Temperature records worsen epidemics of infectious diseases like dengue

Temperature records worsen epidemics of infectious diseases like dengue

Climate change brings consequences much greater than excessive heat, increasing the number of deaths and generating unpredictability in disease control. 

We don’t even need to read the news to know that there’s something abnormal in the weather. Marked by an unusually warm winter and monthly high-temperature records being broken, 2023 is already positioned by climate authorities as the hottest year in the last 125,000. 

As if the direct impact of high temperatures weren’t enough, these climate changes also bring numerous and unpredictable consequences for all inhabitants of the planet, such as droughts, natural disasters, migration, and species extinction, as well as shortages in the availability of water and food. This domino effect causes impacts on microenvironments, such as cities and neighborhoods, and can worsen or even generate epidemics of infectious diseases, which are illnesses caused by microorganisms like bacteria, viruses, fungi, or parasites. 

Temperature changes alter the dengue epidemic pattern.

Dr. José Cerbino, a member of the technical chamber advising on Public Health Emergencies at the Ministry of Health and a medical researcher at the D’Or Institute for Research and Education (IDOR) and the Oswaldo Cruz Foundation (Fiocruz), details that climate changes have already impacted the dengue cycle in Rio de Janeiro. “Except for pandemic years 2020 and 2021, the pattern of the epidemic has been clear for the past 40 years: the dengue season starts in January, reaching its transmission peak in late March and early April, which is the wettest period in the state. The year 2023 interrupted this historical series of four decades. This is the first time we have experienced a dengue epidemic in winter, peaking in July,” explains the scientist, adding that Rio de Janeiro has been breaking temperature records since June of this year. 

The researcher argues that climate change has a close and proven relationship with the increase in the number of dengue cases, but laments that this relationship can currently only be observed retrospectively, as there is no predictive model in the country capable of anticipating changes in the behavior pattern of this and other infectious diseases. 

Heatwaves are overlooked in health studies in Brazil.

According to him, what further complicates the construction of these predictive models is that, in Brazil, climate change is a neglected factor in health data collection. In many countries in the Northern Hemisphere, where the four seasons are well defined, the occurrence of heat waves is reported and associated with an increase in various health problems besides infectious diseases. “We don’t have much data on the heatwaves that have been progressively occurring in Brazil, but in European countries and the United States, there are records of increased overall mortality, increased deaths from cardiovascular diseases, increased demand for emergency services, among other aggravating factors,” he exemplifies. 

The lack of robustness in data associating climate change with an increase in diseases in tropical countries is also explained by the neglect of environmental policies, which are rarely a priority in government plans and suffer from low investments, resulting in fewer research efforts and, therefore, less data that can serve as a basis for addressing health crises caused and exacerbated by climate changes. This puts the population in a position of incalculable vulnerability, as climate changes generate different simultaneous impacts, and the consequences of this can be very unpredictable. Dr. Cerbino shares the information that, of the 375 infectious diseases recorded in all of human history, 218 of them have been impacted due to climate change, a percentage of almost 60%. 

Why is it so much hotter?

There are fewer and fewer opinions defending that the changes we are witnessing now are natural factors. It is a fact that the planet has undergone various climate changes over its 4.5 billion years, and the high temperatures faced in 2023 are being exacerbated by El Niño, a natural climate phenomenon with global consequences that occurs at intervals of 2 to 7 years, causing abnormal warming of the waters of the Pacific Ocean. However, Dr. Cerbino argues that scientific evidence no longer leaves room to discredit that human activity is greatly accelerating the processes of climate change on the planet. 

We know that the Earth is a planet in constant change, having experienced ice ages and periods of temperature increase. But we also have ample evidence in various scientific fields that the changes we are witnessing now are happening much faster than they would if human activity did not exist. The increase in the concentration of greenhouse gases, the way we consume resources and occupy spaces altering the environment, all of this is causing the temperature to rise very quickly. Floods and droughts have always existed, but they are more frequent and more intense now. For infectious diseases, such as arboviruses [infectious diseases mainly caused by mosquitoes], there are changes in the life cycle of vectors, and this generates unpredictability in the control of epidemics.” 

How is the transmission of infectious diseases impacted by climate change?

When it comes to controlling infectious diseases, climate changes and environmental imbalance create worrying uncertainties. According to the researcher, these changes can cause disease vectors to migrate to areas that were not affected by these infections, finding an even more vulnerable population without defenses against the problem. 

Another aggravating factor is that climate change can affect the containment measures of these diseases in environments where they have already stabilized. A great example of this is the spread of Aedes aegypti mosquitoes infected with Wolbachia, a bacterium harmless to humans but which in insects blocks the replication of dengue, Zika, chikungunya, and yellow fever viruses. As infected mosquitoes transmit the bacterium to their peers, the method has proven to be very effective in controlling arboviruses worldwide and is already planned to be implemented in several Brazilian municipalities in 2024. 

The bad news, according to Dr. José Cerbino, is that the bacterium is sensitive to excessive heat, and it is not yet known what the result of this implementation will be with the high and increasing temperatures we are reaching. And this is another limitation caused by the lack of predictability in climate-related information. 

Environmental policies are also health policies

“We are advocating and suggesting that health surveillance also starts monitoring climate data. Nowadays, we look at the number of cases, adverse events in medications, and infection data in hospitals, but we don’t look at the climatic issues and their impact on health. If there is an increase in emergency room visits due to a heatwave, surveillance won’t capture it. We need to know how many deaths, and how many hospitalizations arise from a heatwave. These are fundamental data because they form the basis for arguments for the creation of public policies with an environmental focus,” defends the researcher. 

For many decades, environmental impacts intensified by human action have been topics of international conferences and works of fiction. The subject is taught in schools and has adorned posters of different generations of activists, so it is curious to think about how unprepared we are to face these adversities. If there is a lesson to absorb from the recent high temperatures, it is that this is just the beginning of a complex chain of changes that can generate crises never seen in our history as a species. And if the discourse seemed exaggerated during all these years of signaling, nature now makes it clear the urgency for the creation and enforcement of global public policies aimed at containing climate change. 

Written by Maria Eduarda Ledo de Abreu. 


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