In zoos around the world, and throughout Australia’s eucalyptus groves, koalas are dying of cancer. More than half of all captive koalas eventually die of leukemia or lymphoma, although some zoos see more of the blood cancers than others. Now, researchers at the CDC, NIH, San Diego Zoo, and Los Angeles Zoo have pinpointed a new virus subtype that helps explain the disease—at least in L.A. koalas. Their findings, reported in a new PNAS Early Edition article, have already allowed the development of a test for the virus, so koalas can be tested before being bred or moved between zoos.
In 2000, researchers first discovered a virus, koala endogenous retrovirus (KoRV), that had been integrated into the genomes of koalas within the past hundred years. The similarity of the virus to cat and monkey viruses known to cause leukemias and lymphomas suggested that KoRV could explain the high prevalence of the cancers in koalas. In many species, certain viruses can cause cancer by encouraging abnormal cell growth or introducing genetic mutations into cells. But researchers weren’t convinced that the KoRV virus explained everything. The sequence of KoRV was almost identical between all koalas, whether or not they belonged to a family or zoo more prone to cancer.
“If this virus they all have causes cancer, that would be really bad news for koalas,” says Maribeth Eiden, a virologist at the National Institutes of Health. “But we wondered whether there could be a variant that causes cancer. We don’t usually see cancer-causing retroviruses that exhibit so little variation.”
Eiden and her colleagues decided to compare koalas from the San Diego Zoo, who have been cancer-free for multiple generations, with koalas from the Los Angeles Zoo, where multiple animals have died from lymphoma and leukemia. Every koala tested had the original KoRV virus in their genome, but one koala named Brooklyn—a female from L.A. who turned out to be on the verge of becoming sick and being diagnosed with cancer—also had high levels of another virus.
“Normally finding this other virus would be like finding a needle in a haystack,” says Eiden. “But this girl was so loaded with the virus that we easily pulled it out.”
The virus that Brooklyn had, the researchers found, was a version of KoRV with alterations in some genes for viral receptors. They named the new virus KoRV-B and designed a molecular probe to test whether other koalas had KoRV-B. While no koalas from the San Diego Zoo had KoRV-B, 6 of 13 animals from Los Angeles had the virus. All but two of the KoRV-B koalas, Eiden says, have died of leukemia or lymphoma. And when the team isolated KoRV-B viruses, they found that the viruses had unique abilities to infect koala cells.
When they studied the inheritance patterns of KoRV-B, Eiden and her collaborators discovered that the virus was passed from mother to joey, but never from the father—indicating that the virus wasn’t yet integrated into the koala’s DNA, likely being passed from the mother in utero or through breast milk.
“The fate of the koalas is actually much rosier than we originally thought,” says Eiden. “If the KoRV in their genomes was causing these koala deaths, they would have been doomed.”
More work is needed to fully understand the transmission patterns of KoRV-B, and to determine whether animals with cancer in other countries and zoos also have KoRV-B, or a different variant of KoRV. But since the cancer-causing KoRV-B is exogenous (not endogenous, in the koala’s genomes, like the originally discovered KoRV), the researchers can easily develop vaccines to prevent the spread of the virus, Eiden says. Moreover, they’ve already designed a kit to test whether a koala—being brought to a zoo from Australia or being moved between zoos—has the KoRV-B virus. If it tests positive, the zoo can keep the animal in containment or prevent breeding it to contain the virus. Good news for the sleepy, eucalyptus-loving marsupials from Australia.