Human and equine West Nile virus infections in France, August
– September 2003
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
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
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
||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.
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.