An attempt to pinpoint the source of a marsupial scourge fingers an inveterate villain
By B.D. | NEW YORK
VIRUSES are nasty. The very word stems from Latin for "poison". A class of them called retroviruses is particularly insidious. These, which include AIDS-causing HIV, inject their genetic material into a host’s DNA, where it is replicated along with the healthy genes. They can affect which genes are "expressed", or switched on or off, with potentially profound consequences for health. Human retroviruses understandably get most attention, but they threaten other animals, too. One has recently been wreaking havoc in the marsupial world. In some places, it has wiped out 80% of the koala population.
When a retrovirus infects cells involved in reproduction, bits of its genetic code can get passed down to the host's offspring. These remnants are called endogenous retroviruses (ERVs). Koala retrovirus is in the process of becoming one. Generations later, what was once virulent becomes just part of the species' genome. But right now, it is causing immune-system problems and cancers.
Retroviruses can also pass between different species (as HIV jumped from primates to humans). Similarities in the ERVs found in different animals might shed light on how chains of transmissions occurred. In a paper published by the Proceedings of the National Academy of Sciences, Alex Hayward and Patric Jern of Uppsala University in Sweden compared the genetic motifs in thousands of known ERVs in 60 species. They looked in particular at a class called "gamma-ERVs"—a type commonly found in mammals and to which koala retrovirus belongs.
The patterns they uncovered suggested that one factor affecting transmission might be diet: no gamma-ERVs were seen in the genome of six large vegetarian mammals studied, but carnivores had an abundance of them. Geography and ecology might play a role, too, as more gamma-ERVs were found in animals dwelling close to the ground. Combine these two factors and it is little wonder that more than half of all ERVs were found in rats. Genetic patterns suggest that infections jumping between rodents and other animals (most notably primates) are commonplace. Rats are infamous for passing diseases. As it turns out, they are repositories for retroviruses, too, on top of more pedestrian germs.
It had been suggested that koala retrovirus was transmitted from gibbons by way of Asian mice. Drs Hayward and Jern think this might not be the case. They found ERVs in laboratory mice more closely related to the koala strain. Even indirect contact between gibbons, koalas and lab mice seems dubious. So the similarity between koala retrovirus and ERVs in mice may be incidental, rather than being indicative of a link in passing the disease.
The researchers would now like to use their comparative mapping technique to look at the genome of black “ship” rats (not included in this study). This might yield patterns showing a closer relationship with koala retrovirus. If so, they imagine a scenario of infected rats hitching their way aboard ships to Australia, and spreading the virus to koalas through another vector, such as a bat. This is speculative; the exact mode of inter-species retroviral transmission is still unknown. But these findings provide provocative new avenues of enquiry—and perhaps another reason to dislike rats.
