On the hunt with the malaria men of Kent
Mosquito huntersers warn an outbreak ak of deadly insect-borne orne diseases is now a real risk. By Sarah Newey
Nestled in Cooling churchyard chyard in Kent are the 13 “little stone tone lozenges” said to have inspired the opening of Charles Dickens’ ens’ classic novel Great Expectations.
But for the scientists from the UK Health Security Agency (UKHSA) HSA) now scanning the Thames estuary y for mosquitoes, the significance of gravestones is not literary or abstract.
They are a stark reminder that malaria, an ancient killer which ich once plagued north Kent’s wetlands, ds, and the rest of Britain, could return as the climate shifts. Already, clinicians ians in Essex and Kent are being warned rned to check for signs of another potentially otentially fatal disease: West Nile virus. .
“Just before Dickens’ day, malaria was endemic here,” said Alexander ander Vaux, a senior medical entomologist mologist at UKHSA, waving at the grassy marshland that merges with the Thames. s. “The children in the graveyard [Pip’s p’s younger brothers in the novel], el], they would all have died of malaria.” a.”
Up until the late 1800s, the e disease – then known as ague or marsh sh fever – was common across England. nd. Now, as Europe’s climate changes, the e UKHSA’s mosquito trackers are hunting ng for old and new threats lurking amid d the bulrushes, fearful that insect-borne t- diseases may surge.
Swapping the nets favoured ed by 18th-century botanists for electric ectric traps, the small team of entomologists mologists and ecologists traipse across wetlands catching mosquitoes (and sometimes metimes ticks) for testing. Each week, they collect hundreds of insects, freeze reeze them in polystyrene boxes full ull of dry ice, and take them back to labs bs at Porton Down for analysis.
They are looking for evidence – or the lack of it – of the arrival of “exotic vector-borne diseases”, or VBDs, a threat considered “immediate” by the Government’s national contingency plan for invasive mosquitoes.
“A considerable increase in the spread of invasive mosquito species has been observed in Europe since the late 1990s… this can create potential new routes for introduction into the UK,” the report says, adding that climate change, rising trade and travel, and urbanisation all add to the risk.
A particular concern is the Aedes family of mosquitoes, which carry viruses including dengue, chikungunya, yellow fever and Zika. Aedes albopictus – commonly called the Asian tiger mosquito because of its trademark black and white stripes – has been spotted or become established in 30 European countries including France, Italy and Croatia. Its cousin, Aedes aegypti, which is not so well adapted to temperate climates, has been found in a handful, including Russia and Georgia.
Both species have been implicated in a spate of disease outbreaks across the continent in the last decade, triggered by mosquitoes sucking blood from infected travellers, picking up the pathogen and spreading it locally. For example, 436 people caught chikungunya in Italy in 2017, and more than 2,000 were infected with dengue in Madeira in 2012. Meanwhile, Europe’s first locally transmitted Zika cases were reported in southern France in 2019.
Although the UK is yet to report a mosquito-driven outbreak of malaria or other exotic diseases, the risk is growing. The Asian tiger mosquito, known for its tolerance of frost and ability to adapt to new environments, has been spotted six times in the South East since 2016.
“Invasive mosquitoes take advantage of man-made containers, litter, anything that can fill up with water and provide a habitat,” said Mr Vaux. “The Aedes albopictus mosquito was first established in Italy in the 1990s, and since then has spread to 30 European countries, mainly via vehicles, but also through particular goods like shipments of tyres.”
As a result, UKHSA works with local councils to operate small mosquito traps which attract egg-laying, disease-carrying females at truck stops, service stations, ports and airports, in an attempt to identify and control the arrival of Asian tiger mosquitoes before they settle.
If they do, there is a risk that the insects could spread diseases brought back by travellers. According to the official contingency plan, 2,127 dengue cases and 588 chikungunya infections were imported into the UK between 2012 and 2016. With the right mosquito living here, those cases could well have sparked broader clusters of the sort already seen in Europe.
The surveillance teams have found that other invasive species have already colonised parts of England - one of them carrying the potentially lethal West Nile virus.
The Culex modestus mosquito – known as a ‘bridge vector’ because it can spread West Nile virus from birds to humans – is now established in parts of Essex and Kent, including the narrow, shallow ditches dotted across the Thames estuary. It is here that UKHSA has focused its summer surveillance efforts for the past 12 years, gathering samples weekly from traps left in metal dog cages to protect them from prying geese.
“It was in 2010, we were running the traps at sites in north Kent and found a species unusual to the UK, the Culex modestus, for the first time,” says Vaux. “This doesn’t mean West Nile virus is now spreading in the UK. A lot of things need to come together, including an assemblage of migratory birds moving the virus around,” he added. “That’s why our virologists also test mosquito samples for pathogens.”
But clinicians in these infested areas who spot patients with viral encephalitis, or brain inflammation, have been asked to send samples to UKHSA for testing – especially as the pathogen has spread in parts of Europe, including the Netherlands.
“[Transmission is] definitely a real risk, and that’s only going to increase as the climate changes,” Mr Vaux said. “We know that a changing climate
‘Transmission is a real risk and that’s only going to increase as the climate changes’
means the UK will become more suitable for fo certain species… and it’s critical to track t these shifts.”
Across the th globe, climate change is having major maj ramifications on the spread of disease. d Research published in Nature this t month, found 58 per cent of the 375 i infectious diseases known to affect humans hum – including malaria, cholera and an anthrax – have been made worse by one o of 10 extreme weather hazards connected co to climate change.
In the UK, U experts have warned that warming temperatures t could make Britain more mo suitable for a range of disease-carriers, ca including mosquitoes, mosquitoe parasites and ticks – which can carry pathogens including Lyme disease disea and Crimean Congo haemorrhagic haemorrha fever.
“Modelling Modell studies of likely spread or incursion incursio of VBDs clearly show that the problem of emerging or reemerging VBDs V may intensify and spread to the t UK soon,” according to the contingency conting plan.
A warmer warme summer would also “theoretically theoretica increase the local malaria risk”. ris Six species of the anopheline mosquito which carry the malaria parasites pa are indigenous to the UK, though the parasites themselves are more temperature te sensitive.
“It’s quite quit hard to quantify, but we do expect the t number of vector incursions and detections to increase in a warming warmi climate,” says Vaux, as he gathers a netted n bag crammed full of squirming mosquitoes from a trap tucked into the hedgerow.
“Our work is to ensure that we have the surveillance in place to identify these populations should they arise.”
This time, the unit found nothing unexpected in the woodlands and reedbeds dotted across Kent. But there’s always a tomorrow.
“We understand that if invasive species are missed, then potentially it could lead to the establishment of a species we don’t want in this country. We’re quite committed to ensuring that we’re not in that position.”