Gerbils, Tree Rings, and Plague, oh my!

Fig.1. This adorable great gerbil (Rhombomys opimus) found in Central Asia could be to blame for the spread of plague into Europe during the 1300s (Credit: Yuriy75, 2009).

When traveling to New York City, you might want to avoid touching surfaces within the subway. According to a recent study, nearly 15,000 different microorganism species were found along railings, ticket kiosks, seats, doors, and poles. Of that number, about half the organisms were bacteria, including the causative agent for the bubonic plague, Yersinia pestis. Although the infamous New York City rats can carry this deadly disease, they might not be the rodents to blame for spreading the Black Death in the 14th century. A climatological study published in the Proceedings of the National Academy of Sciences might overturn this long held belief about how the pandemic killed millions in Europe, Asia, and Africa.

Disease follows the rain

From 1346 to 1353, the Black Death swept across Europe, claiming the lives of nearly 60 percent of Europe’s population. After the initial introduction from Asia, many believe that the disease persisted in local black rat (Rattus rattus) populations. Research done by scientists at the University of Oslo and the Swiss Federal Research Institute proves this hypothesis wrong. Study author Nils Christian Stenseth and his colleagues analyzed tree ring records and data on more than 7,700 historical plague outbreaks to understand the link between climate and this disease. They found that the plague arrived in Europe 10 to 15 years after Central Asia experienced a wet spring followed by a warm summer. Although these weather conditions are poor for black rats, they are perfect for for Asia’s great gerbil (Rhombomys opimus) population.

Stenseth et al.’s model challenges two long-held beliefs. First, it shows that gerbils carried the plague into Europe instead of rats. Second, the model argues that the disease arrived in waves from Asia instead of in one push in 1347. Although tree ring records are good proxies for climate, this hypothesis needs more supporting evidence. Stenseth hopes to track genetic variation between plague victims across time to create a map of the appearance of different Y. pestis strains in Europe. The use of tree ring records to create a climate-model of disease is vital for understanding how environmental variability affects human history.


Fig.2. A trunk disk belonging to a juniper tree older than 1,000 years in Tien Shan mountains of Kyrgyzstan. These tree rings help to link climate fluctuations in Asia to plague in Europe (Credit: Jan Esper, University of Mainz, Germany)

 

Measuring past change

This study is not the first that connected climate fluctuation to the emergence of a disease. Today, epidemiologists keep a wary eye on ticks, the vector of Lyme disease, and their spread across the United States in warmer temperatures. Healthcare professionals also watch other climate-dependent and vector borne diseases, like dengue and West-Nile virus. Although tree ring data cannot track these diseases day to day, yearly climate fluctuations cause disturbances in tree growth and a change in the shape of tree rings. Collections like those at the Laboratory of Tree Ring Research (LTRR) at the University of Arizona - with almost 2,000,000 wood research specimens spanning 9,000 years - allow dendroclimatologists to study localized weather patterns over time. Although those specimens were not originally collected to connect disease to a change in climate, Stenseth et al. show that unique applications of specimens can yield surprising results.


References:

Akst, J. (2015, February 9). Subway Microbiome. Scientific American. Retrieved from http://www.the-scientist.com/?articles.view/articleNo/42117/title/Subway-Microbiome/

History: Plague. (2014, November 18). CDC online. Retrieved from http://www.cdc.gov/plague/history/.

Kaplan, S. (2015, February 24). After 8 centuries, rats exonerated in spread of Black Death. Gerbils implicated. Washington Post. Retrieved from http://www.washingtonpost.com/news/morning-mix/wp/2015/02/24/after-8-centuries-rats-exonerated-in-spread-of-black-death-gerbils-implicated/?tid=hp_mm.

Schmid, B.V., Büntgen, U., Easterday, W.R., Ginzler, C., Walløe, L., Bramanti, B., & Stenseth, N.C. (2015). Climate-driven introduction of the Black Death and successive plague reintroductions into Europe. PNAS. doi: 10.1073/pnas.1412887112

Mathez, E.A. (2000). Case Study: Studying Tree Rings to Learn About Global Climate. In Earth: Inside and Out. Retrieved from http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/cs_tree_rings.html

Glossary

Black Death
A pandemic that peaked in the years 1346-1353 and killed an estimated 60% of Europe’s population. The disease came from Central Asia and traveled to the Mediterranean and Europe, causing serious social and economic upheavals. The plague is caused by the pathogen Yersina pestis.
tree ring records
Tree-ring dating involved measuring the space between tree rings (growth rings). Each ring represents one year and the shape or width of the ring show the state of the climate that year.
vector
An organism that transmits a pathogen from a reservoir to host. In the case of Lyme disease, the deer tick is a vector that transmits Borrelia burgdorferi from the deer reservoir into the human host.
dendroclimatologist
A scientists who reconstructs historical climate conditions through the study of tree rings (growth rings).