Rainfall and temperature shape mosquito fauna in Atlantic Forest bromeliads, including malaria vectors

(Press-News.org) The transmission of malaria by the Anopheles cruzii mosquito in the South and Southeast of Brazil was so alarming in the 1940s – with approximately 4,000 cases per 100,000 people – that the disease became known as bromeliad malaria. This is because the Kerteszia subgenus of the mosquito, which transmits the disease in the Atlantic Forest, develops only in bromeliads, plants that accumulate water and maintain conditions favorable for the development of this and other species.

Although malaria is now a minor concern in the region, it still has epidemiological importance. There were 77 confirmed cases in the state of São Paulo alone between 2017 and 2024. Therefore, understanding the life cycle of the vectors and the conditions necessary for their survival is essential to preventing the disease from ravaging this part of Brazil as well, since it is endemic in the Amazon.

In a study published in the journal Scientific Reports, a group led by researchers from the University of São Paulo’s School of Public Health (FSP-USP) monitored the presence of larvae in bromeliads in the Capivari-Monos Environmental Protection Area (APA) for two years. 

The results indicate that rainfall and temperature directly affect the volume of water accumulated in the bromeliad tanks. This volume, in turn, modifies the physical and chemical characteristics of the water, such as pH and dissolved oxygen. These changes can influence which species of mosquitoes can develop inside the plants and in what quantities. The data may be useful for future epidemiological studies and for predicting possible disease outbreaks.

“There are studies that point to a change in malaria transmission patterns as a consequence of climate change. In these projections, some regions of East Africa and South America would become more prone, while current endemic areas could experience declines in rates due to excessive warming. For these and other reasons, it’s necessary to understand the factors that contribute to the success of vectors,” says Antonio Ralph Medeiros de Sousa, the first author of the study and a researcher at FSP-USP who received a scholarship from FAPESP. 

The work also received support from the Foundation through two projects coordinated by Mauro Toledo Marrelli, a professor at FSP-USP: “Investigation of the climatic and landscape variations effects on the vectors and on the spatial and temporal dynamics of sylvatic yellow fever and autochthonous malaria in fragments of the Atlantic Forest in the State of São Paulo, Brazil” and “Biodiversity of mosquitoes (Diptera: Culicidae) in Cantareira State Park and the Capivari-Monos environmental protection area, State of São Paulo”. The latter project was funded through FAPESP’s Research Program on Biodiversity Characterization, Conservation, Restoration and Sustainable Use (BIOTA). 

Data

The data were collected from 2015 to 2017 during Sousa’s doctoral studies. During this period, the researcher visited the same nine bromeliad specimens at three points in the Capivari-Monos Environmental Protection Area in the Parelheiros neighborhood in the southernmost part of the municipality of São Paulo. 

During each of the ten collections, the amount of water accumulated by each plant was measured, as well as the pH, salinity, and dissolved oxygen levels. Larvae from the present mosquitoes were collected. Later, in the laboratory, the larvae were developed, and the species, or when this was not possible, the genus of each mosquito, was identified. The dataset also included rainfall and maximum and minimum temperatures in the 30 days prior to collection.

The researchers analyzed the data using statistical models that tested a cascade effect, in which an initial disturbance (variation in rainfall and/or temperature) affects other components in a sequence of connected processes. First, they analyzed the effect of accumulated rainfall and average monthly temperature on the volume of water stored in bromeliads. Then, they examined the relationship between volume and variations in the physical and chemical properties of the water, such as pH, salinity, and dissolved oxygen. 

Next, they investigated the relationship between these parameters and the occurrence, richness, and abundance of mosquitoes in bromeliads, considering only species that occurred five or more times during the study period. Finally, they explored the direct and indirect effects of precipitation and temperature on physical-chemical parameters and mosquito fauna.

A total of 523 individuals belonging to 23 species were collected, including Anopheles cruzii, a malaria vector; Culex; and Wyeomyia. The latter two genera are not involved in disease transmission cycles, but their bites can cause discomfort when they are abundant. Each bromeliad contained between seven and 15 species, only ten of which occurred five or more times throughout the study.

The richness and abundance of mosquitoes varied in relation to pH, salinity, and the interaction between these two parameters. In general, pH was the parameter most associated with the presence of five of the ten species tested, including the malaria vector. 

“In a scenario of changing rainfall and temperature patterns, there may be an increase in the abundance of the malaria vector, with consequences for public health. However, it’s important to remember that Anopheles cruzii is wild, unlike the dengue vector, Aedes aegypti, which is urban. Therefore, the way to deal with it is different,” Sousa points out.

The researcher refers to control measures since insecticides cannot be used, as with the dengue, Zika, and chikungunya vectors, nor can bromeliads be uprooted, as was done during past malaria outbreaks in the Southeast to control mosquitoes. 

On the other hand, it is important to consider the natural controls of mosquitoes in the wild, such as predation and competition with other species. “Perhaps the effect of the increase in Anopheles isn’t as drastic as it’d be for an urban species,” the researcher notes.

Nevertheless, it is important to understand and monitor mosquitoes because malaria in the Atlantic Forest may be zoonotic, meaning it can infect non-human primates, such as howler monkeys. These monkeys can then infect mosquitoes that could transmit the disease to humans.

While the study does not suggest an imminent risk of malaria spreading, it sheds light on how the environment influences vector populations. This knowledge is essential for predicting possible future scenarios, especially in the context of climate change.

“Climate change, urban expansion, deforestation, and biodiversity loss are all factors that could interfere with transmission dynamics in the future. That’s why it’s important for public authorities to be vigilant,” the researcher concludes.

About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe.

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