Climate change heightens risk of mosquito-borne disease outbreaks

Scientists use mathematical models to predict distribution areas by 2100 of four arboviruses: Oropouche, Mayaro, Rocio, and St Louis encephalitis.

By Karina Toledo  |  Agência FAPESP – Rising global average temperatures due mainly to greenhouse gas emissions will contribute to increases in the distribution areas of four mosquito-borne viruses: Oropouche (OROV), Mayaro (MAYV), Rocio (ROCV), and St Louis encephalitis (SLEV).

This finding is from a study published in the journal PLOS Neglected Tropical Diseases. The research was conducted at Butantan Institute in São Paulo, Brazil, during Camila Lorenz’s PhD with support from FAPESP and supervision by Lincoln Suesdek, a professor in the Parasitology Department at the University of São Paulo’s Biomedical Science Institute (ICB-USP). Flávia Virginio, Thiago Salomão, Breno Aguiar and Francisco Chiaravalloti-Neto, researchers in the same university’s Public Health Department (FSP-USP), also took part.

“We collected data on all outbreaks of these arboviral diseases in Brazil since the 1960s and analyzed how the location of each outbreak correlated with a range of environmental factors,” Lorenz told Agência FAPESP. “Based on the results, we modeled the distribution of the diseases until 2100. Our findings show that the distribution areas of the four arboviruses will increase in the coming years, owing mainly to rising temperatures.”

Seven environmental factors were considered in the multivariate analysis: annual rainfall (how much rain fell during the year in the area where the outbreak occurred), average annual temperature, elevation (altitude), seasonality of temperature (the difference in temperature between the warmest and coolest months of the year), seasonality of precipitation (the difference in rainfall between the wettest and driest months), thermal amplitude (the low-to-high temperature range during the month), and daytime temperature variation.

According to Lorenz, the results suggest that each virus is affected differently by the environmental variables. In the case of OROV and MAYV, for example, outbreaks correlated most closely with average annual temperature and thermal amplitude. Both had similar characteristics and were distributed mainly in the North of Brazil. On the other hand, outbreaks of SLEV and ROCV correlated most with annual rainfall (the more rain, the more outbreaks).

“Although temperature was fractioned in different variables, it was present in all cases to some extent. Rainfall also contributed to the occurrence of outbreaks, since the presence of water is necessary for mosquitoes to reproduce,” Lorenz said.

Elevation significantly influenced only the distribution area of ROCV, she added. A major outbreak of the disease caused by this virus occurred around 1975 in the Ribeira Valley region of São Paulo State.

“Temperature is of course a well-known factor in tropical diseases, but however much common sense points in this direction, we can have scientific certainty only through experiments or statistical validation,” Suesdeck said. “Our study shows that just as the viruses have different characteristics and different lifecycles inside and outside the host, they are also influenced differently by environmental factors. It offers guidelines for the refinement of strategies to detect and control these diseases.”

Maps showing current distribution of four arboviruses (left-hand column), distribution under a low-emissions
scenario by 2100 (middle column) and distribution under a high-emissions scenario by 2100 (right-hand column)

Future climate

In addition to mapping the physical characteristics of all the places in which outbreaks of these four arboviruses have occurred in the past 50 years, the researchers also analyzed climate data since the 1960s and found that the average temperature in Brazil has risen in recent years, especially in the North.

They next used mathematical models to project distribution of the four viruses by the end of the century. The models were based on two future climate scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). 

The first scenario, which assumed a low level of greenhouse gas emissions, predicted a global temperature rise of 1 °C by 2100. The second, which assumed very high emissions, predicted a temperature rise in excess of the 2 °C considered safe by climate scientists.

The modeling work was done by Thiago Azevedo, a researcher in the Geography Department of São Paulo State University (UNESP) in Rio Claro. He calculated the high-risk area for each of Brazil’s state capitals under the two scenarios.

In the case of São Paulo City, for example, the area susceptible to MAYV is forecast to expand from 4% now to 12% in 2050 and almost 20% in 2100 under the high-emissions scenario. For ROCV, the area is set to rise from approximately 1% of the city now to 2.5% in 2050 and almost 4% in 2100.

In Campo Grande, the capital of Mato Grosso do Sul, the high-risk area for MAYV is predicted to rise from 23.8% to 83.6% under the worst-case scenario. In Brasília, the national capital, the area is forecast to expand from 10% to over 57%. In Belo Horizonte, the capital of Minas Gerais, the area will jump from 14.8% to 65%, and in Rio de Janeiro, it will expand from 21.4% to almost 55%.

The greatest increase in the distribution area for ROCV is forecast for Porto Alegre, the capital of Rio Grande do Sul. Currently less than 9% of the city is considered a high-risk area for this virus. The area is predicted to reach 57.3% in 2100 under the high-emissions scenario.

“In the case of MAYV and OROV, we see two future scenarios that are quite different from today, whether with low or high emissions. For SLEV and ROCV, the difference isn’t so stark, but even a small rise is important, as these are little-known diseases and we have no vaccines for them,” Suesdek said.

The four diseases studied, he went on, could potentially cause significant harm to public health and can be considered neglected. They all have acute and intense fever as their main symptom. Because they are easily confused with dengue or malaria, experts believe they are very probably underreported. There are no serological tests for diagnosis (to detect antibodies against the virus in the patient’s blood), and molecular tests are expensive and hard to obtain.

The article “Impact of environmental factors on neglected emerging arboviral diseases” (doi: https://doi.org/10.1371/journal.pntd.0005959) by Camila Lorenz, Thiago S. Azevedo, Flávia Virginio, Breno S. Aguiar, Francisco Chiaravalloti-Neto and Lincoln Suesdek can be read at: journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0005959.