Small ruminant lentivirus diversity in the Fertile Crescent

Since the 1950s, a range of lentiviruses that infect livestock have been characterized, including bovine immunodeficiency virus (BIV) in cattle, equine infectious anemia (EIAV) in horses, and 'small ruminant' lentiviruses (SRLVs) in goats and sheep. SRLVs cause persistent infections associated with chronic debilitating diseases that ultimately lead to multi-organ failure and death, and are relatively common in sheep and goat herds throughout the world. However, due to the chronic nature of disease, often marked by long asymptomatic periods, SRLV infection can easily go undetected.

Domestication of sheep and goats is believed to have taken place ~10,000 years ago in the 'Fertile Crescent', an area which includes regions of present day Turkey, Lebanon, Jordan, Iraq and Syria [1]. During the late summer of 2012 we explored the diversity of SRLVs infecting sheep and goats in the Fertile Crescent area, as part of a broader investigation into the origins of the SRLV pandemic.

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Investigating viral diversity in Malagasy mammals

The West African microcontinent of Madagascar is often described as 'the world's oldest island'. Paleogeological studies indicate that the landmass formed more than 100 million years ago, and has since remained in its present position, approximately 300 miles West of the Southern African mainland.

The extraordinary mammalian fauna of Madagascar reflect this unique biogeographic history. Paleontological evidence indicates that the island originally had no native mammal species. The indigenous terrestrial mammals are derived from a small number of founder populations that reached Madagascar by 'rafting' or 'island-hopping'. These colonizing populations entered an environment in which many mainland species were absent, launching them along a unique ecological and evolutionary trajectory.

We are currently investigating viral diversity in Malagasy mammals. We aim to recover information about the long-term evolutionary history of viral infections in Malagasy mammals through exploration of the the viral fossil record and characterization of viruses infecting contemporary species.

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env-less ERVs are genomic superspreaders

Endogenous retroviruses (ERVs) are a unique combination of pathogen and selfish genetic element. Some ERV lineages proliferate by infecting germline cells like typical retroviruses, while others behave like retrotransposons, replicating entirely within the host cell. In these retrotransposon-like ERVs, the envelope (env) gene required to infect cells becomes redundant and degrades over time, often being lost entirely.

To examine the factors that determine the relative abundance of different ERV lineages, we analyzed over 5000 ERV loci recovered from 38 mammal genomes. This analysis revealed that ERVs lacking the env gene have undergone massive proliferations in their host genomes, and that where ERVs have adapted to replicate intracellularly, their proliferation within the host germ line is boosted by a factor of ~30. This parallels infectious disease epidemics, where commonly ~20% of the infected individuals are responsible for 80% of onward infection.

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Geographic structuring of equine infectious anemia virus isolates

Equine infectious anaemia virus (EIAV) is a lentivirus that infects horses worldwide. Uniquely among known lentiviruses, EIAV can be mechanically transmitted by arthropods (particularly horse flies). EIAV infection is often asymptomatic, but can also manifest as an acute, fulminant disease with high-titre viremia.

An investigation of genetic diversity among globally sampled EIAV isolates revealed clear geographic compartmentalization, with distinct strains being predominant in Asia, Europe and the Americas [1]. In addition, the study found evidence that the majority EIAV strain found in the Americas originated approximately 400 years ago - around the time that horses were reintroduced to the New World by European colonists.

Equids became extinct in the Americas ~12,500 years ago, and were not re-introduced until the late 15th Century [2]. Prior to the development of reliable serologic tests in the early 1970s, it was virtually impossible to distinguish inapparent carriers of EIAV from uninfected animals [3], and European colonists likely introduced the disease to the Americas unwittingly.

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