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West Nile Fever in France

Return of mosquito season puts virus back in focus

01 September 2016, at 12:00am

September 2015 saw the first equine outbreaks of mosquito-borne West Nile Fever in France since 2006. Late summer is the peak time for infected mosquitoes, and OVs working with horses travelling to the Camargue and neighbouring regions should be aware of the disease’s clinical profile and vaccination options.

West Nile Virus (WNv) is one of the vector borne viral diseases, causing equine encephalitis. The vectors are haematophagous mosquito species, with preference for avian hosts. Some of the species will also preferentially bite mammals, including equines or humans. Culex pipiens is the most commonly occurring vector species in the UK. The ubiquitous Cx. pipiens pipiens form have a feeding preference for bird species, while the Cx. pipiens molestus form feeds on mammals (including humans) and birds but is considered to be more local in distribution. Bridging species, such as Culex modestus (which bite horses, humans and birds) are only known to exist in the UK in small isolated populations in coastal wetlands along the Thames estuary (Medlock et al., 20112; Vaux et al., 2015). Horses (and humans) represent dead end hosts. That is, they are not a risk for further transmission of disease. 

The France outbreak assessment

In September 21015, France reported two outbreaks of WNv in horses in the Bouches du Rhone region (OIE, 2015). In one case, a three-year-old Lusitano horse exhibited neuro
logical signs on August 11 and was humanely destroyed on August 19. Of the 60 horses remaining at the premises, two more showed suspect clinical signs and all were tested. In the second case, an Anglo-Arabian mare, also exhibiting clinical signs, was humanely destroyed on August 17.

Of the remaining 19 horses at the premises, all were clinically well and undergoing tests. The farms were both on the outskirts of the Camargue National Park. In the following days, a further five outbreaks were then reported in the same region. Disease control measures, including movement restriction of susceptible horses and vector control, were put in place. These were the first outbreaks reported in France since 2006.

France has not reported WNv in horses in the last few years, yet has been undertaking a comprehensive surveillance programme since 2003, particularly focusing around the Mediterranean region between June and November for clinical cases in horses (RESPE 2015). Horses are dead end hosts for WNv and therefore the trade in horses is not considered a risk of further transmission within the UK.

The West Nile Virus risk to the UK

One significant pathway for entry of WNv into the UK would be through the movement of infected migratory birds from an affected region. The passerine or corvid species and raptor species are frequently found to be susceptible to WNv in some areas, but more populations of corvidae in the UK are sedentary although some species (e.g. nutcracker, jackdaw and rook and some raptors) are migratory in the northern part of their continental range. A small number of these may come to the UK. Other routes of entry include movement of infected vectors through transport or trade but what is unknown at present is the species of mosquito responsible for transmission in France and its presence or survivability in the UK.

As of August 2016, no new cases had been reported in France, although surveillance and reporting cannot be 100 per cent accurate. In Europe, two lineages of WNv are circulating: Lineage 1 and Lineage 2. Both have been identified the last few years in Italy but generally Lineage 1 is present in Spain and Portugal and was the strain found circulating in France, while Lineage 2 is present in Central and Eastern Europe.

The Met Office monitors the global meteorological events, and heavy rain and flash flooding events, experienced by Northern Spain and Southern France in autumn 2015, promote increased mosquito abundance. If these weather patterns are seen more frequently due to climate change, and the incidence of infected mosquitos increases, the possibility of vectors being carried to UK via wind patterns also increases. 

Conclusions and information for OVs

The UK carries out passive surveillance for all suspect cases of neurological disease in horses as well as in birds targeting species associated with WNv mortality such as corvids, other small passerines, raptors and water birds to include samples for mass wild bird mortality incidents and those showing neurological signs. No cases of autochthonous infection in either humans or horses have been reported in the UK. There have been no confirmed cases in wild birds. A vaccine against WNv is available and licensed in the UK and Europe for use in horses.

These cases in France and other parts of southern EU in the Mediterranean basin do not increase the risk to the UK for incursion of WNv. Nevertheless, horse owners and OVs considering sending animals to the South of France for long periods of time should be aware of the risk to horses travelling in these areas. There are options for preventive vaccination against WNv which should be arranged well in advance of any travel, and handlers should be aware of the clinical signs of neurological disease in horses, which can include lack of energy, loss of coordination and weakness in limbs leading to stumbling. 

A forthcoming article in the Eurosurveillance journal will set out efforts being made to co-ordinate between public and animal health agencies across EU member states to represent the different levels of risk of WNv in different areas.

The Public Health England website (via www.gov.uk) provides guidance for the risk to public health. OVs who are interested in outbreaks of the disease are also advised to visit the Centers for Disease Control and Prevention (CDC) website (www.cdc.gov), as screening of human blood donations provides the most accurate surveillance and will most exactly represent the incidence of the relevant mosquito populations.

This article is based on the 2015 disease outbreak assessment by Defra/APHA, available at www.gov.uk. Additional comments were provided by Dr Helen Roberts, Defra International Disease Monitoring and Risk Assessment, Exotics and Risk Team, Veterinary Directorate.