Ebola spread to humans linked to population density and vegetation cover

Date: Jan-22-2015 Ebola is a "zoonotic" disease: the virus starts out in animal populations - believed to be fruit bats - and

then spills over into humans. Now, a new study that investigates landscape features of where spillover occurs suggests

human population density and vegetation cover may be important factors.

The researchers examined landscape features of precise geo-locations of Ebola spillover into humans.

The study is the work of two researchers from SUNY Downstate Medical Center in Brooklyn, NY, who write about their

findings in the open-access journal PeerJ.

First author Michael G. Walsh, assistant professor of epidemiology and biostatistics in SUNY Downstate's School of

Public Health, says they found significant interaction between density of human populations and the extent of green

vegetation cover in the parts of Africa that have seen outbreaks of Ebola virus disease (EVD).

However, he also warns that because of the observational nature of the data, the study cannot prove that the

specific pattern of interaction between human populations and forested land causes the spillover of Ebola from animal

reservoirs into humans. The most it can do is suggest that they may help it along.

We first got to know about EVD in 1976 when it broke out in today's Democratic Republic of the

Congo and South Sudan. Since then, there have been 24 outbreaks of EVD in 10 countries in Central and West Africa.

Due to its high rate of fatality - ranging from 32-90% - EVD causes fear and disruption when it strikes. "Never has

this been more apparent than during the 2014 EVD outbreak in West Africa, which is currently underway," write the

authors.

The current outbreak in West Africa is the largest EVD event ever documented. The latest EVD case counts

from the Centers for Disease Control and Prevention (CDC) show there have been 21,689 lab-confirmed cases and 8,626

deaths from the disease in the three worst affected countries: Guinea, Liberia and Sierra Leone.

EVD spillover tied to interaction between human population density and forestation

For their study, the authors used precise geo-locations of where EVD had spilled over from animals into humans in

West and Central Africa. They applied what they call an "inhomogeneous Poisson process model" to the locations.

Even after accounting for other potential influencing factors, such as climate and altitude, the analysis showed that

while population density was strongly associated with spillover from animals to humans, there was significant

interaction between population density and green vegetation cover.

In locations where human populations were sparse, increasing vegetation cover was linked to a decrease in risk of

spillover. But as human population density increased, increased vegetation cover was linked to increased risk of

spillover of EVD from animals to humans.

Prof. Walsh comments on what they found:

"The reservoir species of the Ebola virus is believed to be fruit bats, with a secondary source being non-human

primates. As human populations increase and move into forested areas that are home to these animals, the risk of humans

contracting EVD appears to increase, judging from our analysis of EVD outbreaks in Central and West Africa."

Meanwhile, Medical News Today recently learned how a new epidemic model predicts Ebola in Liberia could end by June. Researchers have developed a computer model that

not only takes into account features of Ebola virus and how it transmits, but also what is being done to halt its

spread.

Written by Catharine Paddock PhD

Not to be reproduced without permission.

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Courtesy: Medical News Today Note: Any medical information available in this news section is not intended as a substitute for informed medical advice and you should not take any action before consulting with a health care professional.