West Nile Virus (WNV) is a member of the Flaviviridae family, the type species of which is Yellow Fever Virus (YFV) (Latin: flavus = yellow). Other members of this family include the Dengue viruses, Japanese Encephalitis, St Louis Encephalitis and Louping Ill Virus. All members of the flaviviridae are arthropod-borne viruses, known as arboviruses for short, which means that they are spread by insects (particularly mosquitoes, sandflies or midges), and ticks. West Nile Virus is transmitted by mosquitoes.

Transmission of West Nile Virus

Like all arboviruses, WNV multiplies in a haematophagous (blood-feeding) arthropod (in this case, Culex mosquito) and is transmitted by bite to a vertebrate in which it also multiplies. A viraemia ¹is produced of a sufficient level that the virus can be transferred to a subsequent blood-feeding arthropod, and so the cycle is perpetuated.
The arthropod host is known as the vector, and the principal vertebrate host (usually a mammal or a bird) is known as the reservoir. Hosts for WNV include wild birds (the main reservoir), horses and, more rarely, humans. Crows are particularly susceptible to West Nile Virus

Mammals, including humans generally do not develop a viraemia of a sufficient level for the cycle to be perpetuated, and so are known as dead end or incidental hosts.

As well as Culex species, at least 42 other species of mosquito have tested positive for WNV in the United States. These include Aedes, Anopheles, and Psorophora species.

A vector of concern in the United Kingdom is the Asian Tiger mosquito (Aedes albopictus), a tropical species, found primarily in Asia and West Africa. This is of concern because its eggs have been found in used tyres which are imported for retreading or recycling; and it is a mosquito that can survive temperate climates.

Geographical Distribution

The virus is named for the region of its initial detection: it was first isolated from the blood of a febrile woman in the West Nile district of Uganda in 1937. Subsequently the virus was isolated from human patients, birds, and mosquitoes in Egypt in the early 1950s. The virus was soon recognized as the most widespread of the flaviviruses, with geographic distribution including Africa, Europe, West Asia, the Middle East, and North America, including Canada. In temperate zone countries such as the United States and Canada, the disease occurs mainly in late summer and early autumn. Further south, the disease may occur throughout the year.

Signs and Symptoms

As humans are not the primary host, infections in human are rare and the symptoms are mild - being likened to those of influenza. Most people infected with WNV have no signs or symptoms at all. About 20 percent of people develop a mild infection called West Nile fever.

Apart from the fever, the symptoms of West Nile Fever also include headache and muscle aches similar to influenza. Occasionally there is a skin rash on the trunk of the body and swollen lymph glands. People thus afflicted tend to be those who are elderly, young, or have compromised immune systems.

These symptoms last only a few days and a full recovery normally ensues.

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Stupor or coma
• Tremors or muscle jerking
• Signs and symptoms of Parkinson's disease (Parkinsonism)
• Incoordination
• Convulsions
• Partial paralysis

Symptoms of encephalitis or meningitis may last several weeks and certain neurological effects, such as paralysis, may be permanent.

Human Outbreaks of West Nile Fever

Since the original isolation of WNV, human outbreaks have been infrequent and sporadic. Notable examples include:

• 1951-1954 and 1957 -- lsrael
• 1960's -- France
• 1962-1964 -- Mediterranean and central Russia
• 1974 -- South Africa
• 1970's and 1980's -- Belarus and Ukraine
• 1994 -- Algeria
• 1996 -- Morocco
• 1996-1997 -- Romania
• 1997 -- Czechland
• 1998 -- Italy
• 2000 -- Israel
• 2000 -- Southern France. (Detected in horses. First recorded case since the 1960's)
• 1998-2005 -- United States (Endemic)*

* By 2005 WNV had spread to virtually every State of the USA. Throughout the USA in 2003, there were 9122 cases of West Nile Fever, resulting in 223 deaths. The virus strain afflicting the USA seems to be more virulent than that affecting Europe, Asia and Africa.

Of concern are the facts that, since the mid 1990's, there has been an increase in the frequency of the disease in humans, an apparent increase in the severity of the human disease and high avian death rates associated with the human outbreaks in Israel and the United States.

Treatment

As the symptoms are so mild, treatment is not generally sought; and the infection is allowed to run its course and resolve itself.
ln more serious cases, supportive measures are the ones most commonly taken, as West Nile has no specific antiviral treatment. The patients are kept hydrated and attempts are made to reduce fever. Treatment with ribavirin, an antiviral drug, can be helpful within the first few days of infection.

Prevention

As there is no cure, prevention is obviously the best policy. ln areas of known outbreaks, wild bird species (particularly crows) are monitored to assist in the tracking of the virus. Birds found dead are examined for signs of the virus, and spraying of pesticide and other mosquito-abatement measures are taken in areas where infected birds are found.

Other precautions against infection where West Nile Virus is known to be present involve prevention of exposure to mosquitoes, including:

• Avoiding areas where mosquitoes live at dusk and at dawn (their peak feeding/biting times)
• Wearing long-sleeved shirts and long trousers when going outdoors for extended periods
• Using insect repellents
• Draining areas of standing water (mosquito breeding grounds)
• lnstalling screens on the windows of your home

What About a Vaccine?

A vaccine is available to protect horses from WNV. However, no vaccine is yet available for humans, although work to develop one is in progress.

The Situation in Britain

The increased prevalence of WNV in mainland Europe and America, which has claimed hundreds of lives in recent years, has led to fears that it could become endemic in Britain. Indeed, in 2004, two Irish tourists returned home from a holiday in Portugal, infected with WNV.

In the past, malaria, another mosquito-borne infection used to be prevalent in the Kent and Essex marshes, although no cases have been reported since the early 20th Century. However, global warming has led to fears that the climate will become warm enough for mosquitoes to proliferate. Indeed, scientists say that the climate in southern parts of Britain is already wet and warm enough to enable mosquito eggs to survive for months, even over winter.

Summary

Although WNV is not a serious threat to most humans at the present time, it can cause severe epidemics, and an attitude of caution is warranted. lf the virus is reported to be in your area, take steps to protect yourself from it, and pay attention to local news regarding movement of the virus. Though currently your location may not be threatened by it, the increase in air travel has made the spread of disease far easier and, thanks to a trans-Atlantic airliner harbouring a mosquito, or an infected traveller unwittingly carrying the disease to a new location, this virus could soon come to a mosquito near you!