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Imperfect Immunity Drives Evolution of Pathogens

As annual flu shot patrons know, immune systems are not perfect and must be constantly reinforced to protect against rapidly evolving pathogens.

New research 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.  The findings from a multi-university team that included USD Assistant Professor of Biology Arietta Fleming-Davies -- were published March 2 in the journal Science.

Unlike humans, most wild birds do not receive vaccines and must rely on their natural immune systems to protect them from pathogen attacks. Immune systems have ‘memories’ that allow organisms to recognize past abusers and ward them off.  However, in the case of partial immunity, these memories aren’t always perfect and some pathogens make it through the door.

The research measured how bacterial strains of varying strength fared in finches with or without pre-existing immunity to the pathogen. The 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

“The shift to favor more harmful pathogens that we observed in the modeling results is a very dramatic increase, suggesting that immune responses have key effects on the evolution of this pathogen and others,” said Fleming-Davies. “What I found unique about working on this study was that the patterns in the experimental data were so strong—no matter how we looked at it, the same important immune effect popped out.”

She and her fellow researchers from Virginia Tech and other universities say the work provides important insights not only for finches but on how many animal and human pathogens cause only incomplete protection against reinfection and that further work should be done on the relationship between primary infection and the strength of the response that the host generates. 

The work was funded by a $2.3 million grant from the National Institutes of Health and the National Science Foundation on the evolution of infectious diseases.

— Liz Harman

Video by Carleen Syth