“Laboratory experiments using bacteria and their viruses have shown that when hosts evolve resistance against infections, viruses can rapidly overcome host immunity,” says Lotta-Riina Sundberg, from the University of Jyväskylä. “But monitoring these long-term coevolutionary arms races in natural settings with such accuracy is challenging.” That’s why the myxoma story is so important, she adds.
The same dynamics played out in Europe, where a different strain of myxoma was used to control rabbits, following the Australian success. There, too, the virus evolved into milder forms. And there, too, new immunosuppressive strains have emerged. No one knows what will happen in the future. In South America, myxoma’s birthplace, the virus causes an innocuous disease in the local cottontails. But there’s no indication that the Australian or European strains are heading in that direction.
“The broad lesson is that there’s a variety of evolutionary trajectories that pathogens can take,” says Read. “There are situations, no question, where virulence can go quite low. Sexually transmitted diseases, for example, require hosts to be sexually active and that requires that they stay alive for some time. But there’s no reason to think that the average long-term state will be coexistence, and that’s a mistake that’s permeated the public.”
Consider rabbit hemorrhagic disease—another infection that Australia considered as a way of controlling rabbits, and that escaped from a quarantine facility in 1995. The virus behind the disease is transmitted by corpse flies, which are attracted to cadavers, so this virus actually benefits by killing its hosts in spectacular fashion. It’s present in huge numbers at the time of death. As such, it started off lethal and has only become more so with time. In the United States, West Nile virus has become more virulent in house sparrows, in response to the birds evolving resistance. And Marek’s disease—an illness of fowl—became fouler after farmers treated birds with a “leaky” vaccine, which stops them from developing the disease, but not from becoming infected or spreading the virus.
These consequences are relevant to various companies and researchers who are trying to make farmed animals more resistant to diseases. Some are doing it by traditional breeding. Others are looking to genetic engineering. Whatever the route, the myxoma example shows that such measures could drive the evolution of more potent viruses. These may not be a problem for the resistant animals, just as immunosuppressive myxoma strains aren’t especially deadly to wild rabbits. But if the viruses spread to naive animals, they would suffer.
“If you had a bunch of companies in one river system, and one is creating more resistant fish, causing pathogens to become more virulent, what does that do to the wildlife and the fish belonging to other companies?” says Read. “You have to ask about the long-term consequences. Maybe there are some types of resistance that are less likely to provoke this arms race than others. We need to understand that.”