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West Nile fever (West-Nile) is an arbovirus (ARthropod BOrne VIRUS = virus transmitted by a biting arthropod). It is an emerging, transmissible, non-contagious disease. The vector is generally a mosquito of the Culex or Aedes genus.

The disease affects equids, humans and certain birds. In its clinical form it causes a febrile state and encephalomyelitis symptoms.



- Medical: some severe forms in horses.

In 2002, in the USA, 15,000 horses were affected with a mortality rate of 30% causing an impact on international trade.

- Hygiene: impact on human health (serious zoonosis).

Risk of complications in the central nervous system, mortal cases. In 2002, in the USA and Canada, 3,500 people were affected with 200 deaths.

Situation in America: 

The disease was introduced to the USA in 1999 (New York) causing numerous bird, horse and human deaths.

It spread to Canada and the Caribbean in 2002 affecting Mexico, the Cayman Islands, Jamaica, Dominican Republic and Guadeloupe.

Susceptible species: 

Mainly horses and man.

Some domestic or wild birds may be affected such as Passeriformes, Columbiformes, Anseriformes. Asymptomatic infection occurs the most often but several fatal cases were reported during the epizootic in 1999-2002 in the USA.

Rarely (USA): alligators, dogs, cats.

Etiological agent: 

The West Nile virus is an arbovirus from the family Flaviviridae, of the flavivirus genus. It is part of a set of viruses causing encephalitis.

It is a small, enveloped single-strand RNA virus of wide genetic variety. Two strain groups have been identified by genetic analysis.

A relationship appears to exist between the genetic group and virulence. Since 1998, the emergence of highly virulent strains has been observed for equids, man and birds.

Methods of transmission

Virulent matter: Blood, during the viremic stage.

Reservoir: Birds. Significant and lasting viremia in the absence of other symptoms of the disease. Migrating species play an important role in virus dissemination.


Transmission cycle

• Infection through mosquito feeding on a bird in the viremic stage.

• Multiplication of the virus in the arthropod organism.

• Transmission of the virus to a receptive vertebrate during mosquito feeding.

Receptivity and susceptibility factors

Horse are more susceptible than donkeys and mullets. Under nourishment, fatigue and intercurrent infections increase their susceptibility.

Method of contamination: 

Indirect, vectorial, through the bite of a haematophagous arthropod:

- Mosquitoes, mainly of the Culex or Aedes genus.

- Ornithophilic ticks: Amblyomma spp. especially.

Transovarion transmission of the virus in the vector species enables the viral cycle to last over time.

Horses and man are epidemiological cul-de-sacs. They do not intervene in virus transmission.


The disease progresses in 3 phases: an initial febrile phase, a stable phase and a terminal phase with varying outcomes.

In man

Incubation period: 3 – 6 days

The infection is often not apparent. Symptomatic forms range from a simple flu-like simple syndrome to severe or even fatal encephalitis.

- Flu-like syndrome: sudden appearance, high fever. Symptoms persist for 3 to 5 days. Clinical signs: headache, joint and muscle pain, rash, lymphadenopathy, abdominal pain, diarrhea and sometimes respiratory symptoms.

- Complications (in 15% of cases): aseptic meningitis or encephalitis which may lead to coma and death in elderly people or young children. Generally the subject recovers spontaneously with occasional relapse.

In horses

Incubation period: 3 to 15 days.

As a particularly susceptible species horses reveal virus circulation.

Subclinical infections are frequent. Clinical forms take the shape of flu-like symptoms followed by encephalomyelitic disorders with hind limb paresis followed by paralysis. The death rate is moderate to high with progression towards death or recovery with or without sequelae in 20-30 days.

Macroscopic lesions: 

Absents or uncharacteristic.

Microscopic lesions: 

Non-specific inflammatory lesions: poliomyelitis affecting motor axon collaterals of the thoracic and lumbar spinal cord.

Clinical diagnosis: 

Very delicate as several causes of nervous and polymorphic disease are possible. The epidemiological context is important (equids in enzootic areas, abundance of vector mosquitoes, etc.). Laboratory diagnosis is essential.

Differential diagnosis: 

In horses. West Nile fever is to be distinguished from other central nervous system disorders:

- other forms of viral meningoencephalomyelitis (MEM) in equids: Venezuelan equine encephalitis, North American Eastern and Western equine encephalitis

- other viral MEMs: rabies, Borna disease, Aujeszky's disease

- bacterial disease: strangles, tetanus

- dourine

- neurotropic poisoning

Laboratory diagnosis: 

Virus isolation and serological diagnosis.

Samples: blood (during viremia), cerebrospinal fluid, liver, brain.

Virological diagnosis: 

Virus identification through:


- Intracerebral inoculation of new born mice

- Cell culture

- Intrathoracic inoculation of the mosquito.

Serological diagnosis: 

- Immunoenzymatic method (ELISA)

- Haemagglutination inhibition (HI)

- Seroneutralisation (SN)


No effective treatment exists to date. Hygiene measures should be taken and symptoms treated (anti-inflammatory drugs, antipyretics, tranquillizers, vitamins, etc.).


Difficult to implement given the epidemiological complexity of the disease.

Sanitary prophylaxis

In disease-free areas:

Control of the introduction of animals from infected areas, systematic destruction of vectors having possibly been transported via plane or boat. Spatial study of favourable biotopes and potential vector population monitoring, serological monitoring of susceptible species.

In infected areas:

Limit proliferation of vector mosquitoes (deinsectisation). Isolation of viremic horses away from mosquitoes.


Vaccines are available for horses. Trials are currently underway for man in the United States.


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