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How deforestation poses dengue pandemic risk for Africa and Asia

Some diseases are extending distribution ranges
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Aedes albopictus: the Asian tiger mosquito. Used under Creative Commons license. Flickr/Ian Jacobs

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As the world continues to battle COVID-19, scientists are sounding the alarm over the rise of other zoonotic diseases. One of these is dengue, a mosquito-borne viral infection, found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas.

Increased deforestation and climate change continue to add to the predisposing conditions that may thrust Africa and Asia into other pandemics, scientists say, urging better assessments of the disease and predictions on how far it could and has spread throughout the world.

A group of researchers from the Center for International Forestry Research (CIFOR) and Spain’s University of Malaga believe more regions previously viewed as lower risk areas for dengue are now more prone to it, in part due to the rapid loss of forest cover pushing the increased spread of vectors of the virus.

“In Africa and in Asia, there is high potential for the re-emergence of sylvatic dengue in the human transmission cycle because of deforestation, climate change, and vector geographic expansion,” said Alisa Aliaga-Samanez, the lead author of a paper on the subject published in Plos.org. “While the existence of sylvatic dengue cycles has not been demonstrated in the Neotropic realms, there are reasons to foresee this possibility.”

The two regions bear the heaviest burden of the disease, according to the World Health Organization (WHO).

Africa is among the top regions with high forest loss, with 9 percent of forest cover lost between 1995 and 2005 across sub-Saharan Africa, representing an average loss of 40,000 sq kms of forest a year according to CIFOR and World Agroforestry (ICRAF).

The disease is prevalent in a variety of tropical eco-regions including forests, savannas, mangroves, montane grasslands and xeric lands, as well as by low elevations and a high minimum temperature in the coldest month.

Increasing favorability during the early 21st century occurred outside the tropical regions in temperate grasslands, and in areas with high maximum annual temperatures, high pluviometric irregularity but low annual temperature ranges and rainfall.

“The rate of disease emergence is increasing globally, and many long-existing diseases are extending their distribution ranges,” said Julia Fa, an associate scientist with CIFOR-ICRAF, who is also an author on the paper. “This is the case for dengue, a global pandemic whose mosquito vectors are currently occupying ever-increasing numbers of regions worldwide.”

The researchers say the potential role of primates and sylvatic vectors could be increasing the risk of disease transmission in tropical forests.

With its new characteristics, dengue transmission is likely to affect many areas in Asia, Africa, North and South America, Oceania, and regions with no or very few cases of occurrence, including cities in Europe and Japan, according to the research.

“So, the geographical extent of this spread needs to be assessed and predicted,” said Aliaga-Samanez, adding: “There are also reasons to suggest that transmission of dengue from non-human primates in tropical forest cycles is being underestimated.”

Dengue is a viral disease caused by the dengue virus, a group of four Flaviviridae serotypes. The pathogen is principally transmitted by female mosquitoes of the genus Aedes to humans. In most cases, the pathogen causes mild illness, but is also known to cause flu-like symptoms, occasionally producing severe complications that are fatal.

WHO reports that annual dengue cases have been on the rise with one modeling estimating  390 million dengue virus infections per year, of which 96 million manifest clinically (with any severity of disease)  while another study on the prevalence of dengue estimates that 3.9 billion people are at risk of infection with dengue viruses.

The number of dengue cases reported to WHO increased over eightfold over the last two decades, from 505,430 cases in 2000, to over 2.4 million in 2010, and 5.2 million in 2019. Reported deaths between the year 2000 and 2015 increased from 960 to 4032,” wrote WHO in the latest update on the disease.

The scientists believe, the estimates could be conservative since many cases go unreported.

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This research forms part of the CGIAR Research Program on Forests, Trees and Agroforestry, which is supported by CGIAR Fund Donors.
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