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That is, viral sequences from members of the same species form a monophyletic clade in evolutionary trees.This host-specific clustering indicates that the great majority of transmissions occur among members of the same species; however, there are also numerous documented instances when SIVs have crossed between species. Thus, it is clear that in addition to more long-standing virus/host relationships, a number of naturally occurring SIVs have emerged more recently as a result of cross-species transmission and recombination.Common chimpanzees have traditionally been further subdivided into a number of geographically differentiated subspecies (Groves 2001). First, only two of the four chimpanzee subspecies were found to harbor these viruses. The absence of SIVcpz from two of the four subspecies suggested that chimpanzees had acquired this virus more recently, after their divergence into different subspecies.Four subspecies were defined on the basis of mitochondrial DNA sequences (Gagneux et al. To determine the distribution of SIVcpz among these populations, fecal (and in some cases urine) samples were collected at different field sites and tested for the presence of virus specific antibodies. SIVcpz was detected at multiple sites throughout the ranges of both central and eastern chimpanzees in an area ranging from Cameroon to Tanzania, but there was no evidence of infection in western and Nigeria-Cameroonian chimpanzees, nor in bonobos, despite testing of multiple communities. Indeed, phylogenetic analyses of full-length proviral sequences revealed that SIVcpz represents a complex mosaic, generated by recombination of two lineages of SIVs that infect monkeys (Bailes et al. In the 5′ half of the genome, as well as the ) monkeys (Bailes et al. Chimpanzees are known to hunt and kill other mammals, including monkeys (Goodall 1986), suggesting that they acquired SIV in the context of predation.Acquired immunodeficiency syndrome (AIDS) of humans is caused by two lentiviruses, human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2).Here, we describe the origins and evolution of these viruses, and the circumstances that led to the AIDS pandemic.Exogenous viruses are depicted in black, with HIV-1, HIV-2, and SIVmac highlighted in red; endogenous viruses are shown in purple.
Gombe is located in northwestern Tanzania on the shores of Lake Tanganyika. SIVcpz also appears to be transmitted from infected mothers to their infants, and in rare cases, possibly by aggression (Keele et al. Migration of infected females constitutes a major route of virus transmission between communities (Rudicell et al. Behavioral and virological studies also provided insight into the pathogenicity of SIVcpz.The great majority of lentiviruses are exogenous, meaning that they are transmitted horizontally between individuals. These “viral fossils” are of particular interest because they provide direct evidence of the timescale of lentivirus evolution.However, it has recently become clear that, on several occasions in the past, lentiviruses have infiltrated their hosts’ germlines and become endogenous, vertically transmissible, genomic loci (Fig. Examples include the rabbit endogenous lentivirus type K (RELIK), which became germ-line embedded approximately 12 million years ago (Katzourakis et al. 2009), and two prosimian endogenous lentiviruses, which independently invaded the germ-lines of both the grey mouse lemur (p SIVgml) and the fat-tailed dwarf lemur (p SIVfdl) about 4 million years ago (Gifford et al. Molecular clocks derived from extant SIV sequences suggested that ancestral SIVs existed only a few hundreds of years ago (Wertheim and Worobey 2009), but it has long been suspected that such analyses may grossly underestimate deeper evolutionary timescales (Sharp et al. Recent studies of SIV-infected monkeys on Bioko Island, Equatorial Guinea, partly substantiated this conclusion, showing that geographically isolated subspecies have been infected with the same type of SIV for at least 30,000 years and probably much longer (Worobey et al. The endogenous viruses in lemurs reveal that the span of evolutionary history of primate lentiviruses as a whole is at least two orders of magnitude greater still.Acquired Immune Deficiency Syndrome (AIDS) was first recognized as a new disease in 1981 when increasing numbers of young homosexual men succumbed to unusual opportunistic infections and rare malignancies (CDC 1981; Greene 2007). 2011); however, 80% of adults acquire HIV-1 following exposure at mucosal surfaces, and AIDS is thus primarily a sexually transmitted disease (Hladik and Mc Elrath 2008; Cohen et al. Since its first identification almost three decades ago, the pandemic form of HIV-1, also called the main (M) group, has infected at least 60 million people and caused more than 25 million deaths (Merson et al. Developing countries have experienced the greatest HIV/AIDS morbidity and mortality, with the highest prevalence rates recorded in young adults in sub-Saharan Africa (A retrovirus, now termed human immunodeficiency virus type 1 (HIV-1), was subsequently identified as the causative agent of what has since become one of the most devastating infectious diseases to have emerged in recent history (Barre-Sinoussi et al. Although antiretroviral treatment has reduced the toll of AIDS- related deaths, access to therapy is not universal, and the prospects of curative treatments and an effective vaccine are uncertain (Barouch 2008; Richman et al. Thus, AIDS will continue to pose a significant public health threat for decades to come. These relationships provided the first evidence that AIDS had emerged in both humans and macaques as a consequence of cross-species infections with lentiviruses from different primate species (Sharp et al. Indeed, subsequent studies confirmed that SIVmac was not a natural pathogen of macaques (which are Asian primates), but had been generated inadvertently in US primate centers by inoculating various species of macaques with blood and/or tissues from naturally infected sooty mangabeys (Apetrei et al. Similarly, it became clear that HIV-1 and HIV-2 were the result of zoonotic transfers of viruses infecting primates in Africa (Hahn et al. In this article, we summarize what is known about the simian precursors of HIV-1 and HIV-2, and retrace the steps that led to the AIDS pandemic. Old World monkeys are naturally infected with more than 40 different lentiviruses, termed simian immunodeficiency viruses (SIVs) with a suffix to denote their primate species of origin (e.g., SIVsmm from sooty mangabeys).
Antibody positive fecal specimens were then subjected to RNA extraction and reverse transcriptase polymerase chain reaction (RT-PCR) amplification to molecularly characterize the infecting virus strain. In addition, SIVcpz prevalence rates among central and eastern chimpanzees varied widely, ranging from 30% to 50% in some communities to rare or absent infection in others. Nonetheless, the puzzle of why SIVcpz was so scarce among captive chimpanzees was finally resolved: As it turned out, most of these apes were imported from West Africa and thus were members of the ) gorillas have been sampled are shown (each site is identified by a two-letter code; because of space limitations, only a subset is depicted). The current range of the central chimpanzee overlaps those of red-capped mangabeys and the various species, and so it is likely that the cross-species transmission events that led to the emergence of SIVcpz occurred in that area, and that SIVcpz later spread to eastern chimpanzees, although it is unclear whether this occurred during or subsequent to their divergence from the central subspecies.