West Nile Fever

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West Nile Fever

Nature of the disease
West Nile Fever (WNF) is a emerging zoonotic disease, caused by a Flavivirus, and transmitted by mosquitoes (arbovirus) from birds to susceptible mammals. The development of the disease is variable from influenza-like illness to fatal meningitis and encephalitis.
Classification
Not classified by OIE, List D of SPC disease, in humans notifyable disease in most countries.
Susceptible species
West Nile Virus primary affects wild birds (at least 110 species) which are considered to be the reservoir of the disease. Domestic birds, such as chicken, can be infected without developing the disease but ducks and pigeons exhibit clinical signs.  

Mammals can be infected by mosquito bites, with horses and humans appearing to be the most susceptible species. The virus has also been identified in bats, rabbits, rodents, cattle, camels and dogs and cats.

Very recently, evidences of West Nile Virus has been found in alligators in Miami. The epidemiology of the disease in this species is still unknown. 

Distribution
The present outbreak of WNF originates from Africa, but is also endemic in the Middle East and West Asia. In 1996 the virus started to spread in Romania (Europe), since when it has become established in Europe. At present, the virus is spreading through United States of America, where it was first recognised on the East Coast (New York) in 1999 and has now reached the West Coast (Los Angeles) in 2002, and can be considered to be a threat for the Pacific Region.
Clinical signs 
In birds
In wild birds the disease is not systematic, making them good reservoirs as they can carry the virus along their migrations. Signs are generalized and often include neurologic abnormalities and emaciation. Clinical signs caninclude encephalitis, pneumonitis, nephritis, myocarditis and massive die-off (specially corvids).

Domestic birds such as chicken do not seem to develop the disease, however ducks and pigeon develop similar signs to those observed in wild birds. 

In mammals
Among mammals infection includes: fever, listlessness, stumbling, lack of coordination, ataxia, partial paralysis, and death. Usually horses develop more severe form of the disease but without fever.
In humans
In humans clinical signs are not systematic, in most of the cases there is only mild infection with an influenza-like illness that includes fever, headache, and body aches, often with skin rash and swollen lymph glands. In more severe infections there is headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and paralysis. In some individuals, especially the elderly, West Nile Virus cause aseptic meningitis and encephalitis which are often fatal.
Post-mortem findings 
No lesions are pathognomonic for West Nile Virus infection.

In birds, there is usually no pathology indicative of West Nile Virus infection on necropsy.

In mammals there usually are no macroscopic lesions, histologicaly there are slight to moderate non suppurative encephalomyelitis, primarily in the spinal cord and lower brainstem affecting both grey and white matter. The most severe lesions were in the thoracic and lumbar spinal cord. 

Differential diagnosis 
In birds differential diagnostic includes Newcastle disease

Generally in mammals differential diagnostic includes other diseases causing neurological disorders such as rabies, botulism, and other causes of encephalitis. 

Particularly in horses differential diagnostic includes 

  • Equine Protozoal Myeloencephalitis, 
  • Cervical Vertebral Myelopathy, 
  • Equine Herpes Virus 1,
  • Equine Degenerative Myelopathy, 
  • Other encephalitis such as Western Equine Encephalitis; Eastern Equine Encephalitis, and Venezuelan Equine Encephalitis.
Specimens required for diagnosis 
Whole blood, serum, and cerebrospinal fluid (if collected) samples that are processed by PCR and immuno-histochemistry.
Transmission   
There is no direct transmission of West Nile Virus from animal to animal - passage through an arthropod is necessary. In most cases, the vector is a mosquito from the Culex genus, however the virus has been isolated from 28 different species of mosquitoes and has also been is isolated from ticks in Asia.

Mosquitoes get contaminated by biting infected birds. It has not yet been demonstrated that mosquitoes can become contaminated after biting mammals. Mosquitoes become infective after 10 to 14 days of incubation. Vertical transmission of the virus in mosquitoes has been shown.

Horses, humans and chicken seem to be dead-end hosts. That is, the level of viremia in these species is to low to cause the infection of mosquitoes. 

Risk of introduction   
Recognising that mosquitoes do not become contaminated from biting mammals, West Nile Virus can only be introduced into the region through:
  • migratory  birds
  • imported live-birds from an infected country
  • mosquitoes (or ticks) from an infected country
Control / vaccines  
There is no specific treatment to West Nile Fever.

There is a killed vaccine for horses, however it must be given in two doses initially, three to six weeks apart. Efficacy data is not available at this time; however, the vaccine is considered very safe.

Destruction of mosquito breeding sites; decrease exposure to adult mosquitoes; provide screened housing; use insect repellents; and reduce outdoor exposure are also employed.

Monitoring sentinel birds such as chicken ('dead end' hosts having an immune response) and the reporting of clinical suspicion or abnormal die-off of wild birds is essential to initiate an early response to the disease.

References
  • Center for Disease Control and Prevention (CDC)
  • Guideline for surveillance of West Nile Virus by CDC (2001)
  • Promed
  • West Nile Fever, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 2184
  • West Nile Virus, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 953
  • West Nile Virus, Fact Sheet for Veterinarians - Ohio State University, 2002, WNV-1005-02
  • West Nile Fever - West Nile Virus website