West Nile fever

West nile fever is due to an arbovirus of the  Flaviviridae family, and is also known as West Nile encephalitis, or West Nile meningoencephalitis.

It was first identified in Uganda in 1937, where a woman was infected, near the Nile, hence the name

The WNV life cycle involves the following :

• Common hosts who are reservoirs^[1] of the virus : in this disease birds are the reservoirs.
• Mosquitos are the carriers^[2]  (particularly the Culex type).
• Accidental hosts^[3] : horses and Man

The disease is mostly spread in summer and autumn. Areas which are considered at risk, are areas where the carriers thrive. To date, cases have been noted in France in humid areas, and around the Mediterranean basin.

Once they have been infected, mammals and birds are considered immune and protected against another infection for their entire life.

^[1] reservoir species = species contributing mainly to the reproduction cycle of a pathogenic agent (in this case a virus), and from whom the virus can then be trasmitted to other species.

^[2] carrier = an organism who disseminates the infection by transporting the pathogens, or who transmits the pathogens  between the different hosts of the disease.

^[3] accidental host = species who is receptive to the vitus, but only exceptionally affected by it.

In horses, the disease starts with hyperthermia (teperature > 38,5°C) and general loss of condition. In most cases the disease is limited to theses symptoms.

The virus can then migrate to the brain via the blood vessels. 30 to 30 % of horses affected show signs of nervous disorders, usually only slight :

• Despondancy, or on the contrary hyperexcitability.
• Muscle twitching.

These clinical signs can sometimes be more serious :

• Ataxis^[4] and locomotor disorders, which can go as far as paralysis .
• Coma and death of the animal (around 30 % of the equids affected by nervous disorders).

In most cases , recovery occurs within 20 to 30 days. Some after effects may however persist.

^[4] Ataxis = movement coordination disorder

To diagnose a West Nile Virus infection in a more or less recent past, the sanitary veterinarian can carry out blood tests and request serological tests. These tests will determine and quantify the anti-bodies directed against the virus and present in the horse’s blood.

The sanitary veterinarian can also detect an infection in a horse from various samples, looking for the virus or the virus genome : cerebrospinal liquid, urine or samples from the brain of a dead animal.

There is no specific treatment against the virus. The treatment is only symptomatic :

• Isolation of infected animals in a closed quiet space, dim lighting (to limit the consequences of nervous disorders).
• Administering supportive treatment : administering fluids, associated to NSAIDs.

Transmission occurs from wild birds via mosquito bites, and this makes fighting the disease more difficult. Infected horses are not a source of the virus, it is the avian reservoir, and the mosquito carriers which are the source.

Prevention consists in limiting contact between horses and mosquitoes during the risky periods (summer and autumn) :

• Bring horses into closed buildings before the end of the day, and turn them out after daybreak (after high mosquito activity periods). Mosquito nets can be installed at the entrance to any buildings. Insect traps can also be used in the horses’ environment.
• Restrict stagnant water (tarpaulin, ruts, trampled areas, flower pots, tyres…) by drying them out or by restricting access.
• Regularly clean out the water trough.

Vaccinating is possible and ensures good protection against the disease in the majority of horses. Seeral vaccines exist on the market in France.

West Nile fever is subject to state regulations : it is ranked as a category 1 sanitary hazard^[5].

When there is a suspected case, the sanitary veterinarian isolates the infected horses and take the necessary samples enabling to establish a diagnosis. The  DDecPP ^[6] of the area is warned.

The stud or yard is placed under surveillance by local decree (Arrêté Préfectoral de Mise sous Surveillance (APMS) ), while waiting for the results of the tests. If the disease is confirmed, the premises are then categorised by local decree as Premises carrying an infection (Arrêté Préfectoral Portant Déclaration d’Infection (APDI) ), which incurs the following restrictions :

• Census of all the horses present.
• Any infected, or suspectedly infected horse is forbidden to move from the premises.
• All the animals and stables housing the animals must be treated with an insecticide.

The decree is revoked 15 days after the recovery or the death of the last animal infected.

Epidemiological surveillance is carried out in France according to the following components  :

• Human → surveillance in the areas deemed at risk, and during the hazardous periods, when carrying out blood transfusions, or organ donation….
• Equine → State regulated surveillance and voluntary surveillance by the neurological syndrome sub-network of the RESPE ^[7].
• Avian → follow up of the causes of death in the wild bird population.
• Entomological (mosquitoes).

This surveillance aims mainly to follow the circulation of the virus and identifying any hazardous areas to protect human health.

^[5] Category 1 sanitary hazards = The present classification is defined by the ministerial decree dated 29/07/2013. Category 1 defines « sanitary hazards which could potentially cause harm to public health, or damage national prodction capacity, or the salubrity of the environment. These dangers require preventative, and surveillance measures, or defined and imposed control measures, implemented by the state, in the general interest ».
^[6] DDecPP= Local authority (Department) management in charge of the protection of populations (veterinary department).
^[7]  RESPE =  Equine pathology epidemio-surveillance network.

In France

Since 2015, West Nile fever clusters have regularly been declared in the South east of France, the previous clusters dating back to 2006.

* : number of premises accomodating horses where at least one case was detected
** : number of horses declared as being infected

In Europe

Every year, human and equine cases of West Nile fever are diagnosed in Europe. The countries which are the most affected are : Italy, Spain, Hungary, Greece and Romania.

The 2018 situation is a cause for worry due to the increase in the number of human cases.