On 6 October, a case of human West Nile virus (WNV) infection was notified by the Centre National de référence des Arbovirus (CNR, national reference laboratory for arboviruses, at the Institut Pasteur, Lyon). The patient is a 41 year old man living in the administrative department of the Var in southeastern France. Onset of symptoms occurred in late August (fever, myalgia, diarrhoea). After seven days, he was admitted to hospital with a diagnosis of meningo-encephalitis. Initial blood and CSF samples were positive for WNV IgM antibodies by enzyme linked immunosorbent assay (ELISA). Blood samples taken 3 weeks later were positive for both WNV IgM and IgG. The neutralisation test was positive on both blood samples reverse transcript - polymerase chain reaction (RT-PCR) was negative and no virus was isolated. The patient has now been discharged from hospital.

His wife reported having had a mild fever a week before her husband developed onset of symptoms. She tested positive (IgM and IgG) for WNV antibodies by ELISA on a blood sample taken in mid-October. Neutralisation test results are pending.

During the two weeks before symptom onset, the patient had stayed in the Var for four days, and had spent two days in the department of Alpes-de-Haute-Provence (see map). He also had driven along the French Riviera from his home in the Var to Alicante (Spain), where he spent one week. He then returned to the Var in France, where he developed symptoms 2 days after his return
His wife remained in the Var during the 3 weeks before onset of her symptoms.

On 9 October, the Agence Française de Sécurité Sanitaire des Aliments (French food safety agency, which, in collaboration with the Ministry of Agriculture, is responsible for equine WNV surveillance in France) reported WNV infection in a horse living in the Var, approximately 20 kilometres from the residence of the human cases. The horse had suffered from encephalitis in mid-September and subsequently recovered. A blood sample was positive for both IgG and IgM by ELISA and neutralisation.

In addition, two suspected equine cases were notified in the same department, both with meningo-encephalitis with onset in mid-September. One horse was euthanised early in the course of the disease but no cerebral biopsy was performed, and the other horse recovered.

The occurrence of two human cases (one confirmed and one probable) and three equine cases (one confirmed and two probable), in the same area within a 5 week period, strongly suggests that infection was contracted in the Var.

In France, national West Nile virus surveillance comprises:

	notification to the Institut de Veille Sanitaire (http://www.invs.sante.fr) of confirmed human cases by the CNR and its associated laboratory,
	WNV testing and notification of all horses presenting with encephalitis or meningo-encephalitis,
	A voluntary national network of wildlife observers (the SAGIR network, managed by the Office National de la Chasse et de la Faune Sauvage (French game and wildlife agency)) collects ill and dead birds which are sent to the CNR for WNV testing
	In addition, seroconversion is monitored in sentinel birds (mallards and chickens) and horses.

 

Moreover, since 2001, the national system has been reinforced during the ‘at risk’ period (from May to October), in the departments of the Camargue and Corsica where West Nile virus circulation has been documented in the past (see map). In these departments, suspected human cases are notified to the local health authorities and tested for West Nile virus. In addition, sentinel chickens and horses are monitored for seroconversion. No seroconversion has yet been detected in the sentinel horses or birds in 2003.

From October 8 onwards, surveillance of WNV infections has been reinforced in the departments where the patient travelled or stayed prior to illness onset. Retrospective and prospective case finding of human meningo-encephalitis cases is now being carried out by the Cellule Interrégionale d’Épidémiologie (regional epidemiology unit) and the local health authorities in all hospitals of the departments on the Mediterranean coast. Moreover, in the Var, a seroprevalence study is being carried out among 600 horses living within 30 kilometres from the human or equine cases. An ecological study of the vector competence and potential of the mosquito populations around the residences of the cases is being carried out.

All physicians, laboratory biologists, veterinarians and the volunteers in the SAGIR wildlife network involved in the local reinforced surveillance were alerted about possible virus circulation. The French agencies for blood transfusion and donor organ transplantation safety were alerted to the virus circulation on French territory and the occurrence of human cases.

Conclusion
The investigations indicate that West Nile virus was circulating in the Var in France in August and September 2003. It is yet not possible to determine whether virus circulation has ceased. However, taking into account the weather conditions, the reduction in mosquito populations and the southward migration of wild bird species, the probability of persisting virus circulation is decreasing every day.