FAQ - Frequently Asked Questions


Frequently asked questions on West Nile Disease


  • WNV infection
    • What is West Nile Virus?

      West Nile virus (WNV) infection is a mosquito-borne zoonosis. The virus is transmitted among birds via the bite of infected mosquitoes and incidentally humans and other mammals may become infected.
      About 80% of WNV infections in humans are asymptomatic. West Nile fever (WNF) is the most common clinical presentation but some develop severe neurological illness which can be fatal.

    • Is there a vaccine available for horses?

      To date, three vaccines have obtained the marketing authorization in EU member countries for horses:
      - an inactivated vaccine, produced from the VM-2 strain (Equip® WNV, Zoetis, Belgium previously Duvaxyn® WNV, Pfizer, US),
      - the recombinant canarypox virus vCP2017 strain, that expresses the WNV prM/prE genes (Recombitek equine WNV vaccine, Merial),
      - and the inactivated chimaeric flavivirus strain of Yellow fever virus presenting the genes for the structural proteins E and prM of WNV (Equilis® West Nile, Intervet International BV, Netherlands).

    • Is West Nile virus seasonal in its occurrence?

      Between latitudes 23.5° north and 66.5° south transmission of WNV occurs when mosquitoes are active (i.e. between spring and autumn) and most infections in humans and equids are observed between July and September.

    • What is the transmission cycle of the WND?

      WNV is transmitted in an enzootic cycle between mosquitoes and birds that respective acting as vectors and amplifying hosts. Mammals can become infected from the bite of an infected mosquito, but are considered dead-end hosts.

    • Can humans and horses transmit the infection?

      Humans and horses are considered accidental and dead end of the virus and do not contribute to the transmission cycle as they develop a low and transitory viremia not considered able to infect competent mosquito species.

    • Can humans be infected by eating the meat of infected animals?

      There is no evidence that WNV can be transmitted to humans through the meat of infected animals.

    • Are transplants and transfusions dangerous for the transmission of WND?

      The possibility of human-to-human transmission through blood transfusions or organ or tissue transplants is documented. In infected areas donors are subjected to regular checks to exclude the state of infection.

  • Host species susceptible to infection
    • Which species are susceptible to WND?

      West Nile Virus has an extremely broad host range. Many species of birds act as primary hosts for WNV, though its vertebrate host range includes also species of mammals, amphibians, and reptiles.
      Not all infected hosts transmit the virus, but only those in which the virus replicates efficiently enough to reach viremias sufficiently high to infect mosquitoes through blood feeding. Birds are the main vertebrate hosts of the WNV.
      It is generally acknowledged that Passeriformes (especially Corvidae, Fringillidae, and Passeridae families), Charadriiformes (Laridae) and Strigiformes are considered highly competent hosts, although differences in viremic levels vary depending on the species and the viral strain.
      Horses and humans are considered dead end hosts of the virus and do not contribute to the transmission cycle as they develop a low and transitory viremia not considered able to infect competent mosquito species.

  • Prevention and control
    • Is there a vaccine available to protect humans from West Nile virus?

      Currently there are no vaccines available.

    • Is there a vaccine available for horses?

      To date, three vaccines have obtained the marketing authorization in EU member countries for horses:
      - an inactivated vaccine, produced from the VM-2 strain (Equip® WNV, Zoetis, Belgium previously Duvaxyn® WNV, Pfizer, US),
      - the recombinant canarypox virus vCP2017 strain, that expresses the WNV prM/prE genes (Recombitek equine WNV vaccine, Merial),
      - and the inactivated chimaeric flavivirus strain of Yellow fever virus presenting the genes for the structural proteins E and prM of WNV (Equilis® West Nile, Intervet International BV, Netherlands).

    • Is useful vaccinate an WNV-positive horse for WND?

      The natural immunity developed after natural infection can persist for variable periods, so if the horse resides in infected areas, it is advisable that this immunity be re-established and stimulated annually through vaccination.

    • How to prevent WND?

      In addition to vaccination, direct health prophylaxis can be used. In endemic areas the main strategy to reduce human risk infection is based on vectors control and to personal protective strategies to limit mosquito bites. Preventive activities against vectors require an integrated approach or Integrated Vector Management (IVM).
      In free countries, the infection can be introduced by migratory birds or through the introduction of infected mosquitoes. In these areas, therefore, rapid alert systems represent the fundamental tools for the early recognition of viral circulation and for the implementation of all those control measures capable of limiting the spread of the infection.

    • How do you know the affected areas?

      In Italy WNV spread is continuously monitored by a surveillance system. West Nile (WNV) and Usutu (USUV) viruses surveillance activities are included in the National Plan for Prevention, Surveillance and Response to Arbovirus 2020-2025.

    • Can a WND positive horse be moved?

      Horses are dead end hosts of the infection, are unable to transmit the virus to mosquitoes. Therefore it is possible to move horses, even if infected, without causing the spread of the infection. In fact, no particular restrictions are applied to the movement of horses from infected areas. On the other hand, it is advisable that horses, who move to infected areas, especially in the late summer-autumn period, in which the circulation of the virus is maximum, are vaccinated or otherwise protected from infection.

  • Clinical signs
    • What are the commonly symptoms of WND in animals?

      Birds
      Most species of birds can become infected with WNV. Incubation period usually is 3-4 days and the clinical outcome of infection is variable. Some species appear resistant while others suffer fatal neurologic disease.
      Commonly symptoms observed are:

      - depression,
      - lethargy,
      - ruffled feathers,
      - weight loss ataxia,
      - paralysis,
      - handling movements,
      - pedaling,
      - stiff neck,
      - opisthotonos,
      - motor incoordination.

      Death usually occurs 24 hours after the onset of nervous symptoms.

      Horses
      The incubation period for equine WN encephalitis following mosquito transmission is estimated to be 3-15 days. A fleeting viremia of low virus titre precedes clinical onset. WN viral encephalitis occurs in only a small per cent of infected horses; the majority of infected horses do not display clinical signs. The disease in horses is frequently characterized by mild to severe ataxia. Additionally, horses may exhibit weakness, muscle fasciculation and cranial nerve deficits. Fever is an inconsistently recognized feature. The clinical signs can resolve with healing in 5-15 days or progress rapidly with death of the subjects. Treatment is supportive and signs may resolve or progress to terminal recumbency.
      The mortality rate is approximately one in three clinically affected unvaccinated horses.

    • What are the commonly symptoms of WND in humans?

      Most people infected with West Nile virus do not develop any symptoms.
      The incubation period for clinical illness generally ranges from 2 to 14 days, but prolonged incubation periods of up to 21 days have been observed among immunocompromised patients. West Nile fever can range from a mild infirmity lasting a few days to a debilitating illness lasting weeks to months. Symptoms are of sudden onset and often include headache, malaise, fever, myalgia, chills, vomiting, rash, fatigue, and eye pain.
      Less than 1% of people infected via mosquito bite develop West Nile Neuroinvasive Disease Meningitis characterized by clinical signs of meningeal inflammation, including nuchal rigidity or photophobia associated with encephalitis characterized by depressed or altered level of consciousness, lethargy or acute flaccid paralysis. All ages are affected, although very strong predilection is shown with advancing age. Case fatality rates among patients with neuroinvasive disease generally approximate 10%. Advanced age is the most important risk factor for death, ranging from 0.8% to 17% in those aged at least 70 years.
      Treatment is supportive and illness duration varies from weeks to months with possible long-term functional and cognitive difficulties.

  • Treatment