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The following Q&A provides more information on the news story "Continental mosquito with ‘vector’ potential found breeding in UK after 60-year absence" 

9 February 2012

Q&A: The mosquito

1. What is Culex modestus?

Culex modestus (which has no common name in English) is a species of mosquito that is locally abundant in southern Europe, middle and south-west Asia, north India and north Africa. In Europe it has occasionally been found further north in northern France and the Netherlands. The species breeds primarily in wetlands, marshes and inundation areas of meadows, rivers and rice fields and regularly bites both birds and mammals. Like most other mosquitoes adult female Culex modestus require a blood meal before they can lay eggs. After mating and taking a blood meal, female Culex modestus lay eggs on the surface of fresh or brackish water bodies. The eggs hatch into larvae and pass through four larval developmental stages in the water before pupating and emerging as adult mosquitoes.  Culex modestus survives the winter as an adult finding shelter within dense vegetation.

2. Why is it of public interest that Culex modestus has been found in the UK?

Culex modestus is known to transmit West Nile virus (WNV) between birds and people in southern Europe and western Russia. The mosquito is also suspected of transmitting other viruses (Lednice, Tahyna and Sindbis).

One of the main reasons that Culex modestus is able to transmit WNV to people is its preference to bite both birds and humans. WNV is primarily a virus of birds. Humans only become infected when a mosquito like Culex modestus first bites a bird and later bites a human.

The only previous record of Culex modestus was a handful of specimens caught in and around Portsmouth in 1944-45. No established populations were found however. If this British population also feeds on both birds and humans, and is able to transmit the virus, then the presence of Culex modestus may increase the risk of WNV transmission to people, should the virus be introduced to the UK.

3. Where has Culex modestus been found in the UK?

Culex modestus appears to be established in the North Kent marshes and some adult specimens have been found in marshes in south Essex. All of the mosquitoes were found in areas of coastal grazing marsh though not all such areas of marsh were found to be occupied. The specific habitat that this species prefers is currently being determined by scientists from the Centre for Ecology & Hydrology (CEH), and its range by CEH and Health Protection Agency (HPA).

4. How sure are you that the species is Culex modestus?

Species identification was determined independently by experts. To confirm the identification, genetic techniques (DNA barcoding) were used to compare UK specimens with Culex modestus mosquitoes from southern France.

5. Have the Culex modestus caught in the UK been tested for WNV?

Adult Culex modestus have been tested by the Health Protection Agency and have not been found to contain WNV. However further testing is planned that will focus on collecting larger numbers of the species, and following protocols designed to maximise the longevity of the virus in the samples.  The virus degrades quickly and, even during epidemics, the occurrence of WNV in individual mosquitoes is extremely low (<1%). Insufficient numbers of appropriately collected mosquitoes have been sampled to date. CEH and the HPA will work to screen Culex modestus in 2012, though looking for antibodies in birds and mammals may be a better way to find the virus. As described below (question 13) there is no evidence that WNV is present in the UK.

6. How has Culex modestus arrived in the UK?

No one can be sure how the species arrived in the UK. As most mosquitoes (including Culex modestus) are not strong long-distance flyers it is unlikely that it flew here from mainland Europe though this cannot be completely ruled out.

This species, and many others, have previously been accidentally transported over long distances by international shipping. This seems the most likely route of introduction of Culex modestus to the UK given the number of international shipping terminals in the area where the species has become established.

It is not absolutely certain that this species has arrived only recently in the UK. Expert mosquito surveillance has been erratic in the UK and it is conceivable, though unlikely, that Culex modestus has been in the UK since at least 1945 when it was first identified.

7. Has Culex modestus become established because of global warming?

There is currently no evidence to suggest that global climate change has made the UK suitable for Culex modestus. The species is able to survive much colder winters (such as in Volgograd, Russia) than those in the UK. The main restriction on where this species is able to persist appears to be the presence of suitable wetland habitat in which the larvae can breed.

8. Should I send samples of mosquitoes anywhere?

The Health Protection Agency runs the UK’s Mosquito Recording Scheme, details of where and how to send samples of mosquitoes can be found here:

9. What time of year does this species bite people?

Adult Culex modestus mosquitoes appear to be most abundant during July and August, but it is impossible to give a precise ‘season’ at present. Females can be active during the day but they mainly feed at dusk or at night. Culex modestus bite people in continental Europe but have not yet been proven to do so in the UK.

10. There's a lot of wetland in the UK - could the mosquito move into other areas?

Scientists at CEH and HPA are currently working to find out where the mosquito is present and which areas it could potentially move into. As mentioned before, our initial studies indicate that the species breeds in coastal grazing marsh but also seems to have quite specific habitat requirements, so certainly not all areas of coastal grazing marsh are at risk.

11. Can Culex modestus transmit malaria?

Culex modestus cannot transmit human malaria. The species is a member of the mosquito sub-family Culicinae. Only species of the sub-family Anophelinae are known to transmit human malaria.


Q&A: The virus

12. What is West Nile Virus (WNV)?

West Nile Virus (WNV) is a flavivirus that is maintained by wildlife, in particular birds. It is transmitted by a number of different species of mosquitoes. The virus occasionally ‘spills over’ into man and horses when they are bitten by a WNV-infected mosquito.  WNV is related to Japanese encephalitis, Murray Valley encephalitis, and St Louis encephalitis and has occasionally been associated with encephalitis in people, horses and some species of birds.

WNV routinely infects wildlife and occasionally man in parts of Africa, the Middle East and South West Asia. More recently, outbreaks of WNV have been reported in more temperate zones, including Eastern Europe, Russia, France, Italy and Portugal. WNV appeared in the United States for the first time in 1999. Since then it has spread from New York to the whole of North America causing over 1000 human deaths and the death of many thousands of birds and horses.

13. Has WNV ever been found in the UK?

To date, no evidence for the presence of WNV in the UK has been obtained. Every year since 2001 the Veterinary Laboratories Agency (VLA) has performed surveillance for WNV by looking for the virus and for antibodies that bind the virus (indicative of prior infection) in specific species of wild birds in the UK. None of the bird samples tested have proven positive for WNV.

14. What are the clinical signs of WNV infection?

In people, approximately 80% of WNV infections do not cause any symptoms. However when they do occur, the most common symptoms are a fever and headache, but can include a rash, swollen lymph nodes and conjunctivitis. In severe cases, the symptoms include the rapid onset of severe headache, high fever, stiff neck, disorientation, muscle weakness, paralysis, coma or death. Horses and other equines (ponies, donkeys, etc.) show listlessness, lack of coordination, weakness of limbs, ataxia, partial paralysis or death. A fever is not usually observed in equines.

15. Can other species of British mosquito transmit WNV to humans?

There are 34 species of British mosquito. Some may be able to transmit WNV between birds and a few (12 species other than Culex modestus) have the potential to transmit WNV from birds to man (so called ‘bridge vectors’). However elsewhere in Europe, of those British mosquito species, only Culex modestus,  Culex pipiens molestus and Coquillettidia richiardii have been confirmed to have a significant role in transmitting WNV to people, with Culex modestus most often implicated as the bridge vector to people.

16. What can I do to eliminate the risk of getting WNV?

As stated above there is no evidence that WNV is present in the UK so there is no need to change your current behaviour or avoid these wetlands. There is currently no vaccine for WNV so if the virus does appear in the UK the best way to decrease your risk of infection would be to avoid exposure to mosquitoes: cover up, use an effective insect repellent and avoid being in areas with abundant mosquitoes at dawn and dusk when the majority of mosquito species are most active.

17. Cattle, sheep and horses are frequently found on the marshes. Are they at risk?

Horses are susceptible to WNV in a similar way to humans, but sheep and cattle are thought to be much less susceptible. However since there is no evidence that the virus is present in the UK, there is currently very little risk.

18. Could Culex modestus be eradicated from the UK?

We are investigating the current distribution of Culex modestus in the UK and its precise habitat requirements. This knowledge would be crucial in being able to predict possible future distribution ranges of this species and in providing scientific data for policy makers evaluating the risks and benefits of any potential eradication programme.

Back to the news story: Continental mosquito with ‘vector’ potential found breeding in UK after 60-year absence


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