West Nile Virus (WNV)

What is West Nile Virus (WNV)?
West Nile Virus (WNV) belongs to a group of viruses known as 'arboviruses' (viruses that are transmitted by insects). WNV is commonly found in Africa, West Asia, Australasia, the Middle East and in North America. There have been sporadic cases of WNV in a number of European countries in recent decades. The virus can infect humans, birds, mosquitoes, horses and some other mammals; birds, however, are its normal host. It generally produces only a very mild illness in humans, but like many other arboviruses, can produce serious infection of the brain. It is spread by the bite of an infected mosquito.

The virus was first identified in 1937, in a woman in the West Nile District of Uganda and has been a cause of mild illness in Africa and elsewhere since then. The first account of outbreaks of more severe illness came from Israel in the 1950’s. Although WNV tends, in humans, only to produce mild illness, it is quite often fatal for many types of birds. Authorities in the US and Canada have been using the appearance of dead birds as an early warning system for detecting the appearance of WNV in a particular locality.

How do people get infected with WNV?
People are infected by the bite of an infected mosquito. Mosquitoes that carry WNV generally prefer to bite birds but will sometimes bite and infect humans and animals. Less than 1 percent of people who are bitten by mosquitoes develop any symptoms of the disease. In countries in the temperate zones of the world such as Russia and Northern North America, which tend to be warmer than Ireland, West Nile encephalitis cases normally occur in the late summer or early autumn. In tropical climates where temperatures are higher and mosquitoes are active throughout the year, West Nile virus can be transmitted year round. WNV is a notifiable disease in Ireland.

How serious is infection with WNV?
Most cases are not serious. WNV is generally a very mild illness; in fact most people who develop it are unaware that they have been infected. Very occasionally, people will develop serious and, at times, life-threatening symptoms. Children are not particularly vulnerable and serious infection is very rare in youngsters.

What are the symptoms of WNV?
WNV principally affects the central nervous system (the brain and spinal cord). The symptoms vary according to the severity of infection:

a) No Symptoms: After being bitten by an infected mosquito, about 80% of people will have no symptoms at all.

b) Mild Symptoms: Another 20% will develop a mild influenza-like illness, with fever, headache and generalised aches and pains (sometimes known as West Nile Fever). People who develop West Nile Fever are only ill for between 3 and 6 days and generally make a full recovery.

c) Severe Symptoms: A small proportion of infected people (less than 1%) will go on to develop more severe disease. Severe infection may cause inflammation of the brain, the spinal cord and their protective layers (severe forms of the infection are known as encephalitis and meningoencephalitis). This more severe form of illness produces headache, high fever, stiff neck, sore eyes, disorientation, muscle weakness, convulsions and coma. It can be fatal; about 7% of those who developed severe symptoms during the US outbreaks died as a result of complications of infection. Most deaths have been reported in those over 50 years of age, who generally suffer more severe disease.

The risk of developing severe symptoms rises with increasing age. People over the age of 50 are about 10 times more likely than children and young people to develop severe symptoms; the risk for those over 80 years of age is almost 50-fold higher. People with weakened immune systems may be also more vulnerable to severe disease.

Have there been any recent changes in West Nile virus activity?
West Nile Virus disease appeared for the first time in North America in 1999. Over the last decade, the West Nile activity in the US has spread across North America. Each year for example, the US sees between 1200 and 1500 cases of nervous system infection due to WNV. About three thousand cases of West Nile Fever, the milder form of the illness, are also reported in the US. 

Cases of human disease have also occurred in the Eastern Hemisphere. There have been outbreaks in recent years in Bucharest, Romania (1996, over 400 human cases, 17 deaths), Volgograd, Russia (1999, over 800 human cases, 40 deaths), and Israel (2000, over 400 human cases, 29 deaths). In 2003, one travel-associated case occurred in The Netherlands, and two cases in France were thought to have been acquired locally or in Spain. Two Irish cases who acquired their illness in Portugal were reported in July 2004.

What are the risks associated with travel?
In Europe, the risk of acquiring WNV is considered very low. Southern Moravia in the Czech Republic, Southern France, the Danube Basin and Southern Portugal are the only areas recording human cases. In North America, although substantial numbers of cases have been reported during summer months, the risk to Irish travellers is also considered to be low.

Does the appearance of cases in these areas of Europe mean that there is an increased risk of contracting the disease there?
The risk of contracting West Nile Virus throughout Europe is generally considered to be very low. Although the virus may have been circulating among animals and mosquitoes for some years, this remains a very uncommon human disease in Europe. However, while there is a low possibility of contracting the disease in these areas, most people who become infected do not get symptoms and fewer than 1% of those infected will develop a serious illness.

Where else in the world should I be on the lookout for WNV?
WNV and other similar viruses are found throughout the tropics and subtropics, wherever mosquitoes are found. In the recent past cases have been reported in the Middle East and North Africa, and from Western and Central Asia.

Travellers to these areas should be aware of the low risk of WNV and take the necessary precautions to avoid mosquito bites. Anyone developing symptoms and seeking medical attention following return from abroad should tell their GP about their travel itinerary.

What is the public health risk in Ireland?
The public health risk of acquiring WNV in Ireland is considered to be minimal.

What sorts of activities while on holiday increase your risk of being bitten by mosquitoes?
In general, people who spend a lot of time outdoors are more likely to be bitten by mosquitoes. People whose work or hobbies bring them near to water (especially stagnant or still water and water containing leaves and other organic material) will be at increased risk as well. These would include anglers and those walking or working close to ponds, marshes and lakes. Mosquitoes, however, can be found near any still water even birdbaths or pools in urban and suburban areas.

Can WNV be passed directly from one person to another?
No. WNV is not passed from one person to another. The most likely way in which infection can be passed is from the bite of a mosquito, which is infected because it has fed on the blood of an infected bird. However, some people in the United States appear to have become infected with WNV after receiving blood transfusions and organ transplants. These blood donors, although infected with WNV appear to have been asymptomatic.

To protect the safety of transfusion recipients in Ireland, the Irish Blood Transfusion Service (IBTS) have introduced deferral measures to ensure that donors returning from affected areas do not donate for 2 months following their return and any donors who have been infected are deferred from donating for 4 weeks following full recovery. Further information about WNV and blood donation can be obtained from the IBTS on Freephone 1850-731137 or on the IBTS website. 

How long does it take to develop symptoms?
The incubation period (from being bitten to developing symptoms) is usually between 3 and 15 days.

How can you protect yourself against WNV?
The best way to protect yourself against WNV, is to protect yourself against mosquito bites. The risk of being infected, if you travel to North America or other affected areas, is low. To date, there have been only two reports of Irish travellers acquiring infection while abroad. Most people who are infected do not get symptoms; serious illness is rare. However, it is important that travellers to areas where there is a risk of becoming infected, (especially those over 50 years of age, and most especially those over 80, who will be at greatest risk of developing severe disease) are aware of this small, potential risk and take the appropriate anti-mosquito measures.

NB: Mosquitoes carrying WNV are most active early in the morning and in the evening

• Time: Take particular care at times when mosquitoes are most likely to bite: dusk and dawn, from April until October in temperate climates and all year round in tropical areas.
• Risky Areas: Avoid areas where mosquitoes are likely to be found, i.e. near water including ponds, outdoor swimming pools, lakes and marshes.
• Use Insect Repellents: Spray onto exposed skin whenever you are in an area where mosquitoes may be present.
• Dress Safely: If in areas where mosquitoes are likely, wear long sleeves, long trousers, socks and closed shoes. Clothing may also be treated with insect repellents.
• Indoors: mosquito bites can be reduced by air conditioning, insect-proof screens on windows and doors and spraying the room with insecticide.
• Mosquito Nets: Bed nets and cot nets can be used if necessary and are particularly effective if treated.
• Day Biting: WNV has been found in some day-biting mosquitoes too, so reducing bites at any time is a sensible precaution.

Taking these simple measures will also help to protect you against other mosquito-borne diseases, such as malaria, yellow fever or dengue.

What is the treatment for WNV infection?
There is no specific treatment for infection with WNV, nor has a vaccine against WNV been developed. Most infected people do not know they are ill, as they have no symptoms. Those with the mild WNV recover quickly with simple symptomatic (‘cold and flu’) treatment. People, who become seriously ill will need to be admitted to hospital for specialised supportive treatment with intravenous fluids, assistance with breathing and nursing care.

What should I do if I become ill on returning from abroad?
The risk of WNV infection among returned travellers from abroad, and particularly Europe, is considered to be very low. It is not that unusual for travellers to acquire tummy bugs and other mild infections while abroad. Most mild illnesses do not need medical attention. However, if you do become unwell and are concerned, you should discuss your concerns with your doctor making sure to tell him or her in which countries you travelled.

Where can you get further information about WNV?
The following are valuable resources, providing current information on WNV:

Centers of Disease Control and Prevention

European Centre for Disease Prevention and Control

Health Canada

Health Protection Agency

UK National Travel Health Network and Centre

Updated: 10th July 2010