West Nile virus activity in Southern Europe

In October 2008 Austria reported two avian outbreaks of West Nile virus (WNV) in wild birds in North Austria and Vienna [1]. There were no human cases. In the previous month, September 2008, WNV was isolated from horses and wild birds in Northern Italy in the Emilia Romagna region [2]. Subsequently two human cases were reported in the same region [3, 4]. These were the first two laboratory confirmed human cases of WNV in Italy. The first case was a female in her eighties living in a rural area between Ferrara and Bologna who developed symptoms on the 15 September 2008. A second case of WNV neuroinvasive disease was identified on 3 October in a man in his sixties [4].

WNV re-emerged in Italy during 2009 and spread to new areas; new foci of transmission were identified in central regions and the far north-east coast [5]. There were 18 confirmed human cases of neuroinvasive disease reported. Of 38 horses with clinical signs, eight died.

There is some concern that the currently circulating strain may have increased pathogenicity, as there are indications of a genetic mutation which is known to confer stability in avian hosts [6]. The mutation allows high levels of replication and higher viral loads, which may contribute to higher virulence, greater transmissibility and higher human/ equine rates of infection. Although the Italian strain has not been associated with high avian mortality (unlike the USA strain), there has been an increase in human neuro-invasive disease and geographical spread in horses.

People visiting the area should take appropriate measures to avoid being bitten by mosquitoes [7].

There is continuing risk of disease introduction through wild birds into the UK and Defra are continuing to test wild birds for WNV in the UK [1]. Defra's full qualitative risk assessment [8] concludes that there is a very low risk of the establishment of WNV in the UK as a consequence of imported horses from these areas because of the very low levels of virus in an infected horse and the low numbers of suitable vectors.

Since 2002 the HPA has undertaken surveillance for human cases of WNV in the UK and no indigenous cases have been found. In 2006 and 2007 there was one case each year of imported WNV infection in individuals who had visited Canada [9]. The HPA has updated their risk assessment on WNV but the implications for the UK have not changed. As long as effective control measures are in place the probability of WNV infection in the UK is minimal and the impact would be low.

The situation is continually monitored and reviewed - should the risk change the Department of Health contingency plan covers surveillance, prevention and control of WNV [10].

1. West Nile Virus: Austria.
http://www.defra.gov.uk/animalh/diseases/monitoring/pdf/wnv-austria.pdf
See Defras preliminary outbreak assessments at:
http://www.defra.gov.uk/animalh/diseases/monitoring/poa.htm

2. Macini P, Squintani G, Finarelli AC, Angelini P, Martini E, Tamba M, et al. Detection of West Nile virus infection in horses, Italy, September 2008. Euro Surveill. 2008;13(39):pii=18990. Available from: http://www.eurosurveillance.org/ViewArticle.aspx?Articleld=18990

3. West Nile Virus - Italy. 20081007.3177 (7 October 2008)
http://www.promedmail.org/pls/otn/f?p=2400:1202:1987160460011370::NO::F2400_P1202_CHECK_DISPLAY,F2400_P1202_PUB_MAIL_ID:X,74283

4. Rossini G, Cavrini F, Pierro A, Macini P, Finarelli AC, Po C, et al. First human case of West Nile neuroinvasive infection in Italy, September 2008 - case report. Euro Surveill. 2008;13(41):pii=19002. Available from:
http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19002

5. Sabirovic M, Roberts H, Lopez M, Hancock R. International disease monitoring, October to December 2009. Veterinary Record 2010 166:160-162.

6. Barzon L, Franchin E, Squarzon L, Lavezzo E, Toppo S, Martello T, Bressan S, Pagni S, Cattai M, Piazza A, Pacenti M, Cusinato R, Palù G. Genome sequence analysis of the first human West Nile virus isolated in Italy in 2009. Euro Surveill. 2009;14(44):pii=19384. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19384

7. NaTHNaC advice sheet on insect bite avoidance http://www.nathnac.org/pro/factsheets/iba.htm

8. Defra qualitative risk assessment (2009) West Nile virus: Potential risk factors and the likelihood for the introduction of the disease into the United Kingdom. http://www.defra.gov.uk/foodfarm/farmanimal/diseases/monitoring/documents/qra-wnv-090210.pdf
West Nile virus in horses in Italy (2009)
http://www.defra.gov.uk/foodfarm/farmanimal/diseases/monitoring/documents/wnv-italy091023.pdf

9. Surveillance for West Nile virus in the UK. http://www.hpa.org.uk/webw/HPAweb&Page&HPAwebAutoListName/Page/1199451960122?p=1199451960122

10. Department of Health (2004). West Nile virus: A contingency plan to protect the public's health. Available from: http://www.dh.gov.uk/en/Publicationsandstatistics/Publications/PublicationsPolicyAndGuidance/DH_4083548

Last reviewed: 12 March 2010