Travel Health Information Sheets
February 2008
West Nile Virus
Introduction
West Nile Virus (WNV) is a mosquito-borne virus of the Flaviviridae family within the genus Flavivirus. Other Flaviviruses include dengue fever, Japanese encephalitis and yellow fever.
Epidemiology
Global epidemiology
WNV was first isolated from a febrile woman in the West Nile district of Uganda in 1937 [1]. The disease is indigenous to Africa, the Middle East, Asia and Australia. Outbreaks and sporadic cases have also occurred in Europe. In 1999 the virus was first reported in New York and spread rapidly throughout the United States and subsequently to Canada, Mexico, Central America and the West Indies.
WNV is now endemic in the USA and Canada: 3,576 human cases and 115 fatalities were reported in the USA in 2007 [2]. This compares to 4,269 cases with 177 fatalities reported in 2006, 3,000 cases with 119 fatalities in 2005 and 2,539 cases, with 100 fatalities, in 2004 [2]. On the basis of extrapolations from past sero-surveys, the US Centers for Disease Control and Prevention (CDC) estimates that 41,750 cases of WNV occurred in 2006 [3]. Deaths from WNV are usually associated with neuroinvasive forms of the infection.
In Canada 2,353 clinical cases of WNV were reported in 2007 (as of 24 November 2007). This is the highest number of reported clinical cases in Canada since Canada’s first documented case in 2002. In 2006, the Public Health Agency of Canada reported 151 clinical cases, 225 cases in 2005, 25 in 2004, 1,481 in 2003 and 414 in 2002 [4].
Outbreaks and intermittent cases of human WNV infection have occurred in Europe since the 1950s. Countries involved include Albania, Austria, Belarus, Bosnia, Bulgaria, Croatia, the Czech Republic, France, Greece, Hungary, Italy, Kosovo, Moldova, Montenegro, Portugal, Romania, Russia, Serbia, Slovakia, Spain and the Ukraine [5]. Four locally-acquired human infections were reported in the south of France in 2003 [6].
In February 2005, three human cases were confirmed in central Cuba [7].
Notable outbreaks include:
Southern Russia 1999 - 826 cases with (183 serologically confirmed) and 40 deaths [8].
Romania 1996 - 393 serologically confirmed cases and 17 deaths [9].
Israel 2000 - 417 confirmed cases and 35 deaths reported [10].
Other countries with serologically confirmed human infections include Algeria, Azerbaijan, Central African Republic, Democratic Republic of the Congo, Egypt, Ethiopia, India, Madagascar, Nigeria, Pakistan, Senegal, South Africa and Uganda [5]. Evidence of WNV transmission has also been reported in Belize, the Cayman Islands, the Dominican Republic, El Salvador, Guadeloupe, Jamaica, Mexico and Puerto Rico [11]. A serologically confirmed outbreak occurred in a remote region of southern central Sudan in 2002. This outbreak was unique as the disease solely affected children and was characterised by serious neurological involvement [12].
Epidemiology of WNV in European travellers
WNV infection is rarely reported in travellers. Most of those infected are asymptomatic or have mild symptoms. It may therefore be under-recognised.
In 2002, an 82-year old Frenchman was diagnosed with WNV in France on his return from Atlanta, Georgia, USA [13]. In the Netherlands, three cases of WNV infection were reported in travellers who had visited endemic areas in 2002 and 2003 [14-16]. In July 2004, two tourists from the Republic of Ireland (ROI) contracted the virus while on holiday in the Algarve region of Portugal [17]. In 2005 another tourist from ROI acquired WNV during a trip to New York [18].
In 2002, the United Kingdom (UK) Health Protection Agency (HPA) commenced an annual enhanced surveillance scheme for WNV. More information about the scheme can be found at:
http://www.hpa.org.uk/infections/topics_az/west_nile/
In 2006 and 2007 there was one case each year of imported WNV infection in UK travellers who had visited Canada [19]. There were no UK cases reported in 2004 or 2005 [19].
Risk for travellers
There is a very 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 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.
Transmission
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 [5]. 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.
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. 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 [11].
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.
In the UK, although both bird hosts and Culex spp. mosquitoes are present, WNV has not been detected in birds, animals or humans [20]. It is unlikely to become established in the UK mosquito population [21]. The risk of human cases is considered low because there are not thought to be sufficient numbers of Culex spp. mosquitoes in the UK to sustain human transmission [22].
Signs & symptoms
The incubation period is 1-6 days. Most cases (80%) of WNV are asymptomatic or very mild and go unreported. Less than 20% will 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% [23, 24], but may be as high as 15-29% in the elderly [25].
Persistent neurologic sequelae have been observed in individuals who 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 [26].
Extreme fatigue is common following both WNV and WNE. 96% patients in one survey described post WNV fatigue which lasted a median of 36 days [25]. 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 [27].
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 include a travel history when interviewing patients. Specialist advice must be sought when persons suspected of having WNV infection are evaluated. Specific testing should be undertaken (see the HPA link below for guidelines on arranging testing).
Prevention
Prevention is by surveillance, mosquito control, and mosquito bite avoidance.
References
1. Smithburn K, Hughes T, Burke A, et al. A neurotropic virus isolated from the blood of a native of Uganda. Am J Trop Med Hyg 1940; 20:471-492.
2. US Centers for Disease Control and Prevention. Division of Vector-Borne Infectious Diseases. Statistics, Surveillance and Control. 2007 West Nile Virus Activity in the United States. (Accessed 14 February 2008). Atlanta: CDC; 2007. Available at: http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCase
3. US Centers for Disease Control and Prevention. West Nile virus activity – United States, 2006. MMWR Morb Mortal Wkly Rep 2007;56,22:556-559.
4. Public Health Agency of Canada. West Nile Virus Monitor. 2007 Human Surveillance. Ottawa: Public Health Agency of Canada; 2007. (Accessed 14 February 2008). Available at:http://www.phac-aspc.gc.ca/wnv-vwn/mon-hmnsurv_e.html
5. Hubalek Z, Halouzka J. West Nile Fever – a re-emerging Mosquito-borne viral disease in Europe. Emerg Inf Dis 1999:5:643-650.
6. Del Giudice P, Schuffenecker I, Vandenbos F et al. Human West Nile Virus, France. Emerg Inf Dis [letter] 2004; 10:1885-1886.
7. Kouri P. Caribbean: First Human Cases of West Nile Virus Infection in Cuba. Promed Mail 2 February 2005. Archive number 20050202.0355. Available at http://apex.oracle.com/pls/otn/f?p=2400:1202:2467035420427
428::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_
8. Platonov A, Shipulin G, Shipulina O et al. Outbreak of west Nile virus infection, Volvograd region, Russia, 1999 Emerg Inf Dis 2001;7: 128-132.
9. Tsai T, Popovici F, Cernescu C et al. West Nile encephalitis epidemic in southeastern Romania. Lancet 1998; 352:767-771.
10. Weinberger M, Pitlik SD, Gandacu D et al. West Nile fever outbreak, Israel, 2000: epidemiological aspects. Emerg Inf Dis 2001:7: 686-691.
11. Hayes E, Komar N, Nasci R et al. Epidemiology and Transmission Dynamics of West Nile Virus Disease. Emerg Inf Dis 2005:11:1167-1172
12. Depoortere E, Kavle J, Keus K et al. Outbreak of West Nile virus causing sever neurological involvement in children, Nuba Mountains, Sudan. Trop. Med and Int Health 2004:9:730-736.
13. Charles P, Zeller H, Bonnotte B et al. Imported West Nile virus infection in Europe. Emerg Inf Dis 2003; 9:750.
14. van der Klooster J. Female patient with West-Nile fever in the Netherlands. Ned Tijdschr Geneeskd 2002; 146:90-91.
15. Prick J, Kuipers S, Kuipers H et al. Another case of West Nile fever in the Netherlands: a man with encephalitis following a trip to Canada. Ned Tijdschr Geneeskd 2003; 147: 978-980.
16. Vermij P. The Netherlands: Third Imported Case of West Nile Virus Infection. Promed Mail 18 October 2003. Archive number 20031018.2624. Available at: http://apex.oracle.com/pls/otn/f?p=2400:1202:2467035420427
428::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_
17. Connell J, McKeown P, Garvey P, et al. Two linked cases of West Nile virus (WNV) acquired by Irish tourists in the Algarve, Portugal. Eurosurveillance Weekly 2004; 8 (32). Available at: http://www.eurosurveillance.org/ew/2004/040805.asp
18. UK Health Protection Agency. West Nile Virus. Foreign travel-associated illness. England, Wales and Northern Ireland – 2007 report. London: HPA. 67-68.
19. UK Health Protection Agency. Surveillance for Human West Nile Virus in the UK. (Accessed 14 February 2008). Available at: http://www.hpa.org.uk/infections/topics_az/west_nile/
20. Crook P, Crowcroft N, Brown D. West Nile virus and the threat to the UK. Commun Dis Public Health 2002; 5:138-143.
21. Higgs S, Snow K and Gould E. The potential for West Nile virus to establish outside of its natural range: a consideration of potential mosquito vectors in the United Kingdom. Trans R Soc Trop Med Hyg 2004; 98:82-87.
22. UK Health Protection Agency. West Nile Virus General Information. January 2008. Available at: http://www.hpa.org.uk/infections/topics_az/west_nile/
23. Nash D, Montashari F, Fine A et al. The outbreak of West Nile virus infection in the New York City area in 1999. N Engl J Med 2001; 344:1807-1814.
24. Chowers M, Lang R, Nassar F, et al. Clinical characteristics of the West Nile Fever outbreak, Israel. 2000. Emerg Infect Dis. 2001; 7:675-678.
25. Petersen L, Marfin A. West Nile Virus: A primer for the clinician. Ann Intern Med 2002; 137:173-179.
26. Sejvar J. The Long-Term Outcomes of Human West Nile Virus Infection. Emerg Infect 2007:44: 1617-1624
27. Murray K, Resnick M and Miller V. Depression after Infection with West Nile Virus. Emerg Inf Dis 2007:13..Available at: http://www.cdc.gov/EID/content/13/3/479.htm
Reading list
Watts DM, Granwehr BP, Shope RE et al. Chapter 73. Japanese Encephalitis and West Nile and Other Flavivirus Infections. In: Guerrrant RL, Walker DH, Weller, PF, eds. Tropical Infectious Diseases. Principles, Pathogens & Practice. 2nd ed. Elsevier, Philadelphia. 2006: 823-830.
Opinion of the Scientific Committee on Veterinary Measures Relating to Public Health on West Nile Virus (WNV). 14-15 April 2003. Available at:
http://ec.europa.eu/food/fs/sc/scv/out67_en.pdf
Links
West Nile virus: A contingency plan to protect the public’s health. Department of Health, United Kingdom. Available at:
http://www.hpa.org.uk/infections/topics_az/west_nile/
European Commission, Threats to Public Health: West Nile Virus. Available at:
http://ec.europa.eu/health/ph_threats/com/west_nile/wnv_
West Nile Virus: Centers for Disease Control and Prevention. Available at: http://www.cdc.gov/ncidod/dvbid/westnile/site_index.htmDisclaimer | Copyright | Privacy | Sitemap | Accessibility
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