Since the beginning of the epidemic in March 2005, 266 000 cases of chikungunya fever are estimated to have occurred on the island of Reunion, a French overseas territory [2]. Weekly estimates of suspect cases are based on the number of patients with suspected chikungunya seen by sentinel physicians on the island of Reunion and extrapolation of these figures to the entire island population. Numbers of newly estimated cases peaked at 45 000 in week 5 (first week of February 2006), and have since decreased. By June 2006, the estimated weekly number of cases was around 400. The islands of the southwest Indian Ocean (including Comoros, Madagascar, Mauritius, Mayotte, and Seychelles), and several states of India [3] have been affected by chikungunya epidemics in 2005-2006.

Imported cases of chikungunya have been reported from several European countries [4]. The vector, responsible for the transmission in the French territories of the Indian Ocean is known to be Aedes albopictus. This mosquito has been detected on the French Mediterranean coast, between the cities of Menton and Nice, and recently in the Bastia area of Corsica, a French Mediterranean island.

Given the situation and the high number of travellers (304 113 for the year 2004 according to the French Ministry of Tourism) between Reunion and metropolitan France, surveillance of imported cases and vector densities is an important part of the risk assessment of autochthonous transmission in France.

Methods
Chikungunya serological testing in France is carried out by two national reference centres for arboviral diseases and two private laboratories.

Data on positive tests were collected regularly between April 2005 and June 2006 and have been analysed. The variables used are patient and laboratory postcodes, age, sex and date of blood sampling. An imported case is defined as follows:

Results
From April 2005 to June 2006 inclusive, 766 imported cases of chikungunya were identified in metropolitan France. The mean age was 48 years (range: 5 months-83 years), and the male:female sex ratio was 0.9:1.

The temporal evolution of imported cases correlates with the dynamics of the epidemic in Reunion (Figure 1). Between April and July 2005, an average of 20 imported cases was observed monthly, corresponding to the first Reunion outbreak peak and the epidemics on the Comoros islands. Following a decrease between August and November 2005 (winter in the southern hemisphere), the estimated number of cases increased again and peaked in March 2006 (177 imported chikungunya cases).

Since then the trend has been decreasing, with 119 imported cases reported in April, 88 in May and 48 in June. No severe chikungunya cases (neurological signs and/or organ failure) were reported in metropolitan France between April 2005 to June 2006.

Figure 1. Chikungunya: Imported cases to metropolitan France and estimated cases in Reunion (Reunion data source: Cellule Inter Regionale d’épidémiologie Reunion-Mayotte)

Cases were documented in most areas of France (Figure 2), but were concentrated in the greater metropolitan areas of Marseille, Lyon and Paris.

Figure 2. Geographical distribution of imported cases of chikungunya in metropolitan France, April 2005-June 2006

Discussion
The number of chikungunya cases imported to metropolitan France has been decreasing since March 2006. This temporal evolution correlates with the dynamics of the epidemic in Reunion, which has also been going down. The peak of the imported cases in March 2006 was observed one month later than the peak of the Reunion outbreak, which occurred in February 2006. This time lapse may be explained by the different methods used: distribution of estimated cases in Reunion was based on clinical symptoms onset, while imported cases were dated by blood sampling, which was often done several days or even weeks after symptom onset.

The number of imported cases is probably underestimated, since asymptomatic cases cannot be detected by surveillance. Chikungunya IgM antibodies appear 5 days after the onset of clinical symptoms on average, and can last for several weeks. Having measurable IgM titres at a given time does not imply viraemia, and only viraemic patients can cause further transmission. The risk of secondary transmission in metropolitan French areas where the vector is present is therefore probably overestimated, based on imported case data.

A. albopictus is known to be currently spreading around the world [5], and is present in north and central Italy [6] and Albania. The mosquito can transmit the virus, but its actual vectorial capacity (environment-dependent) in metropolitan France is not yet well documented. A. albopictus is also capable of transmitting dengue fever.

Risk assessment of autochthonous transmission is based on surveillance of both the vector and imported viraemic cases. A plan for preventing the circulation of chikungunya in metropolitan France [7] has been developed, and reinforces entomological and epidemiological surveillance. Since 7 July 2006, chikungunya and dengue fever have been added to the list of diseases for mandatory notification in France. Entomological surveillance in France has confirmed the presence of the vector in the city of Bastia in northern Corsica, in addition to the area on the southeast coast of France where it had previously been identified. Alert procedures have been developed and coordination with vector control services strengthened. All ‘suspect’ chikungunya cases detected in these regions with confirmed vector presence must now to be reported immediately, without waiting for biological confirmation. This should facilitate rapid information to the surveillance system and the implementation of control measures.