase of seasonal influenza activity shortly afterwards. As influenza A(H1N1)v is rapidly spreading worldwide, it can be expected that it emerges very soon in the upcoming winter season in the southern hemisphere (as it already has for example in Australia), including Réunion Island. Therefore, the surveillance of influenza on the island has been enhanced to be able to detect the introduction of influenza at an early stage and to monitor the spread and impact of the infections in order to guide the implementation of control measures foreseen in the French national pandemic plan. The usefulness of our enhanced surveillance will be guaranteed by a good collaboration between clinicians, virologists, epidemiologists and public health authorities. Close viral monitoring is of paramount importance since the circulation of seasonal influenza A(H1N1) resistant to oseltamivir with the A(H1N1)v virus is possible during the winter in the southern hemisphere. Such virological approach combined with epidemiologic description of a potential outbreak will assist local public health authorities to adapt control measures to limit the spread of the infection and mitigate the epidemic including use of information on the effectiveness of antivirals. Results of our enhanced surveillance, if an influenza epidemic occurs in Réunion Island, could provide relevant information for continental France or other European countries in preparation for the coming influenza season in the northern hemisphere.
We are very grateful to Jean Claude Desenclos (InVS) for scrutinising the manuscript.
We are thankful to all the sentinel network practitioners, the practitioners of the adult and paediatric emergency departments, Dr Emmanuelle Rachou (Observatoire Régional de la Santé de la Réunion), Dr Marie-Christine Jaffar (Laboratoire de Biologie, Centre Hospitalier Régional, Saint Denis, Réunion), Dr Arnaud Bourdé (Samu Centre 15, Centre Hospitalier Régional Saint Denis, Réunion), SOS Médecins-Ouest Réunion, Drass de la Réunion and the two National Reference Centres for Virus Influenza, Lyon and Paris for their participation in collecting and kindly providing data for this surveillance system.
We thank all the clinicians providing their assistance to patients and for their participation in providing clinical data.
This enhanced surveillance project has received funding from the Agence Régionale d’Hospitalisation de la Réunion.
* Erratum: On 12 June 2009 Figure 2 was replaced and the titles in the References were translated into English.
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