Explosive Relationship: Global Warming and Tropical Diseases

Publicação: 6 de November de 2019

Using mathematical models, scientists estimate what the range of four arboviruses will look like by the end of the century: Oropouche, Mayaro, Rocio and Saint Louis encephalitis virus

The largest increase in Rocio’s distribution area was forecast for Porto Alegre (RS). Currently, less than 9% of the municipality is considered a risk area. In 2100, in the high emission scenario, the index would reach 57.3%

It may seem daunting, but scientific evidence indicates that if drastic action is not taken, we will face irreversible damage to the planet and the collapse of societies in the coming years. So warned British naturalist David Attenborough in the documentary “Climate Change”. But the warning is not recent. Eighteen years ago, the relationship between climate change and health was declared a scientific consensus by the Intergovernmental Panel on Climate Change (IPCC). For years climate change has been before our eyes in real time.

With population growth and globalization, disease-causing agents began to circulate much faster, shifting continents overnight, and Europe became vulnerable to epidemics until then typical of tropical regions. Noted researcher of the subject, Dr. Ulysses Confalonieri believes that global warming can alter tropical diseases. “This may possibly occur in some regions if more humidity and temperature accelerate the development of biological agents, especially vectors”, he points out.

The increase in the average temperature of the planet, induced mainly by the emission of greenhouse gases, should contribute to expand, in Brazil, the distribution area of four mosquito-transmitted viruses: Oropouche (OROV), Mayaro (MAYV), Rocio (ROCV) and Saint Louis Encephalitis Virus (SLEV). The conclusion is from the study of Camila Lorenz, Thiago S. Azevedo, Flávia Virginio, Breno S. Aguiar, Francisco Chiaravalloti-Neto and Lincoln Suesdek, published in September 2017, in PLoS Neglected Tropical Diseases magazine entitled Impact of environmental factors on neglected emerging arboviral diseases.

The future of tropical diseases in face of global warming

Mosquito-borne diseases cause a great deal of disease worldwide. The vital rates of these ectothermic vectors and parasites respond strongly and nonlinearly to temperature and, therefore, to climate change. Dr. Erin A. Mordecai, one of the authors of the article “Thermal biology of mosquito-borne disease” points out that vector-borne diseases are a major concern with change because mosquitoes, ticks and other arthropods that carry the diseases are very temperature-sensitive. “Because these vectors have upper and lower temperature limits, climate warming not only increases transmission everywhere, but global warming is more likely to cause changes in the geographical and seasonal range of diseases such as malaria, dengue or Zika. For example, in cold climates and high latitudes and altitudes (such as much of the US and Europe), global warming is expected to create more conducive conditions for diseases such as West Nile fever, malaria, dengue fever, Zika, among others. At the same time, warming the tropics should make the environment more conducive to dengue, Zika, chikungunya and other pathogens”, warns the researcher.

The studies show that different vectors and pathogens have different ideal transmission temperatures: malaria transmission by the African mosquito Anopheles gambiae peaks around 25ºC, while dengue transmission by Aedes aegypti peaks around 29ºC. “This means that some regions that today have high rates of malaria may experience changes caused by temperature towards Aedes-transmitted diseases such as dengue and other viruses, in line with urbanization and other factors that affect disease dynamics”, he explains. According to her, this should be a major concern for global health because dengue control measures are completely different from those of malaria, and because many people have never had any prior exposure or immunity to these emerging viruses.

Dr. Jamie Melissa Caldwell, also one of the authors of the article, adds. “We expect climate change to shift where diseases occur, the types of diseases that are most prevalent, and the burden of disease. That many vector-borne diseases are affected by climate because the physiology of the vectors themselves and their ability to transmit are linked to temperature, humidity and rainfall. Different vectors are optimized for different conditions, so we expect some diseases to spread with climate change while others may be restricted”, she says. Still according to her, changes are expected to be more prominent in the Tropics where year-round conditions exist for many vector-borne diseases. “Obviously, there are many other factors that determine how and if these changes will occur, such as the size of urbanization, population growth, which also affect disease transmission and are rapidly changing in the tropics”, she adds.

Population exposed to tropical diseases

Experts stressed at the European Congress of Clinical Microbiology and Infectious Diseases, held in Amsterdam in the first half of this year, that encouraged by climate change, travel and international trade, epidemics of vector-borne diseases will develop to reach great part of Europe in the coming decades. “This issue of Europe has been much discussed within the IPCC. Models show these possibilities, especially in relation to migration of North African infections”, recalls Dr. Confalonieri. A study by the Institute of Emerging Pathogens in Florida (USA) also draws attention. According to the document, nearly one billion people could be exposed to tropical diseases as climate change is increasing the variety of mosquitoes scattered around the world. Scientists are concerned about how mosquitoes can disperse as the world continues to warm up.

According to Dr. Confalonieri, we still have time to prevent this from happening. “What can be done is to combat vectors in the built environment, such as cities, since forests and other natural ecosystems are difficult or impractical to reach”, he emphasizes by pointing out that mosquitoes disperse rapidly through human transport (airplanes, etc.), but climate-driven migration takes time. Asked if the incidence of tropical diseases in warm regions such as the Caribbean, West Africa and Southeast Asia could decrease as the climate becomes so hot that it exceeds the upper thermal transmission limits, the expert says it is unlikely that pathogens and vectors have wide temperature and humidity ranges in which they can develop.

Health, air pollution and global warming

Nine out of ten people breathe polluted air every day. In 2019, air pollution (a major contributor to climate change) was considered by the World Health Organization (WHO) to be the greatest environmental health risk and the WHO estimates that climate change causes an additional 250 thousand deaths per year due to malnutrition, malaria, diarrhea and heat stress. “Heat stress and pollution exacerbation are more likely the most direct effects. For the other diseases mentioned there are several non-climatic factors that interfere, and it is difficult to predict”, says Dr. Confalonieri.

The increased incidence of natural disasters that have caused floods, storms and droughts that cause more damage to public health and the risk of reduced agricultural production is another consequence of climate change, which according to Dr. Confalonieri, are the so-called extreme events, difficult to predict, even with complex mathematical and statistical models. More intense heat waves and fires; increased prevalence of diseases caused by contaminated food and water and vector-borne diseases; Increased likelihood of malnutrition resulting from reduced food production in poor regions and loss of working capacity in vulnerable populations are major health risks. Uncertain but potentially more serious risks include: collapse in food systems, violent conflicts associated with resource scarcity and population movements, and exacerbation of poverty, undermining health.

Regardless of the expected changes in the dynamics of tropical diseases or their areas of occurrence, the fact is that Climate change must require new ways of thinking about the control and prevention of tropical diseases in the near future. Thus, monitoring the incidence and geographical spread of these diseases should be part of epidemiological surveillance, focusing on populations that already suffer or may suffer the impacts of climate variation. Human-altered ecosystems not only enhance the transmission of emerging diseases, but also contribute to the onset of other ecotoxicology-related diseases that affect the immune system and harm health in general, even if they are not infectious.

While the topic is noteworthy, experts are concerned that health is not yet an important aspect of media coverage of global warming. To change this, some actions were developed. In 2016, the Lancet Countdown was launched. The project aims to monitor evidence linking climate change and human health by 2030.