Studies of SIVcpz in Tanzania began in 2000, when infection was first identified in Gombe National Park (42
). This discovery prompted surveys of other chimpanzee communities in East Africa, including the M group in Mahale (B), none of which uncovered additional infections (44
). In contrast, surveys of wild apes in the Democratic Republic of the Congo revealed SIVcpz infection at multiple different field sites (19
). Thus, the presence of SIVcpz in Gombe seemed to represent an isolated case, possibly the remnant of a previously more widespread infection that had largely gone extinct. To test this hypothesis, we conducted a survey in the Masito-Ugalla region of the Greater Mahale Ecosystem, which comprises one of the largest remaining chimpanzee habitats in western Tanzania. There were two reasons for selecting this particular area. First, we wished to test populations that were less geographically confined than those in Gombe and Mahale Mountains National Parks (22
). Second, we were intrigued by the ecology of savanna chimpanzees, which provided an opportunity to study a low-density ape population. Testing fecal samples from just two collection sites yielded a number of unexpected results. First, we found that the 67 Issa Valley chimpanzees all shared the same home range, indicating a community size much larger than that of other known savanna chimpanzees (4
). Second, we discovered SIVcpz infection among these apes, indicating that there was, in fact, a second area of SIVcpz endemicity in Tanzania. Third, we found that 21 of 67 savanna chimpanzees were infected, indicating that even low-density ape populations can harbor SIVcpz at high prevalence rates. Finally, we documented two incident SIVcpz infections in Issa Valley, indicating ongoing virus spread among chimpanzees in this area.
Nearly one-third of chimpanzees sampled within an 85-km2
area of the Issa Valley harbored SIVcpz. This rate of infection is at the upper end of prevalence rates previously determined for wild-living populations (16
), with only the Kalande community in Gombe consistently exhibiting a larger proportion of infected individuals (37
). Previous surveys estimated the population density of chimpanzees in the Ugalla region to be 0.08 individual/km2
, with a slightly higher density in the Issa Valley (0.14 individual/km2
). Thus, these savanna chimpanzees live at 20- to 50-fold lower population densities than the forest chimpanzees in Gombe and Mahale (25
). For SIVcpz to be able to spread and persist in a population, the basic reproductive number, R0
, must be greater than 1. For sexually transmitted diseases, R0
is the product of the transmission probability per partnership (β), the rate of partner change (c
), and the duration of infectiousness (D
), i.e., R0
(for a review, see reference 2
). Although population density does not directly factor into this equation, it might be expected to influence the number of sexual contacts and partner turnover. In fact, a recent study in beetles demonstrated that the spread of a sexually transmitted disease (a parasitic mite) was more dependent on the population density of infected individuals than their proportion within the population (38
). Importantly, our data indicate that the low population density in Ugalla has not impeded the spread of SIVcpz among these savanna chimpanzees.
The finding of 67 individuals within the Issa Valley study area, 64 of whom were interconnected by joint nesting episodes or diurnal associations, was surprising. Other known savanna chimpanzee communities are either much smaller (28 to 35 individuals) (4
) or from sites with appreciably greater population densities (0.2 to 0.7/km2
). The significance of such a large community (a minimum of 67 individuals) living at such a low density (less than 0.14 individual/km2
) is that the implied home range is enormous, i.e., greater than 478 km2
, which is nearly twice the size of the largest similarly estimated home range published to date (4
). Such a vast home range would be expected to have major consequences for within- and between-community social behaviors, including those affecting SIVcpz transmission (24
). Nonetheless, the high SIVcpz prevalence implies that the number of contacts between different community members is similar to that of forest chimpanzees. Additional research will be necessary to resolve this seeming paradox.
Molecular characterization of the newly identified viruses revealed a close genetic relationship of Ugalla and Gombe viruses ( and ). Viruses from each community formed well-defined lineages in phylogenetic trees, with one Gombe strain outflanking both the main Gombe and Ugalla clades (). The SIVcpz strains from Gombe and Ugalla share a recent common ancestor, but the historical links between these communities are unclear. Since the 1940s, human settlements and deforestation have significantly reduced chimpanzee habitat between Gombe and Ugalla (35
), and the original distribution of chimpanzees in this area is not known. The Malagarasi River forms the northern border of the Ugalla region and could represent a biogeographic barrier. Only two isolated chimpanzee communities occur between Gombe and this river, i.e., Kwitanga 15 km east of Gombe and Lilanshimba on the north side of the Malagarasi (B) (29
). Our finding of related viruses in Gombe and Ugalla thus suggests either that chimpanzees were previously able to cross the Malagarasi River or that SIVcpz was introduced into Gombe and Ugalla from the west shore of Lake Tanganyika by two separate routes, i.e., from the north through Burundi to Gombe and from the south through Zambia to the Mahale/Masito/Ugalla ecosystem. Among these, the former explanation seems more likely.
Land surveys between Mahale and Ugalla have provided ample evidence for the presence of chimpanzees, suggesting that large parts of this vast area are suitable chimpanzee habitats (B). Although human encroachment, including the Mishamo Refugee Settlement (measuring 55 km by 23 km) between Mahale and Ugalla, may hinder chimpanzee movement, no major geographical barriers exist that would prevent virus spread within the Mahale and Ugalla region. We have previously failed to detect SIVcpz in Mahale Mountains National Park, but our study was limited to only 20 individuals from one particular community (44
). The Greater Mahale Ecosystem—which includes Ugalla—is believed to provide habitat for approximately 2,600 chimpanzees (26
). It will thus be critical to determine to what extent SIVcpz has penetrated this population. In this context, the pathogenic potential of SIVcpz has to be considered. While chimpanzee density in Ugalla does not seem to have changed much since the 1960s (26
), data from the Kalande community in Gombe would suggest that the impact of SIVcpz on chimpanzee population growth has a considerable lag time (37
). It is thus possible that any detrimental effect of SIVcpz on these savanna chimpanzees may have not yet manifested itself.
In summary, we report here that SIVcpz infection in Tanzania is more widespread than previously appreciated. We also provide evidence that a large number of some of the most endangered, yet largely unstudied, ape populations in Tanzania are at risk of SIVcpz infection. Given the value of these communities to different disciplines, it will be critical to determine whether and to what extent other savanna chimpanzees across western Tanzania harbor SIVcpz, in order to be able to gauge what impact this infection might have on their long-term survival.