Britain’s saltmarsh mosquito a potential vector for West Nile virus, study shows

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Electron microscopy of West Nile virus. Photo: PhD Dre CC BY-SA 3.0 via Wikimedia Commons
Electron microscopy of West Nile virus. Photo: PhD Dre at English Wikipedia CC BY-SA 3.0 via Wikimedia Commons

The common saltmarsh mosquito in Britain has been shown to be a potentially competent vector for spreading West Nile virus.

The mosquito-borne virus can infect a range of mammals, including humans and horses. Around 80 percent of humans infected with the virus have no symptoms, but 20 percent develop a fever and flu-like symptoms. Around 1% develop neurological illness which can prove fatal.

In horses, between 10% and 30% of infected horses show signs of illness, which can include neurological problems. Once signs of illness appear, the death rate is about 30%.

To date there has been no evidence of mosquito-borne virus transmission in Britain that would give rise to any public health concerns, despite the occurrence of 34 species of mosquito, 12 of which are known to be competent vectors of arboviruses elsewhere.

The saltmarsh mosquito, Ochlerotatus detritus, is locally common in parts of Britain, where it can be a voracious feeder on people. It is abundant through Britain’s coastal regions.

The study team said they had previously shown that field-collected saltmarsh mosquitoes were competent laboratory vectors of the Japanese encephalitis virus, showing potential for transmission by the mosquito seven days after infection at 23 degrees Celsius.

Given this, as well as the high abundance and biting nuisance of saltmarsh mosquitoes, the researchers said it was important to determine whether it was a laboratory-competent vector of West Nile, Dengue, and Chikungunya virus – three of the most globally important and invasive arboviruses – to determine the risk to Britain from this potential vector.

Researchers used adult mosquitoes reared from young, wild, field-obtained specimens for their study.

In their laboratory tests, they found that the saltmarsh mosquito was potentially competent to spread West Nile virus. Viral RNA was detected in the mosquito’s saliva 17 days after oral inoculation at a temperature of 21 degrees Celsius.

By contrast, there was no evidence of laboratory competence of the mosquito for either Dengue virus or Chikungunya virus.

“To our knowledge, this is the first study to demonstrate competence of a UK mosquito for West Nile virus and confirms that O. detritus may present a potential risk for arbovirus transmission in the UK and that further investigation of its vector role in the wild is required,” University of Liverpool researcher Marcus Blagrove and his colleagues reported in the journal Parasites and Vectors.

They said that further work was required to understand whether this laboratory competence translated into a risk of transmission in natural environments.

The study team noted that, with increasing global travel of both humans and livestock, as well as changing global climatic conditions, the geographic range of many mosquito-borne arboviruses had been increasing in recent decades.

The most prominent examples were the West Nile, Dengue and Chikungunya viruses.

West Nile had expanded its range from a small area of sub-Saharan Africa to the six major continents in the last 25 years. Outbreaks in Europe occurred annually and, given that the virus can be moved around the continent in migratory birds, there appeared to be a route of entry for the virus to Britain. It already occurred in regions with similar climatic conditions to Britain such as Canada, they noted, and antibodies had been detected in migratory and domestic birds in the UK, indicating that the country may be at risk of the establishment of the virus.

New Dengue cases globally are estimated to have increased 30-fold in the last 50 years.

“This can in part be attributed to an increase in the geographical range of the virus and an increase in the human population within, and travelling to, high risk areas,” they said.

Its geographical range had also increased over a similar time period, with more recent expansions believed to be the result of a range of novel mutations increasing the replication rate in the Asian tiger mosquito (Aedes albopictus).

There has been a significant expansion of Chikungunya virus to the Americas, with a large outbreak – more than 1 million cases – in the Caribbean region.

“The occurrence of these viruses circulating in geographical regions where there are increased numbers of UK travellers poses a potential risk for the virus to spread to the UK through infected travellers,” they observed.

“The range expansion of both of these viruses has included an extension into regions with cooler climates, including sporadic autochthonous transmission as far north as France, highlighting the potential future risk to the UK.”

Blagrove was joined in the study by fellow University of Liverpool researchers Ken Sherlock, Gail Chapman, Jolyon Medlock, Tom Solomon, and Matthew Baylis; Daniel Impoinvil, from the US Centers for Disease Control and Prevention; and Philip McCall and Gareth Lycett, from the Liverpool School of Tropical Medicine.

Evaluation of the vector competence of a native UK mosquito Ochlerotatus detritus (Aedes detritus) for dengue, chikungunya and West Nile viruses
Marcus S. C. Blagrove, Ken Sherlock, Gail E. Chapman, Daniel E. Impoinvil, Philip J. McCall, Jolyon M. Medlock, Gareth Lycett, Tom Solomon and Matthew Baylis.
Parasites & Vectors 2016 9:452 DOI: 10.1186/s13071-016-1739-3

The study, published under a Creative Commons License, can be read here

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