A recent study published online in the journal Molecular Biology and Evolution discovered that a common retrovirus, KoRV, found in koalas has been plaguing the species for over 120 years.

Retroviruses are composed of RNA and have the ability to incorporate their genetic material into a host’s DNA. In the case of KoRV, the virus became part of the koala’s germ line, and was passed on from parent to offspring. Therefore, KoRV is actually part of the Koala’s genome. This process is known as “retroviral endogenization”.

Most endogenous retrovirus genetic material found in vertebrates is minimal- traces at most. As the authors indicated, this makes it difficult to learn how endogenous retroviruses become part of vertebrate DNA. That KoRV is still affecting koalas is significant; now scientists can observe this form of infection in real-time.

For this particular study, scientists analyzed the genetic material of 18 koala skins from various European and North American museums, some of which dated back to the 19th century. They were surprised to learn that 15 of the samples were positive for the virus KoRV. The study findings show that this “modern” virus was actually already established, and widespread, among koalas over a century ago.

The virus’ geographic range has changed very little over that time period. Currently, most koalas infected with KoRV are from northern Australia. The sampled koala skins that tested positive were also all from the north. This is likely because koalas are sedentary creatures and therefore have little impact on transmission outside of their local habitat. Transmission of KoRV through the germ-line as a result of endogenization further explains the high prevalence of KoRV in the north, but not in the south.

Infection with KoRV is a significant concern for northern Australian koalas. The retrovirus causes AIDS-like immune deficiency, which makes koalas more susceptible to other potentially fatal diseases, such as leukemia or chlamydia.

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