West Nile virusTransmitted to humans by mosquitoes, the virus is present on every continent except Antarctica
West Nile virus (WNV) is transmitted to humans by mosquitoes; the virus is present on every continent except Antarctica.
WNV is rare in UK travellers with only two cases reported in the UK between 2003 and 2013. It is possible that other cases with mild symptoms have not been reported/diagnosed.
Most people who become infected with WNV do not have symptoms. Approximately 20 percent develop symptoms which may include fever, headache, tiredness, body aches, nausea, and skin rash.
About 1 in every 150 cases develop a more serious neurological illness of meningitis and/or encephalitis. Immunocompromised travellers, those over 50 years of age and individuals with pre-existing medical conditions are at increased risk of developing severe illness.
There is no vaccine to prevent this disease in humans. Travellers should avoid mosquito bites day and night when visiting risk areas.
West Nile virus (WNV) is a mosquito-borne virus of the Flaviviridae family within the genus Flavivirus. WNV belongs to the Japanese encephalitis antigenic group of Flaviviruses, which includes Kunjin, Murray Valley and St. Louis encephalitis viruses.
WNV was first isolated from a febrile woman in the West Nile district of Uganda in 1937 . The virus is found on every continent, except Antarctica, and is indigenous to Africa, the Middle East, Asia and Australia (the Kunjin variant of WNV only). Outbreaks of WNV are also reported in several countries in Europe . In 1999, the virus was first reported in New York and spread rapidly throughout the United States and subsequently through the Americas from Canada to Venezuela . In December 2014, the first human case of WNV was reported in Brazil .
WNV is endemic in the USA and Canada: State Health Departments monitor WNV activity during the transmission season in the USA (which varies from State to State) . Maps showing WNV activity in the USA are available. In Canada, surveillance for WNV is undertaken by Public Health Canada during the transmission season ; maps showing WNV activity in Canada are available.
Outbreaks and intermittent cases of human WNV infection have occurred in Europe since the 1950s [2, 7 ]. In recent years, outbreaks have been reported from countries throughout Europe, including France, Greece, Hungary, Italy, Portugal, Romania and Spain; the transmission season in Europe is June to November . There is so far no evidence that West Nile virus is circulating in the UK . The European Centres for Disease Prevention and Control (ECDC) undertakes surveillance for WNV during the transmission season ; maps showing countries reporting cases in the EU are available.
Risk in travellers
There is a low risk of contracting WNV during travel to areas with WNV activity. The risk is determined by destination, season, length of exposure, and the intensity of WNV transmission at the time of travel. Certain groups, including the immunocompromised, those over 50 years of age and individuals with pre-existing medical conditions, are at increased risk of severe illness.
Travellers to areas where there is WNV activity should be aware of the risk and take appropriate mosquito bite avoidance measures.
West Nile virus in UK travellers
WNV infection is rarely reported in UK travellers. Most WNV infections are asymptomatic or produce mild symptoms; it may therefore be under-recognised generally. Public Health England undertakes an enhanced surveillance scheme for WNV.
In 2006 and 2007 there was one case each year of imported WNV infection in UK travellers who had visited Canada. There were no UK cases reported between 2008 and 2013 .
The main hosts of WNV are birds and the principle vectors are mosquitoes. WNV has been isolated from 43 species of mosquito and is predominantly transmitted by mosquitoes of the genus Culex, most commonly C. pipiens, C. restuans and C. salinariu . The virus is maintained in a mosquito-bird-mosquito cycle. However, humans become infected when environmental conditions are favourable for mosquitoes and there are sufficient numbers of bird hosts.
Culex spp. mosquitoes feed mainly during the hours between dusk and dawn. Humans, horses and occasionally other animals, become accidental hosts when bitten by an infected mosquito. Humans and animals serve as dead-end hosts. There is no person-to-person transmission.
The peak transmission season in temperate regions such as Russia, North America and Canada, is late summer to early autumn when there is high mosquito activity. In tropical countries, transmission is year round.
There have been isolated reports of non-mosquito borne transmission occurring in the USA. An outbreak of WNV amongst turkey farm employees raised the possibility of aerosol spread, and transmission has followed occupational exposure in laboratory workers. Transmission during blood transfusion and organ transplantation has also been documented. Transfusion related transmission in the USA has been reduced following the implementation of screening of donated blood for WNV. Since 2012, the UK blood transfusion also screens donors for WNV . Cases of intrauterine transmission and a single probable case of lactation-associated transmission have been described. Exposure to infected mosquitoes remains the predominant risk factor for acquiring WNV .
Signs and symptoms
The incubation period is 1-14 days. Most cases (80%) of WNV are asymptomatic or very mild and go undiagnosed. Symptomatic persons can experience a mild, self-limiting flu-like illness characterised by fever, headache, myalgia and a maculopapular rash. About 1 in every 150 cases progresses to a more serious neurological illness of meningitis and/or encephalitis. Patients with neurologic disease may have headache, neck stiffness, disorientation, muscle weakness, seizures, flaccid paralysis or coma. In these situations, the case fatality ranges from 4-14%, but may be as high as 15-29% in the elderly . Immunity is considered to follow infection although the duration is unknown . Deaths from WNV are usually associated with neuroinvasive forms of the infection .
Persistent neurologic sequelae have been observed in individuals who have survived acute West Nile encephalitis (WNE). Long-term movement disorders, cognitive complaints, and functional disability have been reported. WNE can result in an acute flaccid paralysis with a wide range of symptoms and degrees of limb weakness. These can range from mild monoplegia to flaccid quadriparesis and acute neuromuscular respiratory failure. Facial nerve palsy has also been observed .
Extreme fatigue is common following both WNV and WNE. In a study of 98 patients, 96% described post WNV fatigue, which lasted a median of 36 days . Depression and personality changes have also been observed. Depression after encephalitis, regardless of etiology, is not unusual: 31% of post WNV patients reported depression post-acute illness .
Diagnosis and treatment
There is no specific anti-viral treatment, but rather supportive intervention.
Healthcare professionals should be aware of the signs and symptoms of WNV and be sure to obtain a travel history when assessing patients. Specialist advice must be sought when persons suspected of having WNV infection are evaluated. Information about diagnostic tests is available from Public Health England (PHE). Appropriate serum and/or cerebrospinal fluid samples from suspect cases should be sent to the PHE Rare and Imported Pathogens Laboratory.
Prevention is by surveillance, mosquito control, and mosquito bite avoidance. There is currently no vaccine to prevent West Nile virus in humans.
First Published : 16 Dec 2014
Last Updated :  25 May 2016
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