New research revealed by scientists within the Department of Biological Sciences and published in the journal Science shows that in the case of a common backyard bird, imperfect immunity to a dangerous pathogen that causes “bird pink eye” actually makes the pathogen stronger and more dangerous for its next victim. That’s bad news for wild birds who must rely on their natural immune systems to protect them from pathogen attacks.
An increase in the severity of finch pink eye in recent years alerted Dana Hawley, an associate professor of Biological Sciences and her collaborators to a potential link between bird immunity and pathogen virulence.
“Our results are not just important for finches,” Hawley said. “Many human pathogens and other animal pathogens also cause only incomplete protection against reinfection. Thus, the potential is there for the host immune response to favor more harmful strains in many types of hosts, but in this case, imperfection can be deadly.”
A healthy male house finch forages in the snow. Photo credit: Bob Vuxicnic.
Lab experiments showed that stronger or more virulent strains have a leg up for several reasons. One of the most surprising was that virulent strains generate more complete memory responses in finches, leaving weaker strains with few hosts to infect. In contrast, weaker strains produce only partial immune memory, leaving the door open for more virulent strains to invade.
Results from that experiment were then modeled to reveal how a pathogen might move through an entire population of finches. The model showed that pathogen strains that came to dominate in an experimental population with incomplete immunity were almost twice as harmful as those that dominate in the absence of immunity. Thus, incomplete immunity is likely what’s driving the evolution of more harmful strains.
“This is really groundbreaking since, most of what we know about host-pathogen co-evolution is in the context of interventions like vaccinations,” said Ariel Leon, a doctoral student in Hawley’s lab and co-author on the paper.