Thinking to outdo nature, scientists began a famous experiment in the wild only to get badly burned. Australia, foxes, rabbits, myxoma virus or hubris, folly, doubling down on folly, unintended consequences, this experiment suggests viruses with a high mortality rate attenuate their virulence to adapt to the selection pressure imposed on them.
: European settlers in Australia interested in fox hunting first introduced foxes. Next, the in 1788 introduced the to serve as food for the foxes as well as hunting targets themselves.
Things didn’t work out as intended in more than one way.
- For one, the introduced foxes preferred native Australian fauna to the introduced rabbits.
- For another, with no natural predators, as is the wont with rabbits, the predictable happened. In short order, Australia was bursting at the seams with too many rabbits. The rabbits destroyed agricultural crops, started competing with the native marsupials for food, and even native Australian mammals started going extinct.
Going out in groups and killing rabbits en masse wasn’t stemming the tide so in 1950, scientists introduced a virulent rabbit-specific South American myxoma virus strain, SLS, into Australia to cull the booming rabbit numbers.
- Native to the Americas, rabbit myxoma virus is transmitted by mosquitoes when they feed on localized skin lesions of infected American rabbits.
- OTOH, myxoma virus was already known to cause a systemic infection fatal in >99% of European rabbits.
Indeed as mosquitoes spread it freely, mortality rate in infected rabbits in Australia was estimated as high as 99% in the first year.
Were European rabbits wiped out from Australia? A resounding no. By the following year, less virulent myxoma virus strains began to emerge and eventually replaced the SLS strain. How could this happen?
- SLS was killing the rabbits within 3 to 4 days from the time they developed infectious lesions, too quick for it to sustain its transmission via mosquitoes. Replicating at lower rates, the less virulent (attenuated) viral strains remained longer within an infected rabbit and thus seemed to get transmitted to new rabbits more successfully compared to SLS.
- Rabbit recovery rates also increased from 1 to 10 to ~55% in subsequent years. More rabbits surviving combined with improved scope of virus transmission by mosquitoes led to a new dynamic among these three.
Since both rabbits and the rabbit myxoma virus multiply rapidly (short generation times), both adapted rapidly to each other. The ensuing co-evolution led to
- Selection of disease resistance in rabbits. Short generation time of rabbits helped in selecting rabbits with increased disease resistance. Rabbits that survived infection with less virulent strains were even immune to SLS.
- More effective virus transmission between rabbits.
What to surmise from this experiment in the wild? The trajectory of a virus so lethal as to kill most of those it infects is inherently unstable. Selection pressure works in both directions, forcing the virus to attenuate its virulence on the one hand while forcing the host to develop resistance on the other.
Recommended: Fenner, Frank John. “The Florey Lecture, 1983-Biological control, as exemplified by smallpox eradication and myxomatosis.” Proc. R. Soc. Lond. B 218.1212 (1983): 259-285.