Wednesday, September 17, 2014

Tracing the History of Disease

In 1894, two research physicians independently identified the bacterium that causes bubonic plague. The bacillus, now known as Yersinia pestis, had finally been found as the pathogen to a disease that killed millions. This discovery was timely because just six years later the plague arrived to the United States on a ship carrying passengers from Hong Kong to San Francisco. Over the next nine years, the bubonic plague swept through the city.

Although a citywide rat-catching campaign brought an effective halt to the San Francisco epidemic, the introduced disease proved deadly to rodent populations across North America. The sylvatic plague, a disease caused by the same pathogen as the bubonic plague, is a modern problem in wild rodent populations. Prairie dogs, in particular, are extremely susceptible and entire communities may be decimated in one cycle of the plague (Sackett et al., 2013). As we mentioned last week, monitoring prairie dog populations allows public health officials to prepare for a human outbreak. But what happens on the animal level? We were able to sit down with a Smithsonian scientist to learn about the animal side of the infamous disease.

 

Conservation and Resistance

Loren Sackett, a conservation geneticist at the Smithsonian, studies wildlife diseases from an evolutionary perspective. Her current work focuses on avian malaria in a Hawaiian bird species known as the Hawaii amakihi. This bird species is of particular interest to scientists because they are surviving at lower elevations of the Hawaii Islands -- exactly where the avian disease is most prevalent.

“They seem to be thriving down there ... we know that they have to have acquired some sort of resistance to avian malaria, so we want to find out how that is based,” Sackett said.

Her interest in understanding the evolutionary side of diseases drove her to complete a dissertation on how the sylvatic plague affects genetic diversity in prairie dog populations. Ongoing work asks whether, like the amakihi, the prairie dogs have or will evolve resistance to the disease that so severely affects those animal populations.

“What a lot of people are interested in, from a practical and manageable standpoint, what can we expect prairie dog populations to do over the long term?” Sackett said.

Fig.1. Michanowski, J. Black-tailed prairie dog

Sackett’s long-term perspective and interest in wildlife diseases is a different take on understanding diseases that can transfer from animals to humans. These zoonotic diseases make up 75% of emerging and re-emerging diseases, so understanding the evolutionary mechanisms behind deadly pathogens would be extremely beneficial to disease response.

Old History, New Technologies

In addition to studying avian malaria in Hawaii, a current project draws her back in time to the introduction of the plague to North America. She is working with Dave Wagner at Northern Arizona University to track genetic diversity in prairie dog populations during and after 20th century plague outbreaks. Her original work encompassed barely a decade of the century long plague presence. Wagner and Sackett are now examining the disease through natural history museums to “compare what genetic diversity is like before plague and how it is now.” New technology to pull more information from fragmented DNA means more information about the plague’s history.

“It’s really astounding the kinds of things we’re able to do with collections,” Sackett said. “I come from a very genomic biased background, but putting our new technologies together with the massive amounts of collections we have is just amazing.”

A truly exciting part of her work is that she can place her dissertation in a century’s worth of disease history and utilize more effective technology to understand old specimens. Sackett hopes to eventually complete genome-wide analyses in both her amakihi work and her prairie dog work.

“We can hardly even imagine the kinds of things that we’re going to be able to do in the near future.”

References:

Bubonic plague hits San Francisco 1900-1909. (1998). In PBS: A Science Odyssey online. Retrieved
from http://www.pbs.org/wgbh/aso/databank/entries/dm00bu.html.

Michanowski, J. (Photographer). Black-tailed prairie dog [Photograph]. Retrieved Sept 16, 2014, from: http://www.bbc.co.uk/nature/life/Black-tailed_Prairie_Dog.

Sackett, L.C., Collinge, S.K., & Martin, A.P. (2013). Do pathogens reduce genetic diversity of their hosts? Variable effects of sylvatic plague in black-tailed prairie dogs. Molecular Ecology, 22 (9). Retrieved from http://www.colorado.edu/ebio/sackett/LorenSackett/Publications_files/Sackett_recolonization_final.pdf.

Zoonotic Disease: When Humans and Animals Intersect. (2014). In Centers for Disease Control and Prevention online. Retrieved from http://www.cdc.gov/24-7/cdcfastfacts/zoonotic.html.




Glossary

genetic diversity
The variation of heritable characteristics present in a population of the same species. Genetic diversity allows species to adapt to a new environment, fend off predators, or fight diseases.
zoonotic disease
A disease that can be passed between animals and humans.

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