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May 3, 2004

Crossing the Barrier 

Persistent attempts by animal viruses to leap the species barrier can eventually spark a new viral epidemic among humans.

In the two decades since the AIDS pandemic began, one critically important question has never been satisfactorily answered: How did the human immunodeficiency virus (HIV) that causes the disease first infect people? Understanding how the virus emerged and spread could help prevent future pandemics of diseases as yet unknown.

DNA sequence analysis of HIV has convinced most scientists that the virus is a mutation of a simian immunodeficiency virus (SIV), commonly found in native African populations of nonhuman primates. The analysis also suggests a chimpanzee SIV first entered human populations and mutated to HIV more than 50 years ago. But how?

Now a team of Johns Hopkins Bloomberg School of Public Health researchers has shown conclusively that hunting and butchering infected nonhuman primates creates a bridge for viral transmission into human populations. In the March 20 issue of The Lancet, the team documents the transmission of simian foamy virus (SFV)—a retrovirus similar to SIV—to individuals living in Central Africa who frequently hunt and consume chimpanzees and gorillas. This “bushmeat” is an essential source of protein for forest-based populations practicing subsistence hunting. Team members say their discovery shows that contact with blood and body fluids of SIV-infected animals during hunting and butchering was probably the fatal spark between species that ignited the AIDS pandemic. Moreover, their results demonstrate that retroviruses are actively crossing into human populations, posing the potential of new and catastrophic human diseases.

Nathan Wolfe (standing) hunts retroviruses in Africa with mentor Donald Burke.

Nathan Wolfe (standing) hunts retroviruses in Africa with mentor Donald Burke.

“The thing about retroviruses is that they never go away,” says lead author Nathan Wolfe, DSc, assistant professor of Epidemiology.

The School’s study was conducted in southern Cameroon, a region of tremendous biodiversity. Recently, as half the population has become urbanized, a vigorous trade from the villages has provided bushmeat to the cities. Although it is illegal to hunt, kill, or sell non-human primates in Cameroon, the practice is widespread.

In their study involving 200 individuals from each of nine villages, researchers found that more than 60 percent of the participants reported direct exposure to fresh blood and body fluids of primates. About 1,100 individuals were screened for SFV antibodies, and test results confirmed 10 had acquired SFV; all reported multiple contacts with blood and body fluid from nonhuman primates.

“These cross-species events have probably been going on for quite awhile creating small groups of humans with low levels of infection that ‘percolate’ but don’t erupt,” says Donald Burke, MD, professor of International Health and founder of the Cameroon research project. “This is a pattern similar to our new concept of ‘viral chatter,’ in which viruses like SARS are continually probing across the species barrier, from animals to humans, until eventually  one ignites into an epidemic.”

Wolfe and his team will soon begin following SFV-infected individuals to screen for secondary transmission to family members and to look for evidence of illness. For now, the study underscores the point that humans “should not be hunting nonhuman primates,” says Wolfe.

“The important thing is not to simply penalize the hunters, most of whom are doing this just to stay alive. We need to provide economic options so it won’t be necessary to hunt bushmeat, and change attitudes [so city-dwellers] do not demand it.”—Mike Field

"Crossing the Barrier" appears in the spring 2004 edition of Johns Hopkins Public Health Magazine, which is available May 18, 2004. Click here to receive a copy.

Public Affairs media contacts for the Johns Hopkins Bloomberg School of Public Health: Tim Parsons or Kenna Brigham at 410-955-6878 or paffairs@jhsph.edu.