Superinfection as a driver for pathogen genomic diversification. Guy H. Palmer, Regents Professor and Director of the Washington State University School for Global Animal Health

A new pathogen strain can penetrate an immune host population only if it has undergone sufficient change to escape immunity generated against the original strain. This model is best understood with influenza viruses in which genetic drift can create new antigenically distinct strains that can then spread through host populations despite the presence of immunity against one or more previous strains. Whether this selection model for genetically distinct strains applies to complex pathogens responsible for endemic persistent infections, which include major diseases such as anaplasmosis, relapsing fever, and sleeping sickness, remains untested. These complex pathogens undergo rapid antigenic variation within an individual host using sets of chromosomally-encoded alleles. Consequently, immunity is developed against a large repertoire of variants, thus dramatically changing the scope of genetic change needed for a new strain to evade existing immunity and establish co-existing infection, termed strain superinfection. Using Anaplasma marginale as a model, we show that diversity in the allelic repertoire encoding Msp2 antigenic variants among strains was equal to the allelic diversity within a strain, reflecting equivalent selection for unique variants to overcome immunity at the host population level as well as within an individual host. This diversity among strains resulte