virology blog

About viruses and viral disease

KoRV

Spread of koala retrovirus in Australia

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friendly-male-koalaThe Koala retrovirus (KoRV) continues to spread within Australia, according to results of a new analysis of a larger sample size from a wider geographical range than was previously studied.

Blood or tissue samples were collected from koalas in different regions of Australia, and polymerase chain reaction (PCR) was used to detect the presence of KoRV proviral DNA, a DNA copy of the retroviral genome integrated into host cell DNA. Most of the koalas from the Australian mainland were positive for KoRV proviral DNA (442/466; 94.8%). All samples from animals in Queensland and New South Wales were KoRV positive. In mainland Victoria 65 of 89 animals contained KoRV DNA (73%). On the Victorian islands prevalence of KoRV ranged from 0% on Philip Island (0/11) to 50% on Snake Island (6/12). On the previously KoRV-free Kangaroo Island (link), 24 of 162 animals (14.8%) were KoRV positive. These results suggest that KoRV initially entered the koala population in the north of Australia and has been slowly spreading to the south. There are also other potential explanations for the results: there may be differences in KoRV susceptibility in northern versus southern animals, and the rate of transmission might differ in the two areas.

The genome of Queensland koalas contain far more copies of KoRV per cell, 165, than animals in Victoria, which ranged from less than one to 1.5 copies per cell. The Queensland koalas are likely fully endogenized – that is, the integrated KoRV DNA is passed from parent to offspring in the germline, and hence every koala cell contains viral DNA. In contrast, in Victoria koalas KoRV has either recently entered the germline (1.5 copies/cell) or has not yet entered this state (<1 copy/cell). In animals with less than one proviral copy per cell, KoRV infection was likely acquired exogenously from one animal to another. The mode of transmission of KoRV among koalas is not known, but might involve animal-animal contact or arthropod transmission.

It seems likely that eventually all wild koalas will be endogenized by KoRV. Whether this process will impact the long-term survival of the species is not known, especially since the disease caused by KoRV infection is poorly understood.

GS Simmons, PR Young, JJ Hanger, K Jones, D Clarke, JJ McKeed, J Meersa. 2012. Prevalence of koala retrovirus in geographically diverse populations in Australia. Austr. Vet. J. 90(10):404-9.

Museum pelts help date the koala retrovirus

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friendly-male-koalaThe genomes of most higher organisms contain sequences from retroviral genomes called endogenous retroviruses (ERVs). These are DNA copies of retroviral RNAs that are integrated into the germ line DNA of the host, and passed from parent to offspring. In most species the infections that lead to germ line ERVs appear to have occurred millions of years ago. The Koala retrovirus, KoRV, is the only retrovirus that we know of that is currently invading the germ line of its host species. A study of Koala pelts preserved in museums suggests how recently the virus infected this animal.

The koala is native to Australia, and all koalas in northern Australia are infected with KoRV. However not all animals in the southeast or on southern islands are infected. It is believed that KoRV crossed into koalas from another species (possibly the Asian mouse Mus caroli) some time within the past two hundred years. To test this hypothesis, DNA was extracted from 28 koala skins that were held in museums and which had been collected from the late 1800s to the 1980s. Polymerase chain reaction was used to detect KoRV DNA in the koala genome. The results show that KoRV was already widespread in Northern Australian koalas by the late 1800s. It has since spread slowly because the virus is not ubiquitous in southern koalas. The slow dispersal may due to the sedentary and solitary nature of koalas. Examination of mitochondrial DNA from the koala skins confirmed that there has been limited movement of the animals with Australia.

The sequence of the KoRV gene encoding the viral glycoprotein, env, was also determined. The results reveal that env sequences from museum specimens are remarkably similar to those of KoRV found in contemporary koalas. At first glance this result might not seem surprising: the endogenous KoRV genomes are evolving at the same slow rate as the host DNA into which they are integrated. However, there appear to have been multiple transmissions and germ line invasions by KoRV, leading the authors to suggest that in all cases very similar retroviruses were involved.

Infection with KoRV in captive animals is believed to cause immunosuppression, leading to fatal lymphomas or Chlamydia infection. A Chlamydia epidemic is believed to have killed many koalas in 1887-1889, consistent with the PCR results indicating that KoRV was widely present at that time.

Update: I had meant to discuss the possibility of dating the invasion of Koalas by KoRV by using older samples, but neglected to include this in the original article. Several days after it was published, Professor Paul Young sent me a note expressing exactly this sentiment:

What would be even better would be to have access to fossilised material that predates European settlement, that we could examine. We collaborated with an “Ancient DNA” specialist and tried this several years ago but we weren’t able to recover usable template DNA. Still worth some future effort though.

Avila-Arcos MC et al (2012). 120 years of koala retrovirus evolution determined from museum skins. Mol Biol Evol. 2012 Sep 14.

A retrovirus is invading the Koala genome

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friendly-male-koalaThere are 62 koalas in Japanese zoos, and 50 of them are infected with koala retrovirus (KoRV). Infection may lead to leukemia and lymphoma, which could negatively impact Koala populations. What is the origin of KoRV?

The koala is native to Australia where they are found in eastern and southern coastal areas. All koalas in the eastern state of Queensland are infected with KoRV, while there are still some uninfected animals in the south. In particular, the koalas on Kangaroo Island do not carry the virus. Koalas were imported to Kangaroo Island in the early 1900s, and apparently those animals were free of KoRV. The island is 8 miles offshore which may contribute to the absence so far of the virus. However, the koalas sent to Japanese zoos likely carried KoRV.

Curiously, 38 koalas have been born in Japanese zoos since the original importation, and 36 of these animals are infected with KoRV. This high rate of infection is a consequence of the fact that KoRV DNA integrates into DNA of koala germ cells. The viral genome is transmitted vertically, from mother to offspring.

The nucleic acid of retroviruses is RNA, but it is converted to a DNA copy during infection and integrates into host cell DNA. If the viral DNA integrates into the germ line, then it can remain in the organism for many generations. The genomes of most higher organisms contain remnants of retroviral genomes called endogenous retroviruses (ERVs). In primates these infections appear to have occurred millions of years ago; in humans, they comprise 6-8% of the genome, more than protein coding sequences (1-2%)! Most ERVs are defective but the koala ERVs are unique because they appear to produce viral particles.

No one has ever observed germ-line infection of a species with retroviruses – until infection of koalas with KoRV was discovered.  This process of ‘endogenization’ can now be studied in real time.

When did the KoRV begin invading the koala genome? It has been suggested that the initial infection occurred less than 100 years ago, but examination of preserved Koala DNA will be required to confirm this estimate.

The origin of most ERVs is unknown because the original infecting viruses disappeared long ago. But it might be feasible to identify the precursor of KoRV, which entered the genome relatively recently. KoRV appears to be closely related to an ERV of the Asian mouse Mus caroli. If this relationship is correct it will have to be determined how the virus was transmitted from mice in Southeast Asia to koalas in Australia.

Infection with KoRV in captive animals may lead to fatal lymphomas or immunosuppression and chlamydial infection. It is possible that infection of wild animals might lead to further decreases in this dwindling population. Should a KoRV vaccine developed to prevent extinction? Development and testing of a vaccine would require the use of koalas. It’s a difficult question, because the spread of KoRV among koalas is a natural part of evolutionary selection. Should we interfere?

Stoye, J. (2006). Koala retrovirus: a genome invasion in real time Genome Biology, 7 (11) DOI: 10.1186/gb-2006-7-11-241

Tarlinton, R., Meers, J., & Young, P. (2006). Retroviral invasion of the koala genome Nature, 442 (7098), 79-81 DOI: 10.1038/nature04841