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West Nile fever
Definition
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.
Significance
- 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 the Americas
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.
Pathogenic 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
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.
Sources
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
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.
Receptivity and susceptibility factors
Horse are more susceptible than donkeys and mullets. Under nourishment, fatigue and intercurrent infections increase their susceptibility.
Symptoms
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, diarrhoea 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.
Lesions
Macroscopic lesions
Absents or uncharacteristic.
Microscopic lesions
Non-specific inflammatory lesions: poliomyelitis affecting motor axon collaterals of the thoracic and lumbar spinal cord.
Diagnosis
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.
Techniques:
Virus identification through:
- PCR
- Intracerebral inoculation of new born mice
- Cell culture
- Intrathoracic inoculation of the mosquito.
Serology:
- Immunoenzymatic method (ELISA)
- Haemagglutination inhibition (HI)
- Seroneutralisation (SN)
Treatment
No effective treatment exists to date. Hygiene measures should be taken and symptoms treated (anti-inflammatory drugs, antipyretics, tranquillisers, vitamins, etc.).
Prophylaxis
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
Vaccines are available for horses. Trials are currently underway for man in the United States.
For more information
Bibliography: link with articles pertaining to West Nile in the Americas.
Results of the surveillance for WN virus in humans, birds, mosquitoes, sentinel animals, and equines:
in the USA: US Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases CDC
- WN human cases
- WN cases in other species
in Canada: Public Health Agency of Canada PHAC
- WN human cases
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