Human action spreads hantavirus infection

Study shows that factors such as deforestation and rising temperatures are associated with the incidence of hantavirus pulmonary syndrome.

By Peter Moon  |  Agência FAPESP – Hantaviruses, one of the most feared groups of emerging viruses, cause a severe and sometimes fatal respiratory disease for which there is no vaccine or treatment. Mortality is high, reaching 41% in Brazil.

Hantaviruses are from the Bunyaviridae family and are transmitted by mice and rats that inhabit natural and rural areas. These viruses are responsible for spreading hantavirus pulmonary syndrome (HPS) in Brazil. Humans are most at risk if they inhale saliva or excreta from infected rodents in the form of dried airborne particles or contaminated dust. 

The epidemiological emergence of hantavirus in Brazil led to a study whose results have been published in the journal PLOS One, providing the first evidence that social, landscape and climate factors are associated with HPS incidence in the Neotropics. 

The principal investigator is Jean Paul Metzger, Full Professor at the University of São Paulo’s Bioscience Institute (IB-USP), who is assisted by his PhD supervisee Paula Prist. The research is part of the Thematic Project “Interface project: relationships among landscape structure, ecological processes, biodiversity and ecosystem services” and FAPESP’s Research Program on the Characterization, Conservation, Restoration & Sustainable Use of Biodiversity in São Paulo State (BIOTA). 

“We set out to identify the socio-environmental variables associated with the disease and to establish infection risk conditions,” Metzger said.

Eight hantaviruses have so far been identified in Brazil, as have eight species of murid rodents that are the viruses’ natural reservoirs. Three of these species are responsible for most cases: the hairy-tailed bolo mouse (Necromys lasiurus) in the Cerrado region of northeastern São Paulo State, the black-footed pygmy rice rat (Oligoryzomys nigripes) in Atlantic Rainforest areas of São Paulo State and the montane grass mouse (Akodon montensis) in Paraná State.

The number of cases reported in Brazil between 1993 and 2012 was 1,537, with 207 in São Paulo State and the rest in Paraná, the Cerrado, Rondônia, Amazonia and Maranhão, according to São Paulo State Epidemiological Surveillance Center (CVE).

Most of the cases reported in São Paulo State occurred in Atlantic Rainforest fragments (150), and the rest (57) occurred in the Cerrado, where they were confined to agricultural areas, as N. lasiurus lives in open areas such as farmlands and feeds on crop residues. 

“The study consisted of quantifying the associations between cases of HPS in São Paulo between 1993 and 2012, climate variables such as annual rainfall and annual mean temperature, landscape structure (percentages of native vegetation cover and planted sugarcane, number of forest fragments, etc.), and social factors such as each city’s human development index and numbers of male rural workers older than 14,” Metzger said.

The researchers used statistical models to analyze HPS incidence data for São Paulo State and the selected variables, modeling HPS transmission risk for the two main biomes: Cerrado and Atlantic Rainforest.

A key finding was that the area occupied by sugarcane was the most significant factor when estimating hantavirus infection risk in both biomes.

In terms of landscape and ecology, the researchers noted that the number of cases may rise as a result of deforestation and expansion of sugarcane plantations.

“When forest areas are cleared, the rodent species that specialize in living in a forest environment tend to disappear and are replaced by habitat generalists, which adapt to ecological changes and become abundant in altered landscapes, such as the species that transmit hantaviruses,” Prist said.

Future climate scenarios

The researchers were surprised by the results of their climate-related analysis of hantavirus transmission. “When we analyzed the data obtained with the aim of predicting HPS transmission risk under future climate and sugarcane expansion scenarios, we found that climate change plays a far more significant role in spreading the disease,” Metzger said. “The higher the mean temperature in the municipalities studied, the greater the risk of HPS transmission.”

Metzger is a member of the Intergovernmental Science Policy Platform on Biodiversity & Ecosystem Services (IPBES), specializing in thematic diagnoses of models and scenarios and analyses of land degradation and restoration, in addition to contributing to regional biodiversity assessment for the Americas.

Metzger and colleagues studied climate change scenarios defined by the Intergovernmental Panel on Climate Change (IPCC), estimating that in the worst-case scenario, the percentage of male rural workers older than 14 who are at risk of dying from HPS could rise by 34% from current levels.

“This percentage is an average for São Paulo State in its entirety,” Prist said. “In some municipalities, the severity of the problem could increase more.”

“We produced a risk map that can be used as a basis for preventive measures and to optimize the availability of resources to stop the disease from spreading, especially in municipalities with moderate to high infection rates.”

Hantavirus was first recognized as a cause of infectious disease in the early 1950s, when a cluster of 3,000 United Nations troops stationed in Korea was struck by a mysterious hemorrhagic fever; the virus was named “Hanta” after the Korea’s Hantaan river. It was later determined that 12,000 Japanese soldiers had caught the disease in Manchuria in the mid-1930s.

The virus was isolated only in 1977. There was an outbreak in the United States in 1993, and in the same year, the first cases occurred in Brazil, where three brothers, all farm workers, fell ill and two died in Juquitiba, São Paulo State.

The article “Landscape, environmental and social predictors of hantavirus risk in São Paulo, Brazil” (doi:10.1371/journal.pone.0163459), by Prist P.R., Uriarte M., Tambosi L.R., Prado A., Pardini R., D’Andrea P.S. and Metzger J.P., can be read at: journals.plos.org/plosone/article?id=10.1371/journal.pone.0163459.