El Niño increases infestations of mosquito that transmits dengue in São Paulo state, Brazil, study concludes

An article published in the journal PLOS Neglected Tropical Diseases reports the findings of a scientific study that shows El Niño to be linked to an increase in infestations of Aedes aegypti, the mosquito that transmits dengue, chikungunya and zika. The authors analyzed data from 645 municipalities in São Paulo state (Brazil) for the period 2008-18, concluding that larval infestations in open-air disused containers increased in response to the effects of the weather phenomenon, especially mean seasonal temperature and rainfall above 23.3 °C and 153 millimeters (mm) respectively.

The larval infestation parameter used was the Breteau Index, which measures the ratio of the number of detected containers with mosquito larvae to the number of properties inspected by health workers.

The analysis showed that the cities most vulnerable to proliferation of A. aegypti are mainly in the central and northern parts of the state and that social disparities also contribute to infestations.

Considered one of the most important climate phenomena on Earth, El Niño is a pattern that involves unusual warming of surface water in the Pacific Ocean due to weak easterly trade winds. It affects atmospheric circulation over the Pacific and results in changes to humidity and temperature in various parts of the planet. In light of the expected increase in the frequency and intensity of El Niño in the coming decades, more effective methods of detecting mosquito breeding sites are needed in order to combat proliferation of the disease vector.

“We didn’t analyze dengue cases in the period. We focused on levels of larval infestation, which on their own don’t serve as indicators of cases. However, the number of cases notified will rise if vector infestation increases, since dengue virus is circulating in São Paulo state. Climate conditions were known to favor an increase or decrease in the vector, but no one had ever analyzed the specific influence of El Niño on this trend,” said statistician and epidemiologist Gerson Laurindo Barbosa, a co-author of the article. He is a researcher at Pasteur Institute, an arm of the São Paulo State Department of Health.

The results of the study showed that the index of positive containers (with larvae) in the cities concerned rose by 1.30 in El Niño years, compared to years with neutral or weak events when sea surface temperatures were neither warmer nor colder than normal. In addition to the Breteau Index, the researchers analyzed temperature and rainfall data adjusted for population density and social inequality.

Although they did not work with national data, the researchers believe their findings for São Paulo may be mirrored by results for the rest of Brazil. “In 2023-24, the state and Brazil overall saw the worst dengue epidemic ever, and it was an El Niño year. Unfortunately, reality showed that the climate phenomenon is a strong influence in these cases,” said Francisco Chiaravalloti Neto, last author of the article. He is a full professor and heads the Department of Epidemiology at the University of São Paulo’s School of Public Health (FSP-USP), as well as its Spatial Analysis in Health Laboratory (LAES). He was supported by FAPESP via the Thematic Project “Use of remote sensing and artificial intelligence to predict high-risk areas for Aedes aegypti and arbovirus infestation”, which also resulted in another study involving a computer model that analyzed photographs of building facades to predict the level of risk for dengue in urban areas (read more at: agencia.fapesp.br/50933). 

Epidemic

A. aegypti plays a key role in the transmission of arboviruses such as dengue, chikungunya and zika. It mainly inhabits urban areas and lays its eggs in clean stagnant water.

This year’s dengue epidemic in Brazil is the worst ever, with the number of probable cases reaching 6.54 million in October, the highest since records began in 1986. The number of deaths so far is 5,696 (with another 1,300 under investigation), according to the Ministry of Health’s arbovirus monitoring panel. São Paulo state accounts for 32% of the total number of cases (2.1 million) and its capital, São Paulo city, for almost 10% (635,000).

The latest El Niño began affecting Brazil in June 2023 and was considered one of the strongest in recent years. It was associated with above-average rainfall in the South region of the country and in southern parts of the states of Mato Grosso do Sul and São Paulo, and with below-average rainfall in parts of the North and Northeast regions, as well as above-average temperatures in much of the country.

“Our study both highlights vulnerable areas and provides a basis for stratifying risk over time. So if an El Niño is predicted – and it’s possible to predict it – we can combine data for all these areas to optimize and target surveillance, and to implement timely control actions,” Chiaravalloti Neto said.

The group of scientists began the study in 2019 and had to surmount a number of methodological challenges, including a lack of information on infestation throughout São Paulo state. “Many cities have neither information nor databases. Doing an experiment in an academic setting, transforming the results into a scientific paper and putting them at the disposal of public administrators for use in policymaking is very important. I work for the City of Santa Bárbara d’Oeste’s Department of Health as the head of public policy, and this kind of contact with academia helps our municipality try to reproduce some solutions that could be successful,” said ecologist and geographer Thiago Salomão de Azevedo, another co-author of the article.

For Barbosa, by supplying new tools to identify high-risk areas for A. aegypti infestation, academia supports policymakers in making science-based decisions that prioritize the most affected regions. “The sharper the focus of our work, the more the human and financial resources involved can be optimized,” he said.

About 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.