POSTED 1 NOV 2000
Familiarity breeds...
How exactly do diseases infect new species? Simple proximity is a critical
factor. The more intimate you are with a pathogen, the more likely it is to
take up residence in your body. The ultimate intimacies -- exchanges of blood
and other bodily fluids -- are the major routes of infections of HIV,
hepatitis, and the long list of mosquito-borne diseases -- including malaria.
So it should not be surprising that hunting and butchering of wild animals
were major risks in a recent
study of disease emergence.
Cooked monkey may look unappetizing to Westerners, but when high-protein food is scarce, poaching is profitable. Due to their intimate contact with bodily fluids, hunters and those who prepare "bushmeat" are prone to catching new infections.Copyright and courtesy Karl Ammann
Study author Nathan Wolfe, a biologist with the Johns Hopkins School of Public Health, wrote us from the field in Cameroon to explain that the links between hunting and Ebola transmission were not experimentally proven, but were based on surveys showing that the more you hunted, the more likely you were to contract the deadly bleeding disease.
In fact, Wolfe says "the best evidence for the link between hunting and transmission of Ebola is among chimpanzees." Studies of chimpanzees in the Ivory Coast by Pierre Formenty and colleagues, he adds, found that "individual chimpanzees who hunted had the highest risk of being infected with the Ebola virus. Hunting chimpanzees, in fact, were more likely to be infected than chimpanzees with other well-known risk factors, such as direct physical contact with individuals who had died from Ebola."
Similarity breeds...
The linkage between Ebola in chimps and humans points to a second factor in
disease emergence: similarity between species. "A great deal of evidence
suggests that the more closely related two species are the greater the chance
that they will be able to share infectious diseases," Wolfe wrote. "As our
closest living relatives, the apes, and to a slightly lesser extent the other
nonhuman primates, are susceptible to many of the same infectious diseases as
humans."
Which makes hunting nonhuman primates all the more dangerous: "Hunting and butchering of these animals is likely to expose humans to a broad range of infectious agents which have the potential to cross and potentially spread among humans."
If this is true, Wolfe argues, monitoring other primates may be a good way of spotting new diseases before they reach the catastrophic stage. Such an effort might have detected the emerging infection HIV among the non-human African primates now considered as the source of the infection.
Treating
patients with Ebola virus during an outbreak in Kikwit, Zaire (now Democratic
Republic of the Congo), in 1995.Courtesy Center
for Disease Control, Special Pathogens Branch
A final point is that viruses often become more deadly when they jump between species. Ebola, for example, apparently lives in a "reservoir" organism that it does not kill. For the past 25 years, it has been causing small but deadly infections when it jumps from this -- what is it, anyway? -- to infect humans. Ebola is about as deadly as a virus gets: In 1995, death came to almost 80 percent of Congolese people with confirmed infections.
Contemporary contagion
As we've learned recently, AIDS seems to reflect many of the ecological and
technological sources of disease emergence: an increasing population that
drives people deeper into the forests, a virus that jumps a small gap between
species, and transportation that quickly spreads the virus around the world.
Because none of these contributing factors seems likely to be mitigated any time soon, many experts expect ever-more emerging infections. And while the constant bleeding of Ebola is a horrific way to die, Ebola remains just a footnote compared to the emerging disease AIDS, which has already filled 19 million graves and subtracted 20 years from the life expectancy in some South and East Africa nations that were once poised for development.
More than 20 years after AIDS emerged from the African forests, the epidemic continues to spread -- accounting for 2.8 million deaths in 1999 alone.
About 34 million people are now infected with HIV, and while Africans have suffered most, some experts predict that within five years, HIV may be killing more people in Asia.
Not sick of emerging infections? Check out our bibliography.
