Combating the threat of West Nile virus

With the news that an outbreak of West Nile virus has again been reported in mainland Europe, transfusion scientist Barry Hill provides an update on the condition for practice nurses and assesses its potential impact on UK blood services

During recent weeks, human cases of West Nile virus - a disease more commonly associated with tropical regions — have been reported in Europe, with three new cases occurring in Greece, additional cases in the Yugoslav Republic of Macedonia and small, but significant numbers recorded in the Russian Federation.¹

The European Centre for Disease Control (ECDC), the EU agency with responsibility for identifying, assessing and communicating current and emerging threats to human health posed by infectious diseases, says that since 2012, and to date this year, there have been a total of 244 cases recorded in Europe and 693 in neighbouring countries.

WNV is transmitted by mosquito bites, and although the risk of contracting WNV during a visit to affected areas remains small, the Health Protection Agency (HPA) has issued advice to travellers,² particularly relating to measures on how to avoid mosquito bites. The HPA has also asked healthcare professionals to be vigilant for signs of WNV in patients returning from these and other areas where WNV is endemic.

AETIOLOGY

WNV is an arthropod borne virus or flavivirus, first isolated in Uganda in 1937. It is transmitted to humans and animals via the bite of an infected mosquito.³ The mosquitoes themselves initially become infective by biting wild birds that carry the virus, which is then transmitted between birds and man who then become 'dead-end hosts' for short time periods after which no further spread occurs. Confined historically to Africa and the Middle East, WNV next began to be reported in Eastern Europe before making an unexpected appearance in the US in 1999.⁴ During the next few years WNV then spread throughout much of the US and Canada, where it is now considered to be endemic, causing major seasonal epidemics. Although cases of WNV have been reported in Europe before, to date there have been no known reported cases in the UK, due primarily to the climate inhibiting the mosquito population and the 'herd immunity' to the condition discovered in the UK's resident bird species.

Consequently the risk of WNV arriving in the UK has been assessed by the Department of Health as low, but the possibility cannot be ruled out, and so plans are in place to be adopted should a UK-acquired case of WNV infection being diagnosed. The 2004 document 'West Nile Virus: A contingency plan to protect the public's health,'⁵ provides a strategy for limiting its impact. Key issues outlined in the plan include the surveillance of people, horses, birds and mosquitoes for evidence of WNV infection, laboratory diagnosis of WNV, patient care and protection of healthcare professionals, public protection and the environmental vector control of mosquito populations to eliminate potential breeding sites.

Since 2002, regular surveillance for human cases of WNV in the UK has taken place. According to the HPA, a case is defined as an adult, particularly those aged 50 years and over with symptoms of encephalitis, meningo-encephalitis, aseptic meningitis or acute flaccid paralysis, who presents with no travel history outside the UK

SYMPTOMS AND TREATMENT

The incubation period for WNV in humans is 3-15 days, and although many infected individuals show no symptoms, around 20% display a mild flu-like condition for 3-6 days, with full recovery soon following. Less than 1% however can develop a severe form of the disease displaying meningo-encephalitis, which may prove fatal in elderly or immunosuppressed patients. In these cases, WNV can lead to encephalitis and meningitis, pancreatitis and fulminant hepatitis.⁵ Patients suspected of severe WNV need to be admitted to hospital and isolated, and exposure to their body fluids kept to a minimum to reduce the risk of transmission to healthcare staff.

THREAT TO THE BLOOD SUPPLY

The risk of transmission of WNV 'person-to-person' either by blood transfusion, organ transplantation or from mother to baby must also be considered. In relation to WNV there are two main situations to be considered for UK blood services, the importation of fresh frozen plasma (FFP) from the US and the risk from UK donors who may have been exposed to the virus overseas. It is the transfusion route, however, that presents the biggest risk, specifically from donations given following the first 1-3 days after infection in the donor. As a result, guidance has been issued to UK blood services by the advisory committee on the Safety of Blood, Tissues and Organs (SaBTO).⁶ New blood donor deferral criteria and blood component transfusion and organ transplantation recommendations were issued in April 2011 stating that any donors who have visited a WNV risk area should defer from donating blood for 28 days after leaving the affected area, unless a screening test which can indicate the presence of WNV has been performed on the donation. Any donation found to be NAT-reactive for WNV would then be discarded, and the donor would be contacted. Also, anyone who has been diagnosed with WNV, or who has had symptoms suggestive of WNV or since their return to the UK will be asked not to donate for 6 months. This change is in addition to existing guidelines relating to anyone travelling to any part of the USA, Canada, North Eastern Italy or Greek Macedonia where WNV is also prevalent. More recently, the NHS Blood and Transplant agency has indicated that it plans to introduce a new screening test for donors who have returned from countries where WNV is prevalent, thereby further ensuring the safety and sufficiency of the blood supply.⁷

TRAVEL ADVICE

To date there have only been two cases of WNV reported in the UK, both of which occurred in travellers returning from Canada during 2006 and 2007. Although there have been no imported cases of WNV in the UK since then, the renewed threat of bringing the disease back into the UK blood supply by returning travellers must not be underestimated.

Modeling of spread of the mosquito that carries WNV has shown that further areas that may become high risk include major tourist destinations, specifically France, Spain, Portugal, South Italy and parts of Turkey — as well as the US and Canada, mainland Greece, Romania, Israel, Albania and Russia.

Practice nurses should therefore remain vigilant to the possibility of WNV in returning travellers, particularly when assessing patients with WNV-type symptoms: a guide containing the latest advice for health professionals on how to recognise and diagnose WNV infections is available on the Public Health England website.^{8 CONCLUSION}

The main WNV risk appears to be via the transfusion route, as travel to areas where WNV is endemic is now commonplace among UK blood donors. Any travellers who may be incubating the disease following their return from high incidence areas are therefore now subject to the new strict donor deferral criteria aimed at removing the possibility of WNV entering the UK blood chain. The UK climate will undoubtedly be a major factor in controlling the possibility of a mosquito-borne outbreak of WNV here, nevertheless, no chances are being taken and the contingency plan to combat it is now ready and waiting.

REFERENCES

1. European Centre for Disease Prevention and Control. West Nile fever — Situation update. Available at: http://www.ecdc.europa.eu/en/healthtopics/west_nile_fever/West-Nile-fever-maps/Pages/index.aspx

2. Health Protection Agency. West Nile Virus. Available at : http://www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/ WestNileVirus/

3. Smithburn KC, Hughes TP, Burke AW, Paul JH. A neurotropic virus isolated from the blood of a native of Uganda. Am J Trop Med 1940;20 (1): 471—92.

4.Nash D, Mostashari 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(24): 1807—14.

5. West Nile Virus: A contingency plan to protect the public's health. Department of Health, May 2004. Publication 40168: DH Publications, PO Box 777, London SE1 6XH.

6. Davis LE, DeBiasi R, Goade DE, et al. West Nile virus neuroinvasive disease. Ann Neurol 2006;60(3): 286—300.

7. NHS Blood and Transplant. Annual review 2012 — 20013: Focus on Safety. Available at: http://www.nhsbt.nhs.uk/annualreview/blood-supply/focus-on-safety/

8. Public Health England. West Nile Virus — advice for health professionals. Available at : http://www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/WestNileVirus/GeneralInformation/wnile030WestNilevirusAdviceforhealthprofessionals/