Simians include diverse species of monkeys which are globally distributed predominantly in the southern hemisphere, and ape species that are restricted to the rainforests of central Africa and Southeast Asia. Types of simian retroviruses have been detected in almost all species of non-human primates which have been studied. As early identification and classification of primate retroviruses was largely possible due to the efforts to establish cell lines from various primate species, there were limitations.

Firstly, the search for simian retroviruses was frequently restricted to only a few species cell lines. Secondly, not all retroviruses were permissive to the available cell lines due to species specific restriction or cell-type specific factors. The earliest discoveries of simian retroviruses were often based on a specific observations which arose from diagnostic workups of cases of then undefined illness or unusual cancers. This chapter focuses on the four main groups of exogenous simian retroviruses; type D simian retroviruses (SRV), simian foamy viruses (SFV) commonly known as spumaviruses, the simian T-cell lymphotropic viruses (STLV), and the expanding group of simian immunodeficiency viruses (SIV).

Initially viruses in these subgroups of simian retroviruses were identified based on the diseases from which they were associated with. Retroperitoneal firbrosarcomas and chronic wasting disease led to the early identification of the SRVs, found to be associated with a spectrum of diseases in different species of macaques. Other simian retroviruses are frequently asymptomatic in their natural hosts and in some cases simply cause cytopathic effect (CPE) in cell culture (SFV).

In the last two decades molecular techniques have provided us with much more insight into the phylogeny of the wide variety of diverse retroviruses, knowledge which has enriched the seroprevalence evidence of widespread retroviral infections in many different primate species.


Further reading: Retroviruses: Molecular Biology, Genomics and Pathogenesis

Labels: BIV, BLV, CAEV, EIAV, FeLV, FIV, Simian Retroviruses, SIV, VMV

Extensive analysis of naturally occurring simian immunodeficiency viruses (SIVs) and comparative phylogenetic studies with human immunodeficiency viruses (HIVs) suggests that the latter are close relatives of the SIVcpz viruses of chimpanzees (HIV-1) or the SIVsmm viruses of sooty mangabys (HIV-2).

Crossing of species barriers resulted in adaptation to the human host and subsequent acquisition of a pathogenic phenotype. Naturally occurring T lymphocyte-tropic lentiviral infections are highly prevalent and productive but are not usually pathogenic for native hosts. Crossing species barriers may produce an abortive infection or, as in the case of the HIVs, may enhance virulence after several cycles of transmission.

The large number of species carrying these viruses may suggest that infection confers an evolutionary advantage to the host. The virulent T-lymphocyte-tropic lentiviruses have a similar genomic structure and exhibit comparable replication strategies. Their major targets are lymphocytes populating lymphoid organs and tissues, and antigen-presenting cells (dendritic cells, mononuclear phagocytes). Within these targets the virus can replicate to very high titres and thereby exhaust CD4+ T cells, producing profound immunodeficiency. Although the infection of lymphoid organs and tissue is the pathologic hallmark of HIV infection, this virus also infects cells of the central nervous system

Further reading: Retroviruses: Molecular Biology, Genomics and Pathogenesis

Labels: hiv, HIV-1, HIV-2, Immunodeficiency Virus, Simian immunodeficiency virus, SIV, SIVsmm