A long-anticipated paper (by me anyway!) has finally been published in this week's issue of Nature. In this paper, we show that wild chimpanzees living in the Gombe National Park in western Tanzania on the shores of Lake Tanganyika appear to die from AIDS-like illness when infected with the Simian Immunodeficiency Virus (SIV). Many African primates harbor their own species-specific strain of SIV and chimpanzees are no exception. The host species for a particular SIV strain is indicated by a three letter abbreviation (all in lower-case) following the all-caps SIV. So, for chimpanzees, the strain is called SIVcpz. It turns out that there are two distinct HIVs, known as HIV-1 and HIV-2. HIV-1 is the virus that causes the majority of the world's deaths. It is what we call the "pandemic strain." HIV-2 is less pathogenic and has a distinct geographic focus in West Africa. The HIVs and the various SIVs belong to a larger group of viruses that infect a wide range of mammals known as the lentiviruses (lenti- meaning slow, referring to the slow time course of the pathology typically caused by these viruses). Collectively, we call the SIVs and HIVs "primate lentiviruses." Both HIV-1 and HIV-2 have well-documented origins in nonhuman primate reservoirs. HIV-2 is most closely related to SIVsmm, a virus that infects sooty mangebeys (a type of West-African monkey). HIV-1, on the other hand, is most closely related to SIVcpz, the virus that infects central and east African chimpanzees. We believe that both HIV-1 and HIV-2 entered humans hosts when hunters were contaminated with the blood of infected monkeys (HIV-2) or chimpanzees (HIV-1). Note that this means that our terminology for the primate lentiviruses is polyphyletic. SIVsmm and HIV-2 are sister species, while SIVcpz and HIV-1 are sister species. Yet we call all the viruses that infect nonhuman primates simian and all the viruses that infect humans human immunodeficiency viruses. It seems to me the best way to fix this would be to call the viruses that infect humans SIVhum1 and SIVhum2. Of course, that will never happen, but I do think that it's important to clarify the evolutionary history of these viruses.
The conventional wisdom regarding primate lentiviruses is that, with the exception of HIV, they are not pathogenic in their natural host. The reasoning for why HIV causes the devastating pathology that characterizes AIDS goes that HIV-1 is a relatively new infection of humans, having just spilled over into the human population recently. Pathogens that have recently crossed species boundaries are frequently highly pathogenic because neither the new host nor the pathogen has a history of coevolution with its new partner. While it is a pernicious myth (that just won't seem to die) that pathogens necessarily evolve toward a benign state, it is true that they frequently evolve a more intermediate level of virulence from their initial spillover virulence. There are a number of problems with the idea that HIV causes AIDS because it is poorly adapted to human physiology.
The first of these is that HIV-1 is not that recent an infection of humans. Sure, we didn't notice it until 1983 but careful molecular evolutionary analysis by Bette Korber of the Santa Fe Institute and my collaborator Beatrice Hahn and her group at the University of Alabama Birmingham puts the most likely date for the emergence of HIV-1 in humans to be 1931. That means that HIV-1 was being transmitted from human-to-human for over fifty years before it was ever noticed by western science. Fifty years, while certainly brief in evolutionary terms, is still long enough to lead to some reduction in virulence or host evolution.
The real nail in the coffin, however, is our new result. Specifically, we show that SIVcpz causes AIDS-like pathology in the Gombe chimpanzees. This result is surprising because (1) given it's pathogenicity, one would expect someone to have noticed it before, and (2) chimpanzees infected in captivity do not show obvious AIDS-like illness. I have been collaborating with Anne Pusey, Mike Wilson and their colleagues at the University of Minnesota's Jane Goodall Institute Center for Primate Studies on the the analysis of the demography of the Gombe chimps for a number of years now. Anne and Mike have, in turn, been collaborating with Beatrice Hahn with her project on monitoring natural SIV infection in wild chimpanzees across Africa. Given my background in HIV epidemiology and statistics, it was only natural that we all join forces to look at the demographic implications of SIV infection among the Gombe chimps. Jane Goodall famously started chimpanzee research at Gombe in 1960 and since 1964, researchers at Gombe have collected detailed demographic information, documenting all births, deaths, and migration events in the central community and eventually expanding to the peripheral ones in later years. As a result, we have an unmatched level of demographic detail (not to mention behavioral and ecological information) against which to assess the impact of SIV infection. Using statistical methods known collectively as event-history analysis, we were able to show that the hazard ratio between SIV-infected and SIV-negative chimps is on the order of 10-16. This essentially means that SIV+ chimps have mortality rates that are 10-16 times higher than uninfected chimps. The analysis controls for the clear potentially confounding effects of age and sex on overall mortality. The reason why no one ever noticed this heightened mortality rate is really because no one has ever looked for it. Even when a mortality rate is 10 times higher for some segment of a population, when that segment is small and when mortality rates quite low (chimps who survive infancy can live in excess of 40 years) it can be hard to detect even a seemingly large difference. This is why we do science: because things that seem obvious once we know they are there can be remarkably subtle when we don't know they're there. Science gives us the framework and the tools for studying nature's subtleties.
This project was absurdly interdisciplinary. The paper has 22 co-authors, each contributing his or her own particular analytical expertise or providing access to crucial data necessary for the larger narrative. There are papers in the literature in which people are made co-authors for pretty thin contributions. This paper has none of that. It was an extremely complicated story to tell and it really required the collaboration of this large team. Such work is not easy to manage and it's not at all easy to do well. I think that Beatrice should be commended for orchestrating all the various major contributions, keeping us in line and on schedule (more or less). It's really gratifying to see the excellent blog piece by Carl Zimmer in which he notes the virtues -- and the difficulty -- of combining various scientific styles in pursuit of an important question. The title of Carl's piece is "AIDS and the Virtues of Slow-Cooked Science." In addition, there is a nice companion piece in this week's Nature written by Robin Weiss and Jonathan Heeney. They too note the strength of the interdisciplinary approach to this problem.
The paper isn't even officially published until tomorrow and it has already been covered on Carl Zimmer's blog for Discover Magazine, The New Scientist, The Guardian, The Scientist, The New York Times and MSNBC. Wow. Weiss & Heeney note a number of questions that are raised by our analysis. Specifically, they ask "why was the progression to AIDS-like illness not more apparent in chimpanzees in captivity?" My co-author Paul Sharp notes "We need to know much more about whether there are any genetic differences among the chimpanzees, or differences in co-infections with other viruses, bacteria or parasites, which influence whether or not SIV infection leads to illness or death. This presents a unique opportunity to compare and contrast the disease-causing mechanisms of two closely related viruses in two closely related hosts." Then, of course, there are the conservation questions that this paper raises. Chimpanzees in the wild have birth rates that are very nearly balanced out by their death rates. This difference, called the intrinsic rate of increase, largely determines the probability of extinction of a small population. When the rate of increase of a population is negative, it is certain to go extinct (assuming the rate remains negative). However, even if the intrinsic rate of increase is greater than zero, the randomness that besets small populations still means that a population can go extinct. So, because their average birth and death rates are so close, individual chimp populations are certainly in potential jeopardy of going extinct, and Gombe is no exception to this rule. Now we add to a population something that increases mortality rates 10-16 times. This is bound to have negative consequences for the persistence of affected chimp populations. This is a topic that we are exploring even as I write...