West Nile Virus General Information
2. When and where was West Nile virus first detected as a cause of disease?
3. Is West Nile virus mainly an infection of humans?
4. What is the transmission cycle of West Nile virus?
5. Are other animals and humans important in transmitting on the infection?
6. How do people get infected with West Nile virus?
7. How long after the bite of an infected mosquito may a person develop symptoms?
8. Can I get West Nile virus directly from an infected person?
9. What are the symptoms of West Nile virus?
10. Where in the world is West Nile virus known to exist?
11. Have there been any recent changes in West Nile virus activity?
13. How does West Nile virus spread to new areas?
14. Is there any West Nile virus activity in the UK?
15. What is the risk of West Nile virus causing human cases in the UK?
17. What is the HPA doing about West Nile virus?
18. Should any precautions be taken to avoid West Nile virus in the UK?
19. What precautions do I need to take against West Nile virus if I am going to North America?
21. What is the risk in the rest of Europe?
22. What is your advice to people travelling in Europe this summer?
23. I have just returned from a European country and am not feeling well, what should I do?
West Nile virus (WNV) belongs to the group of viruses known as arboviruses which are largely transmitted by arthropod insects. Within the group of arboviruses, there are many subgroups or families of virus and West Nile virus comes from the family called the flaviviruses. Other members of this family include the Murray Valley Encephalitis and St Louis Encephalitis viruses. Infection with WNV is a zoonosis, a disease which can be passed from animals to humans.
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2. When and where was West Nile virus first detected as a cause of disease?
The virus itself was first isolated from a woman with a fever in 1937 in the West Nile district of Uganda. WNV was first recognised as a cause of a human illness known as meningoencephalitis (inflammation of the spinal cord and brain) in Israel in 1957 and as a cause of horse disease in Egypt and France in the early 1960s.
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3. Is West Nile virus mainly an infection of humans?
No. WNV is mainly an infection of birds and mosquitoes.
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4. What is the transmission cycle of West Nile virus?
Birds are the normal host which become infected with WNV. The virus circulates in the blood of the bird, so when mosquitoes take a blood meal from an infected bird they may take up the virus as well. The mosquito can then pass on the virus when feeding on another bird. The virus is amplified by continuous transmission in this way between birds and mosquitoes and transmission is therefore increased where large numbers of mosquitoes are close to suitable bird populations.
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5. Are other animals and humans important in transmitting on the infection?
No. They are not important in maintaining transmission cycles since they rarely develop high enough levels of virus in their blood stream necessary to infect mosquitoes. They are incidental victims.
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6. How do people get infected with West Nile virus?
By the bite of an infected mosquito. Mosquitoes that are involved in the transmission of WNV generally prefer to take blood meals from birds but will sometimes bite and infect humans and animals.
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7. How long after the bite of an infected mosquito may a person develop symptoms?
The incubation time is usually 3 to 15 days.
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8. Can I get West Nile virus directly from an infected person?
No. WNV is not transmitted from person-to-person through close contact. However, in 2002 some people in the United States became infected with WNV after receiving blood transfusions. The people who donated the blood had very mild forms of the illness which were not recognised. People wishing to donate blood in the UK are therefore asked to defer if they have recently been to an endemic country. Very rare cases have also occurred following organ transplantation.
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9. What are the symptoms of West Nile virus?
Eighty percent of those infected have no symptoms at all, and 20% have a mild influenza-like illness (fever, headache, body ache). However, a small proportion (less than 1%) develop more severe disease, such as inflammation of the brain (encephalitis), inflammation of the membrane around the brain and spinal cord (meningitis) or inflammation of the brain and the membrane (meningo-encephalitis). Patients may suffer headaches, fever, stiff neck, sore eyes, disorientation, muscle weakness, convulsions and coma. Occasionally, infected people may die. Most deaths have been reported in those over 50 years old, who generally suffer more severe disease.
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10. Where in the world is West Nile virus known to exist?
WNV has now been isolated in insects, animals and humans in many countries, including those in Africa, Europe, the Middle East, west and central Asia, Oceania (where there is the related Kunjin virus), and the Americas. The virus has caused sporadic cases and outbreaks of disease in humans and horses in Europe since the 1960s.
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11. Have there been any recent changes in West Nile virus activity?
Since 1999 when WNV was introduced into North America, there has been significant spread of WNV across North America, and later to Canada, Mexico, Central America, and the West Indies. In 2003 nearly 10,000 cases of human WNV were reported in the USA, however numbers have declined since then: in 2010, 1021 cases were reported. Canada has also seen a notable decline with only 4 cases reported in 2010, compared to the peak occurrence of 2215 reported cases in 2007. WNV is now considered endemic in the USA and Canada.
Over the last decade, disease has also occurred in the Eastern Hemisphere. In 2005 an outbreak of 73 cases (3 deaths) was reported from the Astrakhan region of Russia. In 2008/09 increased WNV activity was reported in Southern Europe, including human cases of WNV in Italy, Hungary and Romania. During 2010, an outbreak of WNV was reported in Greece with over 260 human cases. See here for West Nile activity in 2011.
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12. Is the disease seasonal?
In countries in the temperate zones of the world such as Russia and North America, West Nile encephalitis cases normally occur in the late summer or early autumn, which tend to be much hotter than the UK. In tropical climates where temperatures are higher, WNV can be transmitted year round.
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13. How does West Nile virus spread to new areas?
Much of the expansion of WNV activity is thought to be caused by infected migrating birds flying from areas with known virus activity. However, it is also possible that imported birds, mosquitoes and other animals can introduce the infection into new areas. It is not exactly clear how the virus spread to the USA, but the strain of WNV found in the USA is most closely related genetically to strains found in Israel. Climate and environmental changes, including land use, which serve to increase mosquito populations may favour the establishment of WNV were it to be introduced into a new area. The geographical spread of WNV reflects the global resurgence of arboviruses in general. Population growth, urbanisation, increased international travel, changing agricultural practices and climate change have all been postulated as the cause of their expanding geographical distribution and increased epidemic activity.
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14. Is there any West Nile virus activity in the UK?
No active disease in birds has been detected to date in the UK. However, some work by the Centre for Ecology and Hydrology indicated that certain species of UK birds may have been exposed to WNV. Despite this, there have been no reported human cases of UK-acquired infection. Further work in UK birds, mosquitoes and horses has failed to find any evidence of WNV infection. If WNV is a possible diagnosis in a patient with encephalitis or viral meningitis, information on how to get specimens tested is available here.
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15. What is the risk of West Nile virus causing human cases in the UK?
The risk of WNV causing human cases in the UK is considered to be low. The spread of WNV to the USA has made countries that previously considered themselves at low risk of WNV reassess the situation. It is important to bear in mind that the situation in the USA is very different from the UK. WNV was introduced into the North American continent where it had not been identified before 1999. In Europe, migrating birds from WNV endemic parts of Africa have been regular visitors for many centuries. Despite this, disease due to WNV infection in humans is seemingly rare in Europe. The species of mosquitoes that may transmit the infection (Culex spp.) are present in the UK but are unlikely to be numerous enough to sustain transmission to humans. It is also important to remember that other mosquito-borne diseases are reported every year in the USA, where mosquito biting is considered more of a nuisance, and these diseases are not found in this country. Additionally, it should be noted that dead birds heralded the introduction of WNV into the USA; no bird die-off has been reported in the UK.
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16. Why have we encouraged doctors in the UK to consider West Nile virus as a cause of unexplained encephalitis and viral meningitis?
Although we consider the risk of WNV transmission to humans to be low, it cannot be ruled out, and it would be incautious to do so. We need to know more about the number and biting patterns of mosquitoes in the UK to be more certain about precisely how low the risk is. Further knowledge will come from studies of birds and mosquitoes.
In the first WNV outbreak in the USA, WNV was not initially considered as a diagnosis in cases of encephalitis and viral meningitis. Therefore, to learn from their experience, it is sensible to consider WNV as a differential diagnosis in the summer months in the UK. Of course, encouraging doctors to report more cases of encephalitis and viral meningitis will also improve the information we have on the other infections that are actually more likely to cause these conditions.
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17. What is the HPA doing about West Nile virus?
The HPA carries out surveillance of infectious diseases all the time, and we have no evidence that there are any WNV cases in people in the UK. However, should any human WNV cases occur in this country, we will be able to give early warning so that appropriate action can be taken. If a patient presents with possible viral meningoencephalitis, the doctor should consider WNV as a possible diagnosis alongside the other more likely causes of the disease. Patients aged over 50 years are the most likely to present with neurological symptoms.
Of course, surveillance of human infection is only one part of the picture. Monitoring bird, animal and insect populations also generates important information. This is not the direct responsibility of the HPA, but we are working closely with the Department of Health (DH), who coordinate the response to WNV, and our veterinary and other colleagues who carry out surveillance in these areas. The DH published the Chief Medical Officer's West Nile virus contingency plan in 2004.
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18. Should any precautions be taken to avoid West Nile virus in the UK?
We do not currently advise any particular precautions, as we have no evidence of WNV infection in the UK. This advice will be updated if, in the future, WNV infection is reported in mosquitoes, animals or people in the UK.
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19. What precautions do I need to take against West Nile virus if I am going to North America?
Although WNV is now considered to be endemic in North America, the risk of acquiring WNV infection if you travel to the USA or Canada is low. There have only been two reports of persons coming to the UK with the disease. Most people who are infected do not get symptoms and less than 1% of persons infected with WNV will develop serious illness. However, it is important that travellers, particularly those over 50 years of age who are more at risk of developing severe disease, going to areas where there is active transmission to humans are aware of the potential risk and take the appropriate anti-mosquito measures.
Most of the mosquitoes that carry WNV are especially likely to bite around dusk and dawn. If you are outdoors around these times of the day, do everything you can to reduce mosquito bites. Wear loose fitting, light-weight clothing that covers up skin as much as possible. Use an effective insect repellent on exposed skin and clothing in accordance with the manufacturer's instructions. Indoors, mosquito bites can be reduced by air conditioning, insect-proof screens on windows and doors and spraying the room with insecticide. Bed nets and cot nets can be used if necessary. There has been virus 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, which are occasionally encountered in North America.
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20. Is there any chance of getting West Nile virus infection from a blood transfusion in this country?
This matter has been considered carefully by the UK Blood Transfusion Services, who review the situation each year. Although it is felt that the risk of transfusion-associated WNV infection occurring in the UK is low, testing for WNV travel area donors is being introduced from 1st May 2012. http://www.blood.co.uk/can-i-give-blood/west-nile-virus/. The USA and Canada now screen all donated blood, so any blood products imported from there will have been tested for WNV.
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21. What is the risk in the rest of Europe?
The risk of contracting WNV throughout Europe is generally considered to be low. Various studies have shown evidence of WNV infections in humans, animals and mosquitoes and there have been sporadic cases and outbreaks of WNV in a number of European countries in recent decades. Two clinical cases were reported in 2004 in residents of the Republic of Ireland who had been staying in Portugal in a wetland area of swamp bird reserves where there were many mosquitoes. In recent years (Europe 2010, Europe 2011), human cases of WNV have been reported in Italy, Hungary, Romania and Greece. However, most people who are infected do not get symptoms and less than 1% of those infected develop serious illness.
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22. What is your advice to people travelling in Europe this summer?
Several diseases can be acquired from insect bites in the spring, summer, and autumn months. These include the mosquito-transmitted infections such as WNV and other encephalitis viruses, and the tick-transmitted diseases Lyme borreliosis, tick-borne encephalitis, rickettsial infections and babesiosis.
The advice to people travelling in Europe this summer is that they should take the usual appropriate anti-mosquito precautions. These include:
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Wear loose fitting, light-weight clothing that covers up skin as much as possible.
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Limit outdoor exposure during peak times of mosquito feeding - usually the hours from dusk to dawn - persons are usually exposed in the early morning and evening times. The safest course is to use repellents when outdoors.
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Apply insect repellents to exposed skin and clothing. Always follow manufacturers' directions for use. Repellents that contain DEET are considered the most effective.
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Indoors mosquito bites can be reduced by air conditioning, insect-proof screens on windows and doors and spraying the room with insecticide. Bed nets and cot nets can be used if necessary.
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Mosquito coils can be burned or vaporizing mats used in enclosed areas if needed.
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Avoid areas where there are likely to be large concentrations of mosquitoes or other biting insects.
Further information on insect avoidance can be found at the National Travel Health Network and Centre following the links:
1. NaTHNaC insect avoidance Europe 2. NaTHNaC general insect avoidance
Further information on mosquito borne diseases in Europe.
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23. I have just returned from a European country and am not feeling well, what should I do?
The risk of contracting WNV in Europe is considered to be low and therefore it is unlikely to be the cause of your illness. Most people who come into contact with WNV will have no symptoms, 20% develop a mild influenza-like illness and less than 1% of people develop more severe disease. However, if you do have symptoms you should discuss these with your GP and always remember to tell him/ her about recent travel.
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Last reviewed: 23 April 2012
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