Since the 1980s, Brazil has recorded confirmed dengue outbreaks, but only in 2007 did Rio Grande do Sul have a case that was not imported from another part of the country. The explanation is simple: since the Aedes aegypti mosquito, which transmits the disease, thrives in warm and humid regions, the climate acted as a protective barrier.
That natural defense is now in the past. In 2024, the year Brazil recorded a record number of cases of the disease, Rio Grande do Sul had nearly 210,000 confirmed infections and 281 deaths.
In Bahia, a state accustomed to higher temperatures and with 3 million more inhabitants, confirmed cases were around 163,000, with 188 deaths.
A study by the Oswaldo Cruz Foundation (Fiocruz) shows that this is not happening only in Rio Grande do Sul. From 2006 to 2024, the number of Brazilian microregions with epidemic levels of dengue more than doubled, from 110 to 292, an extent that represents half the country (54%).
The other microregions (46%) showed medium to low incidence, with sporadic or uncertain risk.
The main explanation for this change lies in the climate. The research identified that the greater frequency of thermal anomalies is the climate indicator that best explains the increase in dengue incidence rates in the long term. The concept refers to the recording of temperatures very different from the usual in a given place over a certain period of time.
"What is observed is that the process of expansion of the dengue transmission area appears to be irreversible. Once the virus and the vector have been introduced, it is unlikely that a scenario of zero transmission will return," the authors warn.
According to infectious disease specialist Celso Granato, a member of the Brazilian Society of Clinical Pathology and Laboratory Medicine, the spread of dengue is one of the greatest proofs of how climate change can impact infectious diseases.
The tropical disease has reached not only colder regions of Brazil, but also countries in the Northern Hemisphere. At this moment, the continental region of the United States is experiencing the largest dengue outbreak in its history. Autochthonous cases were also recorded this year in France and Italy.
Granato explains that all stages of the life of the mosquito that transmits dengue are affected by the climate.
"They lay eggs with a very resistant shell, which can remain in nature for years and break open when there is humidity and high temperature. And these mosquitoes can already be born infected, because there is transmission from the female to the eggs."
Infectious disease specialist Celso Granato. Photo: SBPC/ML
Temperature also alters the behavior of adult mosquitoes.
"When it is very hot, they have to beat their wings faster in order to fly. So, they spend more energy and need to bite more to feed, and end up transmitting the disease more as well."
Oropouche Fever
But dengue is not the only disease affected by climate change. Oropouche fever, which circulated almost exclusively in the Amazon Region until 2023, caused more than 12,000 infections last year, in 23 states and the Federal District.
The relationship with the climate is still being investigated, but experts believe that changes in temperature and rainfall patterns, together with environmental degradation, have contributed to the expansion of the disease.
Granato also draws attention to outbreaks that can occur after extreme events, such as droughts and floods, which have become more frequent with climate change. The alert increases with the forecast that Brazil will be affected by the El Niño phenomenon, which favors these events.
Mosquito that causes Oropouche fever. Photo: Brazilian Society of Tropical Medicine
"In Rio Grande do Sul, after the rain disaster, the frequency of leptospirosis increased many times over," he exemplifies.
The disease is caused by a bacterium transmitted mainly by rat urine, which is spread during floods.
"The floodwater also mixes with sewage and can contaminate what people will drink or use to wash their food. There is a great risk of becoming contaminated with bacteria that cause gastrointestinal problems and with viruses, such as hepatitis A," the infectious disease specialist adds.
Droughts, in turn, represent a different and little-known danger: "When there is a fire, the animals that depend on the forest migrate to the homes of people who live nearby, especially small mammals, which have a series of diseases that can be passed to humans."
The most worrying example is the hantavirus. The strain that circulates in Brazil causes a very serious cardiopulmonary syndrome. Since 2016, 530 infections have been confirmed in the country, with 212 deaths, a fatality rate of 40%.
"We have several previous examples, in Bolivia and Paraguay, for instance. When burnings occur, infected rodents enter people's homes and cause this disease," warns Granato.
Fungi
Another little-known threat is that of fungal diseases. At the latest edition of the Brazilian Congress of Clinical Pathology and Laboratory Medicine, professor Sarah Gonçalves Tavares of the Federal University of Espírito Santo warned that climate change is leading fungi to adapt, and with that, some become harmful to humans.
"We have an average body temperature of 37 degrees Celsius, and 95% of the fungi in the environment cannot grow at that temperature. But the temperature of the environment has been rising, and the fungi are adapting to it. So, this barrier that we have ends up being broken."
The professor explains that the change in temperature is one of the "gears" of this process, as is the increase in ultraviolet rays and other ecological changes, which add up and enable the emergence of certain fungi as pathogens.
"We have microorganisms that were in the environment and were not causes of disease in humans, but they have been adapting to various types of stress over time and have begun to appear, mainly, in patients in intensive care."
UFES professor Sarah Gonçalves Tavares. Photo: Sarah Tavares / Personal Archive
The greatest example is Candida auris, first identified in the 1990s in flooded areas of Asian countries, later in rural regions and, finally, in cities. It has two very dangerous characteristics: it can survive at high temperatures and resist for a long time in the environment.
"A healthy person will hardly develop an infection from this type of fungus, but immunocompromised patients are vulnerable," Sarah adds. "That is why outbreaks of Candida auris have occurred in hospital units, with a fatality rate between 40 and 60%."
Aspergillus-type fungus. Photo: Sarah Tavares / Personal Archive
The first record of infection by the fungus in Brazil was in 2020, in Bahia. Since then, there have been 24 occurrences in 15 states, all in healthcare units. Of the 146 colonized patients, 56 died. When the fungus reached the bloodstream, causing a generalized infection, the fatality rate reached 82%.
A more recent example is Trichophyton indotineae, a species of fungus that causes skin problems, first identified in 2022.
"It has already emerged with resistance to antifungals, because it was probably exposed to them in the environment, and the medication ends up selecting isolated strains that are resistant," says the UFES professor.
Resistant fungi do not respond to traditional treatments, and are therefore much harder to combat. Climate change also interferes indirectly in this process: extreme events increase the vulnerability of crops, which leads farmers to apply more fungicides, accelerating the selection of resistant strains.
Environmental alterations caused by the climate can also contribute to the dispersal of fungi that normally exist in delimited areas. One example is Aspergillus, which develops best in warm and humid regions, and has been advancing to other places, following changes in temperature and humidity patterns, including in Brazil.