Emerging Fungal Threats: Drastic Increases and Consequences

By Olivia Ferrari

Image

Little Brown Bat With White Nose Syndrome.

Animals and plants all over the world are experiencing the severe impacts of a sharply rising threat, which will in turn have a major impact on our own future. Over the past few decades, damaging pathogenic fungi have been infecting species at an unprecedented rate. Resulting population declines occur on a wide scale, in diverse groups ranging from crops to bats, from frogs to corals. Worldwide amphibian decline, for example, is connected with the amphibian chytrid fungus. It is considered a major contributor to the catastrophically high mortality rates observed in many amphibian species.1 Another example is White Nose Syndrome in bats, the name of which comes from the visible white growths it causes. This fungal infection, first documented in 2006, poses a major threat to bats in the United States with population surveys suggesting a decline around 75% at confirmed sites.2

An analysis of disease monitoring programs and previous literature has revealed recent positive trends in pathogenic fungi on the whole, displayed in the graphs below. Disease alerts for pathogenic fungi have risen between 1995 and 2010, and have been noted worldwide. Literature shows that fungi comprises a major threat for both animal and plant hosts, but that the impact is more pronounced in animal species. Overall, fungi are the primary cause of pathogen-driven host loss in animals and plants, which has exhibited a significant increase in the second half of the twentieth century. In addition to illustrating rapid recent increase, trends also suggest continued future increase of fungal threats under current conditions.3

Reported worldwide trends in fungal emerging infectious diseases. (a) Disease alerts in ProMED database, (b) Locations of associated reports, (c) Relative proportion of species extinction and extirpation events for classes of infectious disease agents, (d) Their temporal trends for fungal pathogens.3 Figure and caption from Fisher et al. 2012.

Reported worldwide trends in fungal emerging infectious diseases. (a) Disease alerts in ProMED database, (b) Locations of associated reports, (c) Relative proportion of species extinction and extirpation events for classes of infectious disease agents, (d) Their temporal trends for fungal pathogens.3
Figure and caption from Fisher et al. 2012.

 Why Fungi Do So Well

A few fundamental characteristics of pathogenic fungi contribute to their emergence and strong influence. Firstly, fungi can be extremely destructive to host populations due to their high reproductive potential. This allows them to infect all members of a population before it declines to a point where the pathogen cannot spread anymore. Even if not all individuals are infected, a quickly reproducing fungal pathogen may suddenly diminish the population enough that it is still unable to recuperate. Additionally, fungi have the advantage of survival outside of a host. These independent survival stages may be long-lived, enabling their persistence in the environment and thus increasing their chance of transportation to new hosts. Finally, fungi can be incredibly generalist and opportunistic. They have the widest array of potential host ranges for any pathogen group, facilitating infection in a broad variety of hosts and environments.3

 Causes for the Increase

Why has this increase in pathogenic fungi only emerged in recent years? Many claim the causes are anthropogenic. The previously mentioned chytrid fungus, for example, is believed to be spread throughout global amphibian populations primarily by human introduction to previously unexposed populations, or in response to changes in the environment resulting from climate change.4

African Clawed Frog. Displayed the oldest documented case of the chytrid fungus.

African Clawed Frog. Displayed the oldest documented case of the chytrid fungus.

Globalization leads to the distribution of animals and plants to new environments, often carrying with them non-native pathogens. The common practices of international agricultural, domesticated animal, food crop, and timber trafficking, when unregulated, disperse pathogenic fungi to global environments where they may thrive.4 Human activities are also associated with alteration of fungal characteristics, specifically through novel genetic diversity. The mixing of previously geographically separate fungal lineages can create new possibilities for genetic exchange and hybrids, enabling the evolution of new lineages. Furthermore, less direct human impacts can spur the rise of pathogenic fungi. Climate change is a particularly complicated phenomenon, the influence of which on pathogenic fungi is debated. Some argue that warming has contributed to the emergence of fungal disease, but some factors resulting from climate change trends, such as increased ozone, can have the opposite effect. Nonetheless, changing climate factors certainly influence fungal emergence and success. One example is worldwide coral reef decline: the occurrence of a microbial disease threatening hard corals increases with warmer temperatures.3

Potential Consequences

These unique and vital animal and plant species throughout the world are not the only groups experiencing pressure. The emergence of pathogenic fungi puts pressure directly on the future of our species as well. A major concern is its impact on food security. Fungal diseases often occur in crops, with currently threatened species including rice, wheat, potatoes, maize, and soybeans. Estimated food loss based on recent world harvest statistics indicates that persistent disease could lead to losses sufficient to feed 8.5% of the 2011 global human population.3 In addition to food security, the effect on ecosystem services is a primary concern. Invasive tree diseases have been observed in various species worldwide including chestnut, elm, and oak trees. In Canadian pine trees, mountain pine beetles spread a blue stain fungus that severely impacts forest ecosystems. Outbreaks causing tree mortality in British Columbia reduce the forest’s ability to act as a carbon-absorbing sink, while decaying trees increase the amount of carbon released. A total estimated release of 270 megatonnes of carbon is estimated for the period between 2000 and 2020, the worst year of which results in impacts close to 75% of the average annual forest fire emissions from the entire country between 1959 and 1999.5

Pine Beetle Damage. Red trees pictured are dead or dying.

Pine Beetle Damage. Red trees pictured are dead or dying.

Looking to the Future

So what can be done? It’s clear that the rise of fungal disease is a threat to animal and plant survival, global food security, and ecosystem security, and likely to have even more far-reaching consequences. In order to manage the spread of pathogenic fungi, regulations must be enacted on the international transport of goods. Tight biosecurity controls on trade can help us take on the rise of pathogenic fungi and mitigate its effects. Moreover, the facilitation of global discussion on the subject will highlight its importance to those in policymaking positions. With widespread awareness of the weight and scope of this issue, we can take action to diminish this global threat.

References

  1. Retallick, R. W. R., McCallum, H., & Speare, R. (2004). Endemic infection of the amphibian chytrid fungus in a frog community post-decline. PLoS biology, 2(11), e351. Found here.
  2. Blehert, D. S., Hicks, A. C., Behr, M., Meteyer, C. U., Berlowski-zier, B. M., Buckles, E. L., Coleman, J. T. H., Darling, S. R., Gargas, A., Niver, R., Okoniewski, J. C., Rudd, R. J., & Stone, W. B. (2008). Bat White-Nose Syndrome : An emerging fungal pathogen? Science, 323(9), 227. Found here.
  3. Fisher, M. C., Henk, D. a, Briggs, C. J., Brownstein, J. S., Madoff, L. C., McCraw, S. L., & Gurr, S. J. (2012). Emerging fungal threats to animal, plant and ecosystem health. Nature, 484(7393), 186–94. Found here.
  4. Daszak, P., Cunningham, a a, & Hyatt, a D. (2000). Emerging infectious diseases of wildlife–threats to biodiversity and human health. Science (New York, N.Y.), 287(5452), 443–9. Found here.
  5. Kurz, W. A., Dymond, C. C., Stinson, G., Rampley, G. J., Neilson, E. T., Carroll, A. L., Ebata, T., & Safranyik, L. (2008). Mountain pine beetle and forest carbon feedback to climate change. Nature, 452(7190), 987–90. Found here.
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s