The devastating impact that infectious diseases can have on the global population was well demonstrated by the COVID-19 pandemic. Fortunately, pandemics remain unusual occurrences, but research suggests that factors such as climate change may be increasing the risk of such events occurring in the future.
Researcher
Umeå University
In a pre-pandemic study published in 2016, infectious disease researcher Jan Semenza, PhD, currently based at Umeå University in Sweden, and colleagues assessed potential drivers of infectious diseases in Europe. They broke down these factors into three groups: globalization and environment, public health systems, and social and demographic factors. Globalization and environment was by far the biggest contributor at 61%, versus 21% and 18% for the other two groups, respectively.
Semenza was head of the health determinants program at the European Center for Disease Prevention and Control at the time and carried out more work with colleagues to investigate and verify this finding. “We convened experts in the field, and we asked them, ‘What are the underlying drivers of emerging infectious diseases? And then what leads to these infectious disease outbreaks?’” he told Inside Precision Medicine.
“The experts underestimated the contribution of global environment change and climate change, specifically on the emergence of infectious diseases in Europe,” he noted.
According to the U.S. Centers for Disease Control and Prevention (CDC), more than 60% of human infectious diseases can be spread from animals and 75% of new or emerging infectious diseases come from animals.
Predictions suggest at least 10,000 viral species circulating in wild animal populations have the potential to infect humans. Currently most do not, but this could change. A paper published in 2022 predicts that changes in climate and land use could lead to around 4,000 cases of cross-species transmission of such viruses by 2070, based on current statistics.
One way this can happen is when humans and animals use the same land and so-called “spillover events” happen as a result. Another is through climate change. As the weather warms, patterns of animal and bird migrations change and populations of insects such as mosquitoes and ticks move into new areas such as Northern Europe and North America, potentially bringing diseases with them.
For example, the Asian tiger mosquito (Aedes albopictus), known for being a carrier of dengue, chikungunya, and Zika virus, as well as yellow fever, has now become established in Europe and is moving further north each year.
Guest Professor at Umeå University
and Professor at Heidelberg University
“Last year, for the first time, it was shown that in Greece, it didn’t actually overwinter. It just continued being winter active,” explained Joacim Rocklöv, PhD, a guest professor at Umeå University and an Alexander von Humboldt professor at Heidelberg University. His work focuses on infectious disease epidemiology and climate change. “That means you can actually probably have transmission there all year round now.”
The combination of climate and environmental change to disease spread is concerning. However, along with measures to reduce climate change, technologies such as genomic pathogen surveillance, artificial intelligence, and innovative disease control measures like the World Mosquito Program’s Wolbachia method—essentially infecting mosquitos like Aedes aegypti (another dengue fever vector) with a bacteria called Wolbachia that stops them from spreading disease—can help to minimize these potential threats.
Impact of environmental change
Professor
University of Notre Dame
The impact of human activity on the environment influences the spread of infectious diseases in several ways. Earlier this year, Jason Rohr, MD, a professor at the Galvin Life Science Center at the University of Notre Dame in Indiana, and his team published a paper to assess the impact of these factors across a number of studies looking at infectious diseases in humans, wildlife, and plants.
Rohr and team found that loss of biodiversity, chemical pollution, climate change, and invasive species all increased the damage that could be done by emerging infectious diseases, but urbanization seemed to reduce the risk from disease, likely through disease host habitat loss.
Notably, climate warming seemed to increase infectious disease risk to a similar extent in both humans and wildlife. “Even though humans tend to try to control disease in humans more than they would in wildlife, we still saw really strong patterns for human diseases, whether those diseases were in wildlife or actually manifesting in humans,” said Rohr.
Loss of biodiversity increases risk, because rare, more parasite resistant species tend to be lost first. “The more of those rare species that you lose, the greater the proportion of the community is the abundant species that the parasite is really good at infecting,” explained Rohr.
“As you lose species from a community, you end up losing the ones that are most resistant to the parasites, and you get more of those hosts that are least resistant. That increases the risk for humans, because if 75% of the diseases are zoonotic and have a wildlife source, as we lose biodiversity, we increase the risk of infections and, potentially, spillover events to humans.”
Chemical pollution can also have a negative impact. Some chemicals make both humans and wildlife more susceptible to infection through immunosuppression. Others may influence the intermediate host of a parasite such as the freshwater snails, which harbor the worms that cause schistosomiasis in countries such as Senegal.
“We’ve shown that the submerged aquatic vegetation that serves as a habitat for the snails is increasing due to fertilizer use,” said Rohr, who has worked on a number of village trials in Senegal to try and tackle the persistent parasite. “If you remove that aquatic vegetation, you actually can reduce schistosomiasis infections.”
Climate change appears to be a big driver of infectious disease. Camilo Mora, PhD, a professor at the University of Hawaii in the department of geography and environment, and colleagues published a study in 2022 that showed that 58% of infectious diseases have already been made worse by climate disruption or change.
Professor
University of Hawaii
“A lot of people say that we are exaggerating these things, because these are things that might happen in the future, but these are things that already happened,” explained Mora.
All the cases in which Mora and colleagues discovered a negative influence of climate were clustered into one of four categories: the pathogen moved, the people moved, the pathogen got weaker or stronger, or the people became weaker.
“One possibility is for the pathogen to overlap where people are located … for example, in Australia, wildfires are causing bats to migrate and they find shelter around people’s houses. There was a case in which an infected bat landed on a tree in the garden of a house, a kid went to play outside, touched the feces, and got infected,” said Mora.
“We also found some very interesting reports on how people can get stressed out during a flood or a hurricane or a heat wave. These changes in cortisol affect the immune system such that you become more prone to be infected or more vulnerable to disease.”
March to the Global North
Climate change is making Europe and the U.S. vulnerable to infectious diseases that were previously not found in these regions, or only found in very small numbers.
A good example of this is West Nile virus, a single stranded RNA virus carried by birds and transmitted by mosquitos to both humans and horses. Only around 20% of those infected have symptoms, but these can be serious or even life-threatening in some people, at times causing neurological symptoms and encephalitis. The number of infections seems to be increasing in Europe, with 2018 and 2022 having particularly high numbers of reported infections.
Semenza has been studying West Nile virus with Rocklöv and other colleagues and used machine learning to assess the many variables that could be contributing to outbreaks of the disease.
“In 2018, for example, there was this massive outbreak of West Nile. And we were puzzled why the epidemic started so early and why it was such a huge outbreak,” he explained.
“By having a machine learning or artificial intelligence algorithm, we were able to identify the underlying drivers. We were surprised that the most important variables were in fact temperature in the second quarter. This means that you can use that as an early warning system, because we know that if the spring gets really warm, then you will see a fall season with a lot of cases.”
Tick borne diseases are also spreading across Europe and the U.S. A particularly concerning example in Europe is Crimean–Congo hemorrhagic fever. Spread largely by ticks in the genus Hyalomma, this disease has symptoms similar to those of Ebola and can have a 50% fatality rate. H. marginatum ticks are typically found in warmer places such as North Africa and West Asia and in more recent years, Southern Europe. Although still rare, these ticks can now be found in Spain, France, and Italy, and have even been seen in Germany and Switzerland. Statistical models that account for predicted changes in climate suggest that H. marginatum will continue to move north in the coming years.
level, as of August 2023. [EuroGeographics]
“It is traveling from the south further into Europe,” said Rocklöv. “Now it is still in the south, but of course when the ticks get established, and if they can live to adulthood, then there will be potential cases of Crimean Congo hemorrhagic fever.”
Fungal diseases are also adapting to climatic conditions. George Thompson, MD, is a professor of medicine at the UC Davis Medical Center in Sacramento, California, who specializes in fungal diseases.
Professor
UC Davis
“Candida auris is really thought to be the poster child, so to speak, for that emergence and adaptation to the warming environment. It was first described in an ear culture in Japan in 2009. Since then, it’s caused invasive infections, bloodstream infections, big outbreaks in medical facilities, hospitals, and nursing homes, skilled nursing facilities. It’s moved from a local infection to now causing invasive infections,” he explained.
Another example is a fungi called Sporothrix, which typically lives in the environment and on plants such as roses. “In Brazil, it’s adapted and is now highly associated with cats, it lives on their claws, which clearly are warmer than the environment,” said Thompson. “We think that particular species has evolved pretty quickly to the warming environment.”
Fighting climate-driven infectious disease
Fighting infectious diseases is hard at the best of times, but adding environmental factors such as global warming makes a difficult task even more complicated. However, researchers and companies are working on methods to help drive back the spread of such diseases.
Early prediction systems such as the machine learning algorithm developed by Semenza, Rocklöv, and colleagues for West Nile virus are helpful because they allow local health authorities to prepare for new outbreaks.
Rohr and colleagues are using data from the Analytics for Investigation of Disease Outbreaks tool to create mathematical models that predict new outbreaks of specific diseases.
“We’re looking at the statistical properties of these time series to determine whether we could have detected the outbreak before it occurred and what the response window is,” Rohr explained. “Our response window across all these diseases so far is over three weeks, which is enough time to respond and potentially mitigate or prevent the infections.”
During the COVID-19 pandemic, genomic pathogen surveillance was widely used to track the spread of the virus and search for new variants. Some countries had much better access to this technology than others, but Semenza says this is something the European Centre for Disease Prevention and Control (ECDC) is trying to change.
“The ECDC has made quite an effort trying to provide grants to member states to increase their sequencing capacity and to increase their output, sample collection, and analysis to equilibrate the field and try to bring these countries up to speed,” he said.
Wastewater surveillance has been carried out for some time to monitor potential outbreaks, particularly in built up areas. But this has also increased since the pandemic.
“It has been used for monkey pox, different COVID-19 strains and other diseases to try to see if there’s community transmission. If you can’t do community surveillance, you can look at wastewater and see what kind of transmission you would expect,” said Semenza.
“Those are the new technologies that are definitely changing public health and public health surveillance as we know it.”
Rocklöv and colleagues are currently exploring different types of “smart” surveillance including automated mosquito traps and are also collecting bioacoustics recordings from birds to try and better track the movements of wild species.
“We have smart traps for mosquitoes, for example, which could run for six months and give you very good data on mosquitoes,” he explained.
He added that they are developing a new project where people can submit pictures of ticks they’ve observed. “We will classify them using AI-based visual learning to assess what type of tick species they are.”
Many different methods have been tried to target vector species such as mosquitoes, ranging from simple insecticides to molecular solutions such as gene drives.
The World Mosquito Program’s Wolbachia method is a less controversial but effective method of non-insecticide-based mosquito control.
Associate Director of Research
New England Biolabs
“It uses a naturally occurring bacteria called Wolbachia that lives in most insects on the planet but not in a lot of mosquitoes. They engineered that bacteria to live in Aedes mosquitoes, the vector that spreads things like dengue and chikungunya … when Wolbachia lives in these mosquitoes, they are much less likely to spread these viruses,” explained Nathan Tanner, PHD, an associate director of research at New England Biolabs, which produces enzyme-based reagents and tests for use in life science research.
New England Biolabs has worked with the World Mosquito Program to help them create simple tests that can detect whether mosquitos are infected with Wolbachia pipientis or not using loop‑mediated isothermal amplification (LAMP).
LAMP tests fall somewhere between antigen tests and PCR tests in that they are more accurate than antigen tests, but easier and cheaper to carry out than PCR tests because they do not require specific machinery or expertise. They only call for a simple heating step that can be achieved using hot water.
Such easy-to-use and affordable tests are important for the management of infectious diseases, particularly in the Global South, where public health resources can be limited. For example, LAMP tests are being used to assess whether people in Africa are infected with Onchocerca volvulus and/or Loa loa, both nematode worms that can cause blindness.
It is important to assess whether people have both infections as treatment with the antiparasitic drug ivermectin can cause serious side effects in people infected with both parasites. “You have to know which pathogens are present in the population to know what drug to use and the only way to know that is through testing, which has to be widespread, and it has to be cheap,” said Tanner.
Is change on the horizon?
The last 10 years have included nine of the hottest on record and extreme weather events such as droughts, wildfires, serious flooding, and hurricanes seem to be on the increase. The combination of such environmental changes with the increasing spread of dangerous infectious diseases could be devastating in the future and indeed, are already causing problems in many places.
Infectious diseases have historically disproportionately affected the Global South, but increasing temperatures in the Global North mean that clinicians and researchers based in these areas and elsewhere need to stay informed about new diseases, surveillance, and treatment and prevention efforts.
“Global environment change is so complicated and climate change in particular, because it basically preys on infrastructure vulnerabilities. Decaying, aging infrastructure is such a problem in combination with climate change,” said Semenza.
“Trying to come up with resilient public health infrastructure is definitely something that we need to look carefully at.”
Rocklöv believes things are changing for the better. “There is much more funding and more publications in this area,” he emphasized. “I think the interest overall has increased and you see it more and more often in high impact journals.”
Mora believes the best way to deal with the issue of infectious diseases and climate change is to go direct to the source. “In my opinion, there is just one single thing that will fix them all. And that is for us to start taking this issue of climate change more seriously.”
While there is a lot to be said for the direct approach, Rohr and his team are developing truly integrated approaches to deal with both infectious diseases and climate change issues. They recently won the 2024 International Frontiers Prize for innovative public health and sustainability research in recognition of this effort.
In a study published last year, they first attempted to stop disease spread by removing submerged vegetation that is a key habitat for water snails that spread Schistosoma parasites in villages in Senegal. But Rohr and team did not stop there. They also used the aquatic vegetation to create animal feed, fertilizer, and even fuel (in combination with animal dung) for biodigesters to produce cooking gas.
“The value of the biodigesters is that by encouraging these communities to take the vegetation and couple it with cow manure, we will reduce methane gas emissions from cow manure … On top of that, it reduces deforestation because 80% of the cooking fuel in Senegal is wood,” emphasized Rohr.
“This is something that can be implemented by these communities that are marginalized and resource-poor. They can come and grab the vegetation and convert it from a public nuisance that’s increasing disease to a private good.”
If they can be widely implemented, truly integrative projects like this one that simultaneously tackle infectious disease risks, climate change, and other environmental issues could help solve what is undoubtedly a complicated global problem.
Read more:
1. Observed and projected drivers of emerging infectious diseases in Europe
2. One Health – About Zoonotic Diseases
3. Climate change increases cross-species viral transmission risk
4. Aedes albopictus – current known distribution: May 2024
5. A meta-analysis on global change drivers and the risk of infectious disease
6. Over half of known human pathogenic diseases can be aggravated by climate change
7. West Nile virus on the rise in Europe, finds authority
8. West Nile virus keeps on moving up in Europe
9. European projections of West Nile virus transmission under climate change scenarios
10. Hyalomma marginatum – current known distribution: August 2023
11. Predicting climate-driven distribution shifts in Hyalomma marginatum
12. Infectious Diseases in a Changing Climate
15. Impact of climate change and natural disasters on fungal infections
17. Analytics for Investigation of Disease Outbreaks
18. Gene Drives Could Fight Malaria and Other Global Killers but Might Have Unintended Consequences
21. A planetary health innovation for disease, food and water challenges in Africa
Helen Albert is senior editor at Inside Precision Medicine and a freelance science journalist. Prior to going freelance, she was editor-in-chief at Labiotech, an English-language, digital publication based in Berlin focusing on the European biotech industry. Before moving to Germany, she worked at a range of different science and health-focused publications in London. She was editor of The Biochemist magazine and blog, but also worked as a senior reporter at Springer Nature’s medwireNews for a number of years, as well as freelancing for various international publications. She has written for New Scientist, Chemistry World, Biodesigned, The BMJ, Forbes, Science Business, Cosmos magazine, and GEN. Helen has academic degrees in genetics and anthropology, and also spent some time early in her career working at the Sanger Institute in Cambridge before deciding to move into journalism.
