Monday, May 21, 2012

Emerging fungal diseases

Last week, it was my turn to present an article in our Pathology Department Journal Club, and I opted to review a review. Here's the abridged version…

If you are one who keeps a finger on the pulse of global wildlife disease, you may have wondered why we’re seeing an apparent surge in emerging fungal diseases in recent years. A review published in the April 12 2012 issue of Nature provides a fairly comprehensive look at the question: why fungi, and why now? According to the authors, the answers lie in high fungal virulence, long-lived environmental stages, generalist pathogenicity, relative genotypic plasticity, and the effects of environmental drivers.

But first, let’s look at a few key players.

White-nose syndrome in bats in North America. Bats across the eastern US are dying at an alarming rate, and affected caves continue to be identified. As of January 2012, the Federal Wildlife Service estimated the cumulative death toll to be upwards of 6.7 million bats. The little brown bat, pictured above with the white fungus collecting on the muzzle, is one of the most common species in this region, and is considered by many to be at risk of extinction if this trend continues.  The etiologic agent, Geomyces destructans is related to soil fungi that haven’t previously been associated with infection of vertebrate hosts.
Chytridiomycosis, a cutaneous infection of amphibians by the chytrid fungus  Batrachochytrium dendrobatidis has been implicated in dramatic declines in amphibian populations everywhere these animals occur, resulting in 100% mortality in some isolated populations. Here, too, is an example of a species never before associated with infection.

Loggerhead sea turtles, already an endangered species, are further threatened by infections with the fungus Fusarium solani associated with failure of egg clutches and weak hatchlings.

Other examples cited in the article include fungal agents associated with declines in coral species, honeybees, crayfish, and a host of plant species.

Certainly, emerging fungal infections have been making headlines; however, are they really increasing? The authors conducted a retrospective study of reports published by two large epidemic/health monitoring services, ProMED and HealthMap, and found that yes, in recent years, the percentage of reports involving fungal pathogens in animals has increased, by up to 3-fold. A literature review also indicated that of the reported extinctions and local species extirpations directly due to disease, fungal agents were the most commonly implicated pathogens, with chytridiomycosis being the single most common causative infection.

Contrary to the mounting concern of the threat of fungal epidemics, our traditional view of fungal disease typically considers these to be opportunistic pathogens, with immunocompromised individuals being the population at risk. This begs the question: are emerging fungal diseases an indication of other underlying conditions? Researchers have asked whether fungal diseases are becoming more prominent now due to environmental stress associated with such factors as pollutants, habitat destruction and forced migration, climate change, or other primary underlying unidentified pathogens (eg. Viruses). While this question persists, more and more evidence indicates that, at least in the case of Chytrid and WNS, the fungus is looking like the sole culprit.

We don’t know yet why fungi seemed to be finding an expanded niche as primary pathogens in global populations, and whether this trend will continue to increase remains a mystery of some concern.

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