The war against West Nile Virus

In 2002, West Nile Virus arrived in New York and rapidly spread across the USA causing the death of almost 300 people. Barry Hill examines the background behind this disease and looks at its possible repercussions to the UK.

Larvae of Culex Mosquitoes. A shift in the feeding behavior of those mosquitoes helps explain the rising incidence of West Nile virus in North America. (Image: James Gathany, CDC)

WEST Nile virus (WNV) is an arthropod borne virus first isolated in Uganda in 1937, which is transmitted by mosquitoes, the principal host being wild birds. Humans and other animals such as horses however can be infected via mosquito bites and then become ‘incidental hosts’ for short periods. Confined historically to Africa and the Middle East, it then began to be reported in Eastern Europe, particularly the equine form in Rumania and Russia, before making its recent unexpected appearance in the Western World. The disease was first identified in the USA in a 1999 outbreak in Queens, New York. During 2000-2001 WNV then spread throughout much of the USA, Canada and the Caribbean causing a major seasonal epidemic which resulted in almost 10,000 cases - 262 of which proved fatal. As of November 2007, 3,265 cases of WNV related disease and 92 deaths have been recorded in US states in 2007 alone by the US Centers for Disease Control and Prevention (CDC). Although the origin of the initial New York outbreak of WNV is uncertain, possible causes affecting the mosquito population relating to climatic changes, long-haul air travel and land-use appear to be the most likely explanations.

The incubation period for WNV in humans is 3-15 days, most cases are asymptomatic or result only in mild flu-like symptoms for 3-6 days, with full recovery soon following. In around 1 in 150 cases however, a severe form of the disease can develop which may prove fatal in elderly or immunosuppressed patients. In these cases, WNV can lead to encephalitis and meningitis, pancreatitis and fulminant hepatitis. Patients suspected of severe WNV should therefore be admitted to hospital and isolated, exposure to their body fluids then kept to a minimum to reduce the risk of transmission to health workers. Although cases of WNV have been reported in Europe, to date there have been no known reported cases here 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. But according to Chief Medical Officer Sir Liam Donaldson, although the chances of WNV arriving in the UK have been assessed as low, the possibility can not be ruled out, commenting “West Nile Virus no longer respects the geographical boundaries that once restricted it to the Old World”. Because of this, the CMO outlined details of plans to be adopted in the event of a UK-acquired case of WNV infection being diagnosed aimed at enhancing surveillance, alerting clinicians of its symptoms, and controlling mosquito populations. In the document ‘West Nile Virus: A contingency plan to protect the public’s health’, the CMO defines the roles and responsibilities of the parties involved in tackling the mosquito-borne virus, as well as presenting a strategy for limiting its impact.

 Key issues outlined in the plan include:

• Surveillance of people, horses, birds and mosquitoes for evidence of WNV infection.
• Laboratory diagnosis of WNV.
• Patient care and protection of healthcare professionals.
• Public health action and the role of veterinary authorities.
• Public protection.
• Environmental control of mosquito populations.

The risk of transmission of WNV ‘person-to-person’ either by blood transfusion, organ transplantation or from mother to baby must not be overlooked. In the USA epidemic several patients were later confirmed to have acquired WNV via the transfusion of blood components, and a case of an infant being infected via breast milk was also reported, as was laboratory-acquired infection by scientific staff. But by far the biggest fear in this category relates to the transfusion risk, specifically to donations given following the first 1-3 days after infection in the donor, and because of this, guidance has now been issued by the UK blood services aimed at combating this risk. In a statement on WNV issued by the Standing Advisory Committee on Transfusion Transmitted Infections, new blood donor deferral criteria were recommended to update earlier precautionary measures already in place. These state that any donors who have visited a WNV risk area between the peak months of June and November should defer donating blood for 28 days from leaving the affected area, unless a validated ‘WNV NAT’ ( a nucleic acid testing based 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 any donor subsequently becoming ill with WNV-type symptoms following donation would also be followed-up. One other possible transfusion related transmission risk is that posed by imported fresh frozen plasma (FFP) from the USA, which is now being used here in the UK for clinical use as a vCJD pre-cautionary measure for anyone born after January 1996. However, a process known as methylene blue treatment for FFP has also been introduced, which inactivates WNV and other transmissible viruses to make the risk negligible. Although there is currently no human vaccine for the prevention of WNV, this is set to change soon as reported in the December 2007 Laboratory News, which stated that the first ever WNV vaccine is now under development by UK-based company Acambis in collaboration with Sanofi Pasteur. Acambis is currently conducting Phase 2 trials of their ChimeraVax-West Nile vaccine which is showing promising results and could be available as early as next year.

With air travel to the USA and Canada common place amongst UK blood donors however, the main WNV risk therefore appears to be via the transfusion route. Any travellers who may be incubating the disease following their return from high incidence areas in the appropriate season are now subject to strict donor deferral criteria described above, aimed at removing the possibility of WNV entering the UK blood chain. Although 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 planning to combat it is now waiting in the wings. But as the CMO points out - “West Nile virus is no longer limited in its geographic distribution”- as the people of New York have discovered to their cost. As such there is no room for complacency, the warnings are there to see, West Nile virus has mobilised and the UK must therefore be ready for it.

By Barry Hill. Barry has workled in pathology for over 30 years and specialises in blood transfusion and haemotology.