Seven warmer-weather diseases climate change may bring to the UK
A LEADING research institute is warning that the effects of climate change may lead diseases more associated with warmer climates to the UK.
In an email to National Geographic UK, Dr Steven White, a theoretical ecologist with the UK Centre for Ecology & Hydrology, an independent nonprofit research centre, highlights a group of pathogens the UK may become more familiar with if climate change trends continue.
“Most human-to-human transmitted diseases are not affected by climate, except potentially for a few respiratory diseases,” he says. “It is mainly diseases where an intermediate host – for example, a mosquito vector – is required for transmission, that are most affected by climate. And therefore, climate change.”
As the world continues to grapple with COVID-19 – itself likely originated from a zoonotic virus – the consequence of humans’ invasive interaction with potential reservoirs of similar viruses, such as bats, is one we've all felt. Virus hunters agree that contact with infected animal faeces or blood, harvesting for food or capturing live specimens increases the likelihood such pathogens will have the opportunity to jump species.
But as White illustrates, indirect causes – such as deforestation and the emissions cost of expansion – can also have a global knock-on when it comes to the spread of disease. This could simply be by creating favourable conditions across more of the world for potential vectors to thrive.
“Other climate change effects on disease could include the ecology of the host animal where things like migration and phenology affects the species, which in turn interacts with disease transmission,” he says.
“It is mainly diseases where an intermediate host – for example, a mosquito – is required for transmission, that are most affected by climate. And hence, climate change.”
2019 saw the highest temperatures ever recorded for Britain in both winter and summer: 21.6 °C on 26 February in London, and 38.7 °C in Cambridge on 25 July. This November, the United Nations COP26 Climate Conference, hosted this year by the UK, aims to ‘accelerate action towards the goals of the Paris Climate Agreement’ amongst the 195 signatory nations.
This means arresting global temperature rise from pre-industrial levels at a preferable 1.5 degrees C and a maximum of 2 degrees C. All signatory nations must establish individual goals to reduce their carbon footprint, using emission targets called Nationally Determined Contributions (NDCs).
But with nations still reeling from the current pandemic, climate change may not be front and centre in many people's minds – despite the rise of disease and human-caused environmental change being intrinsically linked.
The below, White claims, are just a few of the diseases that could have the biggest impact in terms of human, animal or plant health. “Not only will we have to prepare for the disease,“ he says, ”but it is vital for us to think carefully about how we interact with nature.”
Xylella fastidiosa
Once only found in the Americas infecting citrus, grapevines and coffee plants, this non-specific bacterial plant pathogen was detected in Italy in 2013, where it is currently devastating olive trees – causing economic and biological damage to the region of Puglia. Strains of the bacteria have since been found across more of continental Europe, infecting a range of agriculturally critical crops.
Spread by xylem-feeding insects such as the meadow froghopper, or spittlebug – which are common in the UK – the bacteria cause blockages in these nutrient tissues of a plant, starving it to death. A variant known as multiplex discovered in France, Spain and Portugal, is of most concern, being able to survive in cooler climates and infect broadleaf trees such as oak. To date, there is no known cure.
Olive trees in Italy affected by Xylella fastidiosa. The trees have been heavily pruned in an attempt to limit the spread – and may regenerate – but there is no effective treatment.
“Xylella fastidiosa is not currently present in the UK, and there are tough restrictions in place to stop it entering,” says White, adding that with increased global plant trade, it is only a matter of time before it is introduced. “The question of whether the disease will readily spread in the UK is somewhat open,” he says. “However, the pathogen growth in host plants is highly sensitive to temperature, and climate change is likely to increase the suitability for disease spread in the UK.”
Based at the John Innes Centre in Norwich, BRIGIT is a coalition of scientists and stakeholders set up to model what could happen if vector-borne plant diseases such as Xylella fastidiosa, ever got a toehold in the UK. Professor Saskia Hogenhout is one. “The BRIGIT project has gathered information on the behaviours of insect vectors that may spread the Xylella from infected plants into the environment.” Hogenhout told National Geographic UK in an email. “Current evidence shows that the insects largely remain local. However, this may change in the future –particularly if areas become deprived of water and the insects need to migrate to areas with more green plants.”
She adds: ”If plants with Xylella arrive in the UK, quick and efficient diagnostic, followed by early action, should considerably reduce likelihood of spread and establishment of the disease.” As for the potential impact, while there is still a lot that isn't known, it may not all be bad news. “The project has been running for nearly 3 years and we have successfully filled many of the knowledge gaps, and also ran awareness campaigns for the general public and stakeholders. We estimate that the spread is likely to be less impactful than the outbreak locations in Southern Europe,” Hogenhout says, ”but there is still some uncertainty. We urge caution given the devastation seen around the globe.”
Related: Italy’s olive trees are dying. Can they be saved?
West Nile Virus
The principal hosts of this flavivirus are birds – its occurrence is often associated with dead crows and jays – but humans can be infected too. The chief vector is the culex mosquito, which passes on the disease after gorging on birds infected with the pathogen, then biting a human. First detected in Uganda in 1937, a major outbreak of West Nile virus (WNV) in Europe in 1996 and the United States in 1999-2010 – a strain thought to have originated in North Africa or the Middle East – highlighted the danger posed by an incongruous insect-borne virus to international populations.
While serious in its severe form, the fatality rate is relatively low – with 80% of carriers asymptomatic, and only 1 in 150 developing the most extreme form of disease, which can lead to neurological damage and death. As with malaria, the best treatment is to avoid being bitten by mosquitos – and while the virus has never been contracted in the UK, that could change. (Related: At last, a malaria vaccine has passed important clinical trials.)
“Climate change is having a dramatic effect on the mosquitos and the disease itself – it’s spreading,” says White. “Rising temperatures means that new habitats are available to the mosquitoes, or they are able to sustain higher abundances.”
A glossy ibis is tested for West Nile Virus antibodies in Doñana, Spain.
Key to disease transmission, White says, is the EIP – or extrinsic incubation period – of the virus within the mosquito: essentially, the time it takes for the virus to replicate enough to spread to new hosts. The shorter the EIP, the more likely the mosquito will live to pass on the disease.
“The EIP is crucial for many vector-borne diseases, not just WNV. It is highly temperature sensitive, shortening in [warmer] temperate regions.”
In recent years West Nile Virus has been moving northward in Europe, with 2018 seeing the highest number of cases registered to date, and it has been detected in Germany and the Netherlands. It is thought that migratory birds may carry the virus from areas of continental Europe, where it could be spread to humans by mosquitos such as Culex pipiens or the recent arrival Culex modestus – believed to be colonising south-east England thanks to rising temperatures. This species, which has a higher incidence of feeding on humans, may act as a bridge vector between birds and humans.
As to the likelihood of spreading between humans, unlike COVID-19, casual contact is highly unlikely to cause a spread, with no human-to-human transmission of West Nile virus documented. As for conditions, White thinks that “currently we are unlikely to have an outbreak – but climate change and rising temperatures greatly increase the risk, with plausible outbreaks predicted by the latter half of the century.”
Chikungunya, Dengue and Zika
This trio of diseases share one vector critical to their spread: the Aedes family of mosquitoes, principally Aedes aegypti and Aedes albopictus, both of which are present in southern Europe – with their range increasingly moving north. Zika was first identified in Uganda in monkeys, and in humans causes fever, rashes and malaise – though can have serious implications for unborn infants, with congenital malformities such as microcephaly. 86 countries have reported Zika, with a severe outbreak in Brazil in 2015. Chikungunya, a disease usually found in Africa and Asia, causes severe joint pain and fever – with severe complications typically associated with underlying health issues.
Pesticides are spread in an area of the Amazon that has flooded, to discourage the breeding of mosquitoes carrying Dengue. The insects multiply in standing waters, which combined with poor hygiene and people out in the open can lead to outbreaks of the disease.
Perhaps most concerning is dengue, classically associated with the tropics, with a severe form of the disease manifesting as a wide spectrum of symptoms and complications. While the overall fatality rate is low, recent years have seen an alarming rise in dengue, with cases increasing 8-fold over the last two decades, from 505,430 cases in 2000 to over 5.2 million in 2019.
Local transmission of the virus was reported for the first time in Europe in 2010, with the threat of explosive outbreaks now acknowledged by the World Health Organisation (WHO). With early treatment critical, the combined pressure of COVID-19 and the rise in dengue make its potential consequences 'devastating', according to the WHO – with estimates placing half the world’s population at risk from the disease.
As for the UK, we may not be vulnerable – yet. “Currently it is not thought that the UK is suitable for the transmission of these three diseases,” says White. “Climate change means the mosquitos are more likely to establish, thus permitting the potential for future outbreaks.”
“The combined pressure of COVID-19 and the rise in dengue make its potential consequences 'devastating', according to the WHO – with estimates placing half the world’s population at risk from the disease.”
Avian influenza
Climate change has a significant impact on animal ecology – such as the degradation of prey habitats, changes in breeding behaviour and migration patterns. The movement and potential interactions of birds becomes a critical factor when considering the transport of pathogens such as avian influenza – bird flu – which is often fatal in wild birds.
The majority of avian flu strains do not affect people. The H5N1 strain of the virus, however, has jumped species – 861 human cases of the virus have been recorded by the WHO since 2003, with a mortality rate of 52%, with poultry workers at particular risk. Despite apparently being poorly suited to humans, this strain in particular has been identified as a pandemic risk, due to its high mortality rate amongst those afflicted and supposed ability to reach enhanced transmission potential between mammals in a relatively small number of mutations.
Particularly within the poultry industry, avian flu has had recent impact in the UK, with some 14 outbreaks in the last year alone – including the highly deadly H5N8 strain, and others such as H7N1, H5N1, H5N2 and H5N3. While H5N8 has only affected a small number of humans to date – seven Russian poultry workers – the strain is a concern due to its highly pathogenic nature.
A warning sign to motorists in Wensleydale, Yorkshire, during an outbreak of H5N8 avian flu, late 2020. Following the latest spate of outbreaks, the government confirmed the UK was free of avian flu on 3 September 2021.
There is no evidence of significant human-to-human transmission of avian flu, other than anecdotal reports of limited spread between those in very close and prolonged contact – such as between a sick son and his father in China in 2007.
However, as observed with COVID-19, the ability of avian flu to affect humans at all, and therefore the hypothetical potential for a variant to emerge that can spread amongst both poultry and people as habitats and behaviour change, raises concerns.
Bluetongue
A fast-acting virus that affects ruminants and camelids such as sheep, cattle, goats and llamas, bluetongue is spread by Culicoides midges. Midges, some of which bite humans, are often encountered in great numbers by backcountry hikers in high summer, but pose more than an irritation to livestock if carrying this disease. Symptoms amongst affected animals range from foot lesions to facial swelling, and the eponymous oxygen starvation – cyanosis – of the tongue.
A summer midge swarm in the Scottish Highlands. The biting midge is capable of carrying the virus that causes bluetongue amongst livestock, deer and camelids.
A UK outbreak of bluetongue in 2007 following a European heatwave the previous year affected 135 farms, but was nevertheless considered a near-miss. A later study by the University of Liverpool concluded this was likely due to the notably cooler temperatures encountered by the virus by the time it reached the UK, which limited the spread. Quoted in Science Daily at the time, study author Dr Joanne Turner predicted “the UK outbreak would have been larger had the virus been introduced in a warmer year – something that is likely to occur more frequently in the future due to climate change.”
“Nothing can prevent the incursion of infectious vectors from the near continent if conditions are right, as was the case in 2007,” Turner told National Geographic UK in an email. “Our modelling work suggests that future climate conditions will lengthen the transmission season in the UK, and lead to larger outbreaks should infection be introduced. However, it also shows that animal movement restrictions are effective at preventing very large outbreaks.”
These movement restrictions – necessarily honed by disastrous foot-and-mouth outbreaks in the early 2000s – continue to be the key preventative strategy for controlling the spread of bluetongue amongst livestock. Vaccination is also an option, with numerous serotypes of the virus catered for. But as Turner explains, ”timing is everything. It takes time to administer vaccines and for animals to develop immunity, which is why animal movement restrictions remain a cornerstone of disease control strategies. The earlier interventions can be applied the better, which is why surveillance and post-import testing are also crucial.”
The government, Turner says, is monitoring continental European outbreaks of bluetongue – as well as the risk of infectious vectors finding their way to the UK, but increasingly, vigilance on the ground will be key. “As the climate warms, efficient and timely detection will become even more vital.”
