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Climate Change Is Enabling the Spread of Deadly Mosquito-Borne Diseases

Climate Change Is Making Mosquito-Borne Diseases Deadlier

Many of the long-predicted effects of climate change are now occurring: glaciers have shrunk, plant and animal ranges have shifted, trees are flowering sooner and there are longer and more intense heatwaves. Scientists have now discovered another dangerous side effect – the rise of mosquito-borne diseases. A warmer climate is allowing many species of mosquitoes to move further north, bringing with them a range of deadly diseases like malaria and a number of particularly nasty diseases like the eastern equine encephalitis virus (EEEV). 

Mosquitoes are found all over the world, including in Northern regions like Canada (82 known species) and Northern Europe (nine known species). As global temperatures rise though, scientists fear that two particularly deadly species – the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus) – could extend their range from tropical climates. This would, according to Carbon Brief, expose over 1 billion people to deadly diseases like chikungunya, dengue fever and Zika for the first time.

Country-level suitability range for mosquito species Aedes aegypti and/or Aedes albopictus: suitability ranges from 0 (white) to 100% (deep red). Source: International Journal of Infectious Diseases

Southern Europe is no stranger to deadly mosquito-borne diseases. In the 1970s the region managed to eradicate the local transmission of malaria, spread through the female anopheles mosquito, through a combination of insecticide spraying, drug therapy and environmental engineering. According to a study by two scientists from Greece, “Within the WHO European region, most malaria cases are currently imported by international travellers and immigrants, and occur in Western European countries, chiefly France, Germany, the United Kingdom and Italy.”

However,  successful eradication isn’t always the case. Right now, there is no approved vaccine for the Zika virus, which, in 2019 is present in87 counties according to the World Health Organisation (WHO). Battling a disease without a vaccine is not easy, as we have seen with the COVID-19 pandemic. Some parts of the USA are already experiencing the challenges of fighting a range of mosquito-borne diseases without a vaccine, and this the case with EEEV.

Dangers of EEEV

One of the diseases that have the potential to spread into northern regions and cause deadly outbreaks is the aforementioned EEEV. The virus was first discovered in 1933 in the mid-Atlantic region of the USA. The virus only spreads through mosquitoes, and cannot be transmitted from person-to-person. In mild cases, the virus results in fever, chills, joint pain, and muscular pain. 

In severe cases, however, it results in encephalitis (swelling of the brain) which can be deadly. According to the Centres for Disease Control and Prevention (CDC), “Of those who recover, many are left with physical or mental sequelae, which can range from mild brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction.”

Currently, there is no treatment for EEEV. This makes the disease potentially deadly for anyone who contracts it. According to CDC data, EEEV is fatal in 30% of cases. For context, the fatality rate for COVID-19 is estimated to be about 1%. CDC data shows that in 2019, there were 38 confirmed cases (15 resulted in death) across 10 states. Although these numbers are small, increased temperatures though could lead to a huge rise in the number of cases. To understand how we need to look at the EEEV outbreak in the USA in a bit more depth. 

CDC data tracking the number of cases and deaths due to EEEV in the USA. Data from CDC

EEEV Outbreak in the USA

In a 2013 paper published in the New England Journal of Medicine, the authors Philip Armstrong and Theodore Andreadis found that “Over the past decade, we have witnessed a sustained resurgence of EEE virus activity… in the northeastern United States and northward expansion into regions where the virus was historically rare or previously unknown, including northern New England and eastern Canada.”

Like other mosquito-borne diseases, the survivability of the EEEV virus is highly dependent on weather conditions. Active transmission usually ceases in winter, when it is too cold for mosquitoes to feed and breed. However, climate change has resulted in milder winters, allowing mosquitoes to feed all-year-round. Armstrong and Andreadis add “new strains of the EEE virus may also be reintroduced annually by migrating birds into the northeastern United States from more southerly regions, such as Florida, where transmission is continuous throughout the year.”

The two scientists conclude that while a wide variety of factors contribute to the spread of EEEV, changing weather patterns have the most immediate effect of mosquito presence. Milder winters and warmer summers, they believe, are the key factors to the rate of mosquito replication and feeding. 

Battling New Diseases

EEEV is just one example, right now global warming is opening up more and more of the world to newer diseases. In 1999, New York City reported 25 cases of the West Nile Virus (WNW), the first time the virus was reported outside of the Eastern hemisphere. From 1999 to 2014, the CDC reported 41,762 cases of the virus throughout the USA. A report by the Environmental Protection Agency (EPA) links the spread of WNW to climate change and warmer environments.

Distribution of West Nile virus cases in Europe up to 2018. Source: European Centre for Disease Prevention and Control

Similarly, Europe has also seen a rise in the number of WNW cases yearly. There were 463 infections in 2019 as per the European Centre for Disease Prevention and Control, up from 262 in 2010. Germany and Slovakia reported their first cases of the infection in 2019, showing that the disease is indeed increasing its range on the continent.

Speaking to National Geographic, disease vector biologist Lyric Bartholomay said: “I don’t think there’s any doubt that as temperatures shift, their (mosquito) ranges and places they can go will also change.” This has caused scientists to worry, as there is now a strong belief that other mosquito-borne diseases like the Rift Valley Fever and chikungunya could make their way to the US for the first time in the future.

The world’s first approved Anti-dengue vaccine Dengvaxia, developed by Sanofi. Source: Noel Celis/AFP/Getty Images

If and when these diseases come, battling them isn’t going to be easy. Just like with COVID-19, humans will have to make adjustments to the way they live. Big pharma though, will welcome the host of new diseases – as it could create a huge boom in demand in regions that can afford to pay for vaccination. A report by Morgan Stanley found that “nearly one billion more people—well over 10% of the world’s current population—could be exposed to the threat of dengue, yellow fever, chikungunya and the Zika virus over the next 60 years.” This would create a demand for an additional $125 billion (minimum) in vaccines. The company believes that the cost of vaccines could reach $200 billion after taking into consideration variations and pricing. 

As the world comes to grips with the challenges of the current global pandemic, it is useful to keep an eye out for what could threaten us next. Climate models alone aren’t enough to estimate the impact of mosquito-borne diseases, as seen in the case of malaria in southern Europe, human behaviour can have a huge impact. It is true climate change will expose more people to more diseases, but advances in biotechnology, environmental engineering and public policy planning have been able to mitigate the worst impacts in the past.  So it’s a good bet that all these could work again, but only if we are willing to act quickly.

Sources: Carbon Brief, Centres for Disease Control and Prevention, International Journal of Infectious Diseases, Morgan Stanley, Mosquito World, National Centre for Biotechnology Information, New England Journal of Medicine, Popular Mechanics, Science Direct

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