Environmental change + genetic mutation = new viruses

Posted: 9 August 2004

Environmental alterations, such as replacing forests with ranchland, combined with genetic mutations, can produce emerging viruses like severe acute respiratory syndrome (SARS) and HIV, researchers from the University of Texas Medical Branch (UTMB) in Galveston have discovered.

"Many microbiologists would agree that nature is a more dangerous producer of new microbial threats than any bioterrorist ever will be," said Professor Scott Weaver, director for emerging infectious diseases at UTMB's Center for Biodefense and Emerging Infectious Diseases and senior author on the paper.

Dr. Scott Weaver, University of Texas Medical Branch. Photo: UTMB
Dr. Scott Weaver, University of Texas Medical Branch. Photo: UTMB
Dr. Scott Weaver in his lab at the University of Texas Medical Branch© UTMB
The scientists studied viruses from two late 20th century outbreaks of Venezuelan equine encephalitis (VEE) - a deadly illness that can cause brain inflammation in horses and people - and compared them with a similar virus that does not usually infect horses or people. The outbreaks occurred in 1993 and 1996 in deforested regions of the Mexican states of Chiapas and Oaxaca.

They cite evidence showing that clearing forests for ranchland along a 805 kilometre (500 mile) long, 32 to 80 kilometre (20 to 50 mile) wide stretch of Mexico's and Guatemala's Pacific coastal plains put extreme evolutionary pressure on the VEE virus strain that used to be prevalent there.

Scientists previously believed that this VEE was spread by a subspecies of mosquito called Culex (Melanoconion taeniopus) that infects and feeds on rodents and other small mammals but does not transmit the virus to horses or people.

Genetic mutation

In a paper published earlier this month, Proceedings of the National Academy of Sciences, the researchers show that, as deforestation wiped out the Culex mosquito subspecies, a single genetic mutation in the virus allowed it to move into a new niche, increasing the virus's ability to infect and be transmitted by a different mosquito species, (Ochlerotatus taeniorhynchus). This species prefers to feed on horses and other large mammals.

The mutation also made the virus more infectious and now easily transmitted to horses and people.

"What's troubling," said Weaver, "is that this shows a virus can find a simple genetic mutation that allows it to switch to a new species of mosquito that has the capacity to infect horses and people."

No samples exist today of the VEE virus strain that once circulated between mosquitoes and small mammals in forests and swamps along the Chiapas and Oaxaca coastal plains. But the researchers had access to samples of a similar VEE virus widespread in the nearby coastal Guatemalan community of La Avellana between 1968 and 1980.

Environmental change

By making a DNA copy of that Guatemalan virus genome, the scientists were able to prompt mutations in the lab that resulted in amino acid changes in the envelope glycoprotein. Just one of those changes in the Guatemalan virus, it turned out, controlled the infectivity of the virus for the mosquito species Ochlerotatus taeniorhynchus.

"If the coastal plains were still forested, we wouldn't have this new virus," Weaver said.

VEE, like SARS, HIV and hantaviruses, is an RNA virus, meaning that its genetic material is encoded in a single-strand RNA molecule rather than the double-stranded structure characteristic of the DNA double helix.

"RNA viruses have the capacity to mutate so frequently that they are able to respond very readily to new environmental opportunities we provide them or selective pressures we put on them," Weaver said.

"Environmental changes can create opportunities for viruses and other microbes to change their vectors and their host ranges," he said. "In this case, the environmental change resulted in a change in vectors to mosquitoes that are highly attracted to horses and human beings."

Weaver and his colleagues are also examining the effect of neotropical deforestation on the ecology and epidemiology of several arboviral diseases in the Amazon basin of Peru.

"Humans are now an important component of Amazonian ecological systems," Weaver says. He is researching the possibility that the direct and indirect effects of human disturbance alter the frequency of contact among hosts, vectors and viruses, altering transmission cycles and potentially enhancing human contact and epidemic emergence.

Copyright Environment News Service (ENS) 2004. All Rights Reserved.

Read the study, Venezuelan equine encephalitis emergence: Enhanced vector infection from a single amino acid substitution in the envelope glycoprotein, by Aaron C. Brault, Ann M. Powers, Diana Ortiz, Jose G. Estrada-Franco, Roberto Navaro-Lopez and Weaver.