Introduction - Evolution of Infectious Diseases: Forecasting Pathogen Emergence
Kathryn Hanley, New Mexico State University, Las Cruces, USA
Infectious diseases emerge through a shift of the causative pathogen into a new host population, acquisition of novel transmission mode, manifestation of new symptoms, or escape from previously effective control strategies. With recent advances in genomics technology and mathematical modeling, the goal of research on emerging diseases has shifted from detecting and controlling emergence to predicting and preventing it. However these efforts depend on a detailed understanding of drivers and patterns of pathogen evolution.
In some cases, pathogen evolution can result from direct selection by humans. For example, chloroquine resistance in Plasmodium has evolved due to widespread use of this drug for prevention and treatment of malaria over the last century. Still, anthropogenic selection does not always produce the expected evolutionary result. While the use of therapeutic drugs regularly selects for drug resistance, deployment of vaccines rarely results in the evolution of vaccine escape mutants. Thus a number of vaccines, such as the measles and polio vaccines, have been spectacularly successful in controlling their targets. However there may be a risk inherent to these successes; if a vaccine eradicates its target pathogen, the vacant niche may be filled by a novel pathogen.
Other pathogens seem to lie on “evolutionary fault lines” and undergo unexpected phenotypic shifts. These shifts are most common among RNA viruses, which have fundamentally unstable genomes relative to DNA-based organisms. Host jumps are the most dramatic manifestation of such shifts, with HIV representing the most devastating example of a host shift in recent times. Through its impact on host immunity, HIV has also enabled the emergence of other, co-infecting pathogens.
While there has been considerable progress in forecasting disease emergence, significant challenges remain. Influenza researchers had been pointing out over the last decade that a pandemic was overdue, but it was a surprise when the pandemic originated from swine influenza in Mexico rather than bird influenza in Asia. Additionally, the anticipated impact of global climate change will further complicate the task in predicting disease emergence.