Disease: West Nile Virus

This page provides general information about  this condition. Text can be revealed by clicking on  the green headers. Links to press releases, results from DWHC investigations as well as other useful documents and relevant literature available on the DWHC website can be found at the bottom of the page.

Pathogen

West Nile virus (WNV) causes the disease West Nile Fever (WNF). It is an RNA arbovirus and belongs to the genus Flavirus in the family Flaviviridae. There are different lineages of WNV described, of which lineage 1 and lineage 2 are associated with disease in humans. The virus is for the first time isolated in 1937 in the West Nile district in Uganda  (1).

  1. David S, Abraham AM. Epidemiological and clinical aspects on West Nile virus, a globally emerging pathogen. Infectious Diseases (London, England). 2016;48(8):571-86.

Susceptible species

West Nile virus has a broad variety of bird species as host. In nature WNV is in an enzootic cycle between mosquitoes and birds, but it can also infect and cause disease in humans, other mammals and reptiles.

Birds serve as reservoir for WNV. The WNV can be fatal to a variety of bird species, such as  Sphenisciformes (pinguins), Passeriformes (passerine), Gaviiformes (such as loons or divers), Podicipediformes (grebes) en Pelecaniformes. In a few sporadic instances the great cormoran are reported. Also a big variety of birds of prey are sensitive to an infection with WNV, such as Accipitridae (hawks, eagles, kites, vultures), Strigiformes (owls), Falconidae (falcons and caracaras) en Cathartidae (condors) (2). The disease is also detected in Corvids (3). Even though many bird species can be infected, most birds survive, but especially crows and western jackdaws frequently die of infection

Mammals  are considered as dead-end host, because they do not develop high levels of virus in their bloodstream (viraemia) (4). Amongst mammals WNV causes disease mainly in horses (5).

  1. Stidworthy MF, Denk D. Chapter 27 – Sphenisciformes, Gaviiformes, Podicipediformes, Procellariiformes, and Pelecaniformes. In: Terio KA, McAloose D, Leger JS, editors. Pathology of Wildlife and Zoo Animals: Academic Press; 2018. p. 653-86.
  2. Trupkiewicz J, Garner MM, Juan-Sallés C. Chapter 33 – Passeriformes, Caprimulgiformes, Coraciiformes, Piciformes, Bucerotiformes, and Apodiformes. In: Terio KA, McAloose D, Leger JS, editors. Pathology of Wildlife and Zoo Animals: Academic Press; 2018. p. 799-823.
  3. Suthar MS, Diamond MS, Gale Jr M. West Nile virus infection and immunity. Nature Reviews Microbiology. 2013;11:115.
  4. Spickler AR. West Nile Virus. http://www.cfsph.iastate.edu/Factsheets/pdfs/west_nile_fever.pdf. 2013.

 

 

 

Signs in animals

Clinical signs develop because the virus infects the central nervous system and other major organs such as the liver, spleen, kidney, and heart (6).

Symptoms in birds include cramps, ataxia (uncoordinated movements), abnormal head posture and movement, uncoordinated flight, tremors, muscle weakness, and disorientation. The development of visual impairment and blindness is common in hawks and owls. Nonspecific symptoms include lethargy, anorexia, dehydration, and scattered feathers (7). It is also common for a bird to be infected without clinical signs.

West Nile virus infection in horses is usually not accompanied by clinical signs. However, in several outbreaks, neurological signs and symptoms were observed. Neurological signs include ataxia (uncontrolled movements), paresis (incomplete paralysis), or paralysis (limb paralysis) of the legs. These symptoms are often accompanied by skin and muscle tremors and muscle stiffness. Other symptoms may include incoordination, abnormal mental states ranging from drowsiness to hyperexcitability (increased irritability) or even aggression, facial nerve paralysis, tongue paresis, and dysphagia (difficulty swallowing). Some horses do not recover and die or are euthanized (8).

  1. Ye J, Zhu B, Fu ZF, Chen H, Cao S. Immune evasion strategies of flaviviruses. Vaccine. 2013;31(3):461-71.
  2. Gamino V, Höfle U. Pathology and tissue tropism of natural West Nile virus infection in birds: a review. Veterinary Research. 2013;44(1):39-.
  3. Castillo-Olivares J, Wood J. West Nile virus infection of horses. Vet Res. 2004;35(4):467-83.

 

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