The ongoing Zika virus outbreak in the Americas has now had confirmed cases in the UK and other parts of Europe. The disease appears to be linked to an increase in babies born with microcephaly as well as a serious neurological disorder called Guillain-Barré syndrome. Could mosquitoes here also transmit the disease?
Scientists believe that Zika virus is primarily being spread by the Aedes aegypti mosquito. Although there have also been rare cases of sexual transmission, the overwhelming majority of cases are transmitted by mosquitoes (known as vectors for the disease). The virus replicates and spreads inside the mosquito, eventually reaching its salivary glands, where it can be injected into a human with a bite.
Aedes aegypti is particularly effective at spreading the virus – it preferentially bites humans and is very well adapted to living in urban environments. Aedes mosquitos bite during the day, which makes bed nets useless, and they also show a preference for breeding in artificial water containers. They can lay their eggs in water that collect almost anywhere, including in plastic bags and discarded bottle caps. Coupled with their aggressive biting behaviour, this makes them very difficult to control.
Aedes aegypti mosquitos are very rare in Europe and absent from the UK, living mainly in tropical and sub-tropical parts of the world, but they may not be the only important species in this outbreak. Scientists in Brazil have reported that Culex quinquefasciatus, a more common mosquito species, can be infected with Zika in the lab. So can Aedes albopictus, a species that can be found in many parts of the Mediterranean (but is also absent from the UK).
Researchers are trying to work out how important these different vector species might be in the current outbreak. Different species require different control strategies, depending on their distribution, activity cycles, biting behaviour and habitat preference. For example, you need to know where a species breeds if you want to make larvicide spraying effective. But in practice, establishing which species are involved isn’t always straightforward.
“There are often a whole range of mosquito species you can infect with a pathogen in the lab,” explains Dr Anthony Wilson, who leads a group studying vector-borne diseases at the Pirbright Institute. “But whether they play an important role in the field is another matter.
“It boils down to the species’ behaviour, such as its biting preferences, and also how abundant it is.” In addition, environmental conditions like temperature have an impact on the development of the pathogen inside the mosquito, and on how likely the mosquito is to survive the pathogen’s incubation period.
In previous outbreaks, like the one in French Polynesia in 2013, Zika has been transmitted by a large number of different Aedes mosquito species. Determining categorically whether a species transmits a disease requires isolating the virus from the saliva of wild mosquitoes, but this can be difficult as only a tiny fraction of the population will ever be infected. Even so, demonstrating in the lab that a species can be infected adds to a body of evidence that implicates that vector.
The World Health Organisation has urged Europe to prepare for the potential arrival of Zika virus, saying that any countries with Aedes mosquitoes are at risk. So far, cases of tourists returning with the disease have been confirmed in the UK, Spain, Germany, Denmark and France. The virus hasn’t spread beyond these individual cases because European mosquitoes are inactive during winter. But as spring and summer bring warmer temperatures, the risk of further spread will increase.
As well as this, climate change is predicted to drive the spread of Aedes albopictus to Northern Europe, which could also increase the risk of outbreaks of diseases like West Nile fever, dengue and chikungunya.
UK mosquitoes don’t currently transmit any of these diseases, but Anthony and his team are currently researching the potential risk from other diseases like Rift Valley fever, a serious illness that can lead to blindness, haemorrhaging, brain inflammation and death.
“Rift Valley fever hasn’t spread to Europe yet, but we don’t know if there is something stopping it or whether we’ve just been lucky so far,” he says. “We are working as part of a consortium of laboratories across Europe to see whether local populations of mosquitoes are capable of transmitting these pathogens.”
New high containment facilities at the Pirbright Institute have enabled researchers to study a greater range of mosquito-borne viruses and analyse the threat they pose. “Diseases are moving faster all the time,” says Anthony. “At a national and European level, we have to make sure we are acting proactively to see what threats might be next on the horizon and what infrastructures we need in place to combat them, rather than waiting for them to get here.”