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Incomplete host immunity favors the evolution of virulence in an emergent pathogen

Science

Partially protective vaccination can sometimes select for increasingly virulent pathogens. Fleming-Davies et al. asked what happens in a natural system. In the United States, the house finch population is suffering an increasingly virulent epidemic caused by Mycoplasma gallisepticum. The pathogen induces incomplete immunity that clears less virulent pathogens and offers partial protection against strains of greater virulence. In the birds, the partial immune response does away with competition from the less virulent pathogens. The partial immunity of the host also hinders replication of the more virulent pathogens enough to allow some birds to survive. This allows increasingly virulent forms of the pathogen to be transmitted.


A deadly amphibian disease goes global

Science

Trade routes have shaped human history by connecting distant civilizations, allowing the exchange of materials, technology, and people, but also diseases (1). Pathogens have been frequent hitchhikers, bringing them in contact with new hosts that provide the fuel for epidemic outbreaks of disease. Humans are not the only victims of trade-driven diseases, but scientists have only recently begun to appreciate the risk to biodiversity of inadvertently introducing new pathogens to naïve evolutionary arenas. On page 1459 of this issue, Scheele et al. (2) demonstrate the consequences of globalized pathogen exchange by reconstructing the hidden history of disease-driven declines and extinctions for hundreds of amphibian species.


Pathogens at the limits

Science

Sexually transmitted disease limits the distribution of European alpine meadow flower populations. The factors controlling the geographical distribution of organisms are receiving increasing attention in the context of climate change. Bruns et al. analyze the effects of a different, little-studied factor--the role of disease in limiting the range of species. In alpine plants in northwest Italy, they found that a pollinator-transmitted fungus that causes sterilizing anther-smut infections in meadow flowers occurred throughout the plant's range. Reduced population densities of these plants at their range limits does not affect the distribution of the insect-borne pathogen.


The hidden biodiversity of amphibian pathogens

Science

Since the discovery of the salamander chytrid pathogen [Batrachochytrium salamandrivorans (Bsal)] (1), the world has been preoccupied with determining where it does and does not occur (2) so that policies can be implemented to prevent introduction into unaffected areas (3). Pathogenic chytrids cause chytridiomycosis, a disease of the skin that can cause mortality and die-offs, including population declines and species extinctions. In the United States--the world's biodiversity hot spot for salamanders and currently free of Bsal--a multinational scientific task force has been created to test the susceptibility of native species and to prepare an emergency response should Bsal be detected (4). Meanwhile, attention to Bsal's better-known cousin B. dendrobatidis (Bd), another chytrid pathogen that has decimated amphibian populations around the world, has faded, in part because of perceptions that once Bd is present, conservation actions and policy options are limited. On page 621 of this issue, O'Hanlon et al. (5) remind us that Bd remains a serious threat to global amphibian biodiversity and clarify where and when Bd came from and how it spread.


To grow and to defend

Science

Feeding an expanding world population while sustaining an inhabitable environment represents the greatest challenge of our time. To meet this challenge, the scientific conundrum of increasing crop yield while protecting it from evolving pathogens must be resolved. Rice (Oryza sativa) contributes the majority of dietary energy for more than half of the world's population. The most devastating pathogen of rice worldwide is the fungus Magnaporthe oryzae, the causal agent of rice blast, which results in an estimated yield loss of 30% globally. Therefore, controlling M. oryzae infection is a key battlefront for improving global rice production (1).