Climate change and infectious diseases

Global climate change has resulted a wide range of impacts on the spread of infectious diseases is a prominent instance due to ‘climate-sensitive’ characterization. Like other climate change impacts on human health, climate change both exacerbates existing inequalities and challenges in managing infectious disease while increasing the likelihood of certain kinds of new infectious disease challenges.

Documented infectious disease impacts of climate change, include increased malaria and dengue, which are expected to worsen as the global climate changes directly result in extreme weather conditions and higher temperatures. Not only will it propagate their spread, but climate change will probably bring forth new infectious diseases, and change the epidemiology of many existing diseases.

Despite humanity's recent success at managing diseases such as SARS, coronavirus, and Ebola – historical data is by no means a guarantee of the future success of confrontation given that each new disease brings its own set of challenges as seen in with the ongoing pandemic regarding COVID-19. There is no direct evidence that COVID-19 is worsened or is caused by climate change.

Background

The World Health Organization and United Nations Environment Programme established the Intergovernmental Panel on Climate Change (IPCC) in 1988 because there was little knowledge about health issues caused by global climate change. Intergovernmental Panel on Climate Change has presented three assessment reports. IPCC First Assessment Report, IPCC Second Assessment Report and IPCC Third Assessment Report wrote about climate change, potential health risks caused by climate change and early evidence of actual health impacts. IPPC and other policy-related assessments at the regional and national levels stimulated scientific studies to understand the climate-health relationships.

A noticeable effect of the global climate change is the increase of temperature. In 1895, the record-keeping of temperature in the U.S began and it has increased by 1.3 °F to 1.9 °F. This is because the concentrations of greenhouse gases increase. Based on this information the annual average U.S temperatures are expected to increase by 3 °F to 10 °F having direct effects on human health. The extreme temperatures (hot and/or cold) affect the body by compromising its ability to regulate its internal temperature and by worsening chronic conditions such as cardiovascular and respiratory diseases. Respiratory diseases also worsen by the air quality. Air quality becomes poor because of climate change, which produces higher concentrations of CO2, higher temperatures and changes in precipitation. Climate change affects the growing season and the pollen because the start or duration of the growing season becomes extended, and the quantity, the allergenicity and the spatial distribution of pollen increase. Climate change affects vector-borne diseases by affecting the survival, distribution and behavior of vectors such as mosquitoes, ticks and rodents. The viruses, bacteria and protozoa are carried by these vectors transferring it from carrier to another. Vector and pathogen can adapt to the climate fluctuations by shifting and expanding their geographic ranges, which can alter the rate of new cases of disease depending on vector-host interaction, host immunity and pathogen evolution. This means that climate change affects infectious diseases by impacting their length of the transmission season and their geographical range. Vector-borne diseases are a concern because they have played a significant role in human history by determining the rise and fall of civilizations. This is why the World Health Organization considers climate change as one of the greatest threats to human health.

Dengue fever

Dengue fever is an infectious disease caused by dengue viruses known to be in the tropical regions. It is transmitted by the mosquito Aedes, or A. aegypti. Dengue incidence has increased in the last few decades, and is projected to continue to do so with changing climate conditions. Once infected with the dengue virus, humans experience severe flu-like symptoms. Also known as "break-bone fever", dengue can affect infants, children, and adults and can be fatal. Dengue fever is spread by the bite of the female mosquito known as Aedes aegypti. The female mosquito is a highly effective vector of this disease. Climate change has created conditions favorable to the spread of the dengue virus. Transmission peaks during the rainy season when mosquitoes breed.

The World Health Organization (WHO) has reported an increase from a thousand to one million confirmed cases between 1955 and 2007. Where the mosquito, Aedes aegypti, lives and the amount of mosquitos present is strongly influenced by the amount of water-bearing containers or pockets of standstill water in an area, daily temperature and variation in temperature, moisture, and solar radiation. While dengue fever is primarily considered a tropical and subtropical disease, the geographic ranges of the aedes aegypti are expanding. Globalization, trade, travel, demographic trends, and warming temperatures are all attributed to the recent spread to this primary vector of dengue.

Dengue is now ranked as the most important vector-borne viral disease in the world. Today, an estimated 50–100 million dengue fever infections occur annually. In just the past 50 years, transmission has increased drastically with new cases of the disease (incidence) increasing 30-fold. Once localized to a few areas in the tropics, dengue fever is now endemic in over 100 countries in Southeast Asia, the Americas, Africa, the Eastern Mediterranean, and the Western Pacific with Southeast Asia and the Western Pacific regions being the most seriously affected. Recently the number of reported cases has continually increased along with dengue spreading to new areas. Explosive outbreaks are also occurring. Moreover, there is the possible threat of outbreak in Europe with local transmission of dengue being reported for the first time in France and Croatia in 2010. One country that has seen significant impacts from dengue is Bangladesh.

The cases of dengue fever have increased dramatically since the 1970s and it continues to become more prevalent. The greater incidence of this disease is believed to be due to a combination of urbanization, population growth, increased international travel, and climate change. The same trends also led to the spread of different serotypes of the disease to new areas, and to the emergence of dengue hemorrhagic fever. There are four different types of viruses in dengue fever. If someone is infected with one type of dengue virus, he or she will have permanent immunity to that type of dengue virus, but will have short term immunity to the other type of dengue fever. Some of the symptoms of dengue fever are fever, headache, muscle and joint pains and skin rash.

Dengue fever used to be considered a tropical disease, but climate change is causing dengue fever to spread. Dengue fever is transmitted by certain types of mosquitoes, which have been spreading further and further north. This is because some of the climate changes that are occurring are increased heat, precipitation and humidity which create prime breeding grounds for mosquitoes. The hotter and wetter a climate is, the faster the mosquitoes can mature and the faster the disease can develop. Another influence is the changing El Nino effects that are affecting the climate to change in different areas of the world, causing dengue fever to be able to spread.

"Some 1.8 billion (more than 70%) of the population at risk for dengue worldwide live in member states of the WHO South-East Asia Region and Western Pacific Region, which bear nearly 75% of the current global disease burden due to dengue. The Asia Pacific Dengue Strategic Plan for both regions (2008--2015) has been prepared in consultation with member countries and development partners in response to the increasing threat from dengue, which is spreading to new geographical areas and causing high mortality during the early phase of outbreaks. The strategic plan aims to aid countries to reverse the rising trend of dengue by enhancing their preparedness to detect, characterize and contain outbreaks rapidly and to stop the spread to new areas."

Ebola

Ebola is one of the deadliest viruses, starting out as a small outbreak and eventually turning into a major global issue. The average case fatality rate of the Ebola virus is approximately 40% and to date, there have been more than 28,600 cases with 11,310 deaths. Recent studies are holding climate change liable for the uptick in Ebola. Many researchers are linking deforestation to the disease, observing that change in the landscape increases wildlife contact with humans. Ebola virus is typically found in animals of the wild and can be transmitted from these animals to humans when exposed to infected bodily fluids. The virus can also be transmitted from another human when in direct contact with the virus hosts blood, vomit, or feces. Seasonal droughts alongside strong winds, thunderstorms, heat waves, floods, landslides, and a change in rainfall patterns also impact the wildlife migration. These conditions pull them away from their natural environment and closer to human proximity. One example of an Ebola outbreak caused by climate change or a shift in nature was seen during the drought of Central Africa. This ultimately amplified food insecurity leading West African communities to eat animals such as bats who were infected with the virus.

Coronavirus

Further information: Coronavirus disease 2019

Position of the United Nations Environmental Programme and the International Livestock Research Institute

According to the United Nations Environment Programme the Coronavirus disease 2019 is zoonotic, e.g., the virus passed from animals to humans. Such diseases are occurring more frequently in the latest decades, due to a number of factors, so, as for now, 75% of all emerging disease are zoonotic. Large part of the causes are environmental. One of the factors is climate change. Quick changes in temperature and humidity facilitate the spread of diseases. The United Nations Environment Programme concludes that: "The most fundamental way to protect ourselves from zoonotic diseases is to prevent destruction of nature. Where ecosystems are healthy and biodiverse, they are resilient, adaptable and help to regulate diseases."

In April 2020 the United Nations Environment Programme published 2 short videos explaining the link between nature destruction (including from climate change), wildlife trade and COVID-19 pandemic and created a section in its site dedicated to the issue.

In July 2020 the United Nations Environment Programme and the International Livestock Research Institute published a report named: "Preventing the next pandemic - Zoonotic diseases and how to break the chain of transmission. The report says that the frequency of zoonotic diseases like the coronavirus is increasing due to a number of causes, generally linked to nature destruction: "high demand for animal protein, unsustainable agricultural practices and climate change" All those changes the way in which animals and humans interact.

In the year 2021, the United Nations Environment Programme published an assessment report about climate change, pollution and biodiversity named: "Making Peace with Nature". Significant part of the report was dedicated to the link between nature deterioration and the rise in zoonotic diseases. According to the report, there is a direct link between climate change, deforestation, land use, wildlife trade and the emergence of pandemics like the COVID-19 pandemic.

Position of the World Health Organization

The head of the World Health Organization, Tedros Adhanom Ghebreyesus, in a special video message spoke about the link between the health of the biosphere and prevention of pandemics like the COVID-19 pandemic and highlighted that efforts for improving health will not work without stopping climate change.

The World Health Organization created a special page dedicated to frequently asked questions about coronavirus and climate change, including the link between climate change and the emergence of new zoonotic diseases. As of 24 November 2020 it summarized the current knowledge about the issue as follows:

There is no evidence of a direct connection between climate change and the emergence or transmission of COVID-19 disease. As the disease is now well established in the human population, efforts should focus on reducing transmission and treating patients.
However, climate change may indirectly affect the COVID-19 response, as it undermines environmental determinants of health, and places additional stress on health systems. More generally, most emerging infectious diseases, and almost all recent pandemics, originate in wildlife, and there is evidence that increasing human pressure on the natural environment may drive disease emergence. Strengthening health systems, improved surveillance of infectious disease in wildlife, livestock and humans, and greater protection of biodiversity and the natural environment, should reduce the risks of future outbreaks of other new diseases."

Other studies

Part of the causes of the COVID-19 pandemic can be environmental, like climate change and deforestation. According to the World Bank climate change can increase the risk of an epidemic like the coronavirus by a number of ways including by causing deforestation. Deforestation is responsible for 31% of the zoonotic diseases.

Climate change can increase migration of animals and connection between them and humans. This can facilitate the transmission of viruses from one animal to another and to humans. Viruses generally learn to coexist with their host and became more dangerous when they pass to another. The increase in humidity can also make the transmission easier, even though there are suggestions that increase in humidity and temperature may decrease the expansion of the pandemic.

A 2021 study found possible links between climate change and transmission of COVID-19 by bats. The authors suggest that climate-driven changes in the distribution and richness of bat species harboring coronaviruses may have occurred in eastern Asian hotspots (southern China, Myanmar and Laos), constituting a driver behind the evolution and spread of the virus.

Climate change reduces the number of animals in populations, what lead to less genetic diversity. Such condition facilitates the spread of viruses. Scientists already linked a number of zoonotic diseases outbreaks to floods and droughts and their frequency will increase with climate change. Also, others impacts of climate change can make societies less stable - more wars, human migration, less effective medical and sanitation systems increase the risk of epidemics.

Increase in temperature can reduce the capability of the human body to fight the virus, while bats will be less impacted.

Climate change can cause food insecurity which can make people eat bushmeat, for example bats, that are possibly linked to the outbreak.

Due to environmental and social causes, outbreaks became more frequent, so that in the past decade the World Health Organization declared global health emergency 4 times. Through the 21st century, the number of pandemics like COVID-19 pandemic can increase due to several factors including climate change.

The origins of the virus that caused the COVID-19 pandemic are not yet known exactly, but, probably, it originated in bats. Bats are vulnerable to climate change due to several factors, therefore, increases in aridity make them move closer to human settlements what facilitates virus's transmission to humans

Emergence of new infectious diseases

In recent decades, experts began to sound the alarm that climate change could be associated with changes to the epidemiology of infectious diseases. This viewpoint focuses on a threat where there is a strong possibility that new, previously unknown infectious diseases will emerge from warmer climates as microbes adapt to higher global temperatures that can defeat our endothermy thermal barrier. Human defenses against microbial diseases rely on advanced immunity, which includes innate and adaptive arms and endothermy. This creates a thermal restriction zone for many microbes. Given that microbes can adapt to higher temperatures, there is a concern that global climate change will select for microbes with higher temperature tolerance that can defeat our immune defenses and cause new infectious diseases to emerge.

There is a specific concern about the emergence of new diseases from the fungal kingdom. Mammals are extraordinary among animals in their endothermy and homeothermy, which allows us to maintain elevated body temperature through life; but it can be defeated if the fungi were to adapt to higher temperatures and survive in the body. Research has shown that fungi that are pathogenic for insects can be experimentally adapted to replicate at mammalian temperatures through cycles of progressive warming. This demonstrates that fungi are able to adapt rapidly to higher temperatures. The emergence of Candida Auris on three continents is proposed to be as a result of global warming and has raised the danger that increased warmth by itself will trigger adaptations on certain microbes to make them pathogenic for humans.

If these threats materialize, medicine will need to conform to new infectious diseases for which it has no previous knowledge or experience. As seen with the recent pandemic of the novel coronavirus (COVID-19); control measures, therapeutics, and effective responses take time, where countless lives are lost in the meantime. However, if proper measures are taken today, it could increase the preparedness for confronting new infectious diseases.

Though climate change will almost certainly bring forth new infectious diseases and change the epidemiology of many existing diseases, it is important to note that humanity has successfully confronted new deadly diseases in recent years, such as SARS, coronavirus, and Ebola. Prior success is no guarantee for future success given that each new disease brings its own set of challenges as seen in with the ongoing pandemic, COVID-19. There is no proof that COVID-19 worsens or is caused by climate change, however, we’ve seen how a new infectious disease can change the way we live as humans in such a short span of time and also the number of deaths that resulted from the pandemic. The best solution against future threats is continued investment in surveillance, antimicrobial therapeutics, epidemiology, and research into mechanisms of these microbes.

The rising of new diseases and worsening of diseases due to climate change have been a wake up call for the science community. The collection of data on how climate changes over the years and how these changes are affecting health were stimulated by Science Diplomacy organizations because this information helped scientists to raise awareness about the situation nationally and internationally so action can be taken. This is shown by the actions that were taken during the malaria and dengue outbreak. However, Science Diplomacy organizations need more support from the world so a better solution can be reached to solve the climate change issue because new infectious diseases will emerge. These new infectious diseases could be the end of humanity if they are not addressed properly and on time.

Climate change may lead to dramatic increases in prevalence of a variety of infectious diseases. Beginning in the mid-'70s, there has been an "emergence, resurgence and redistribution of infectious diseases". Reasons for this are likely multi-causal, dependent on a variety of social, environmental and climatic factors, however, many argue that the "volatility of infectious disease may be one of the earliest biological expressions of climate instability". Though many infectious diseases are affected by changes in climate, vector-borne diseases, such as malaria, dengue fever and leishmaniasis, present the strongest causal relationship. One major reason that change in climate increases the prevalence of vector borne disease is that temperature and rainfall play a key role in the distribution, magnitude, and viral capacity of mosquitoes, who are primary vectors for many vector borne diseases. Observation and research detect a shift of pests and pathogens in the distribution away from the equator and towards Earth's poles. A tool that has been used to predict this distribution trend is the Dynamic Mosquito Simulation Process (DyMSiM). DyMSiM uses epidemiological and entomological data and practices to model future mosquito distributions based upon climate conditions and mosquitos living in the area. This modeling technique helps identify the distribution of specific species of mosquito, some of which are more susceptible to viral infection than others.

Beyond distribution, rising temperatures can decrease viral incubation time in vivo in vectors increasing the viral transmissibility leading to increases in infection rates.

Malaria

Further information: Climate change in Africa § Malaria

Increased precipitation like rain could increase the number of mosquitos indirectly by expanding larval habitat and food supply. Malaria, which kills approximately 300,000 children (under age 5) annually, poses an imminent threat through temperature increase. Models suggest, conservatively, that risk of malaria will increase 5–15% by 2100 due to climate change. In Africa alone, according to the MARA Project (Mapping Malaria Risk in Africa), there is a projected increase of 16–28% in person-month exposures to malaria by 2100.

Aedes aegypti, the mosquito that is the vector for dengue transmission.

Climate is an influential driving force of vector-borne diseases such as malaria. Malaria is especially susceptible to the effects of climate change because mosquitoes lack the mechanisms to regulate their internal temperature. This implies that there is a limited range of climatic conditions within which the pathogen (malaria) and vector (a mosquito) can survive, reproduce and infect hosts. Vector-borne diseases, such as malaria, have distinctive characteristics that determine pathogenicity. These include the survival and reproduction rate of the vector, the level of vector activity (i.e. the biting or feeding rate), and the development and reproduction rate of the pathogen within the vector or host. Changes in climate factors substantially affect reproduction, development, distribution and seasonal transmissions of malaria.

Malaria is a mosquito-borne parasitic disease that infects humans and other animals caused by microorganisms in the Plasmodium family. It begins with a bite from an infected female mosquito, which introduces the parasite through its saliva and into the infected host's circulatory system. It then travels through the bloodstream into the liver where it can mature and reproduce. The disease causes symptoms that typically include fever, headache, shaking chills, anemia, and in severe cases can progress to coma or death. "About 3.2 billion people – nearly half of the world's population – are at risk of malaria. In 2015, there were roughly 214 million malaria cases and an estimated 438,000 malaria deaths."

Mosquitoes have a small window for preferential conditions for breeding and maturation. The ultimate breeding and maturing temperature for mosquitoes ranges from 16 to 18 degrees Celsius. If the temperature is decreased by 2 degrees, most of the insects will succumb to death. This is why malaria is unsustainable in places with cool winters. If a climate with an average of approximately 16 degrees Celsius experiences an increase of about two degrees, the mature bugs and the larvae flourish. Female mosquitoes will need more food (human/animal blood) to sustain life and to stimulate production of eggs. This increases the chance of spread of malaria due to more human contact and a higher number of the blood sucking insects surviving and living longer. Mosquitoes are also highly sensitive to changes in precipitation and humidity. Increased precipitation can increase mosquito population indirectly by expanding larval habitat and food supply. These prime temperatures are creating large breeding grounds for the insects and places for the larvae to mature. Increased temperature is causing snow to melt and stagnant pools of water to become more common. Bugs that are already carrying the disease are more likely to multiply and infect other mosquitoes causing a dangerous spread of the deadly disease.

Climate change has a direct impact on people's health in places where malaria was originally not prevalent. Mosquitoes are sensitive to temperature changes and the warming of their environment will boost their rates of production. A fluctuation of two or three degrees is creating exceptional breeding grounds for mosquitoes, for larvae to grow and mature mosquitoes carrying the virus to infect people that have never been exposed before. In communities living in the higher altitudes in Africa and South America, people are at now at a higher risk for developing malaria because of increase in the average temperature of the surroundings. This is a severe problem because people in these communities have never been exposed to this disease, causing an increased risk for complications from malaria such as cerebral malaria (a type of malaria that causes mental disability, paralysis and has a high mortality rate) and death by the disease. Residents of these communities are being hit hard by malaria because they are unfamiliar with it; they do not know the signs and symptoms and have little to no immunity.

The population at risk of malaria in the absence of climate change is projected to double between 1990 and 2080 to 8820 million, however; unmitigated climate change would, by the 2080s, further increase the population at risk of malaria by another 257 to 323 million. Therefore, reducing the effects of climate change in the present would reduce the total by about 3.5%, saving tens of thousands of lives worldwide.

Non-climatic determinants

Sociodemographic factors include, but are not limited to: patterns of human migration and travel, effectiveness of public health and medical infrastructure in controlling and treating disease, the extent of anti-malarial drug resistance and the underlying health status of the population at hand. Environmental factors include: changes in land-use (e.g. deforestation), expansion of agricultural and water development projects (which tend to increase mosquito breeding habitat), and the overall trend towards urbanization (i.e. increased concentration of human hosts). Patz and Olson argue that these changes in landscape can alter local weather more than long term climate change. For example, the deforestation and cultivation of natural swamps in the African highlands has created conditions favourable for the survival of mosquito larvae, and has, in part, led to the increasing incidence of malaria. The effects of these non-climatic factors complicate things and make a direct causal relationship between climate change and malaria difficult to confirm. It is highly unlikely that climate exerts an isolated effect.

External links

• Effects of global warming on human health
• Central Epidemic Command Center
• Public health emergency of international concern