Cases in U.S. Southwest correlate with cyclic changes in Pacific ocean temperature

By Janet Raloff

Web edition: September 9, 2010

Warmer, drier conditions in the Southwest may be bad for gardeners, real estate developers and fish, but this climatic trend promises to depress the risk of bubonic plague, an international team of scientists reports. Their new study correlates changes in long-term climatic patterns with incidence of the deadly bacterial pestilence, one spread by fleas living on and around mice and other rodents.

The El Niño Southern Oscillation and Pacific Decadal Oscillation are two major patterns of Pacific climate variability, which operate on very different scales. El Niño events, also known as ENSOs, tend to last for 6 to 18 months and most directly impact water temperatures in the eastern tropical Pacific. Pacific Decadal Oscillations, or PDOs, persist for 20 to 30 years at a time, bringing long-term swings in sea temperatures, mostly to the northeastern Pacific.

Both drive North American climate, setting in motion relatively long-lasting patterns of warmer, wetter weather.

Nils C. Stenseth of the University of Oslo’s Centre for Ecological and Evolutionary Synthesis in Norway and a team of colleagues (from institutions including the U.S. Centers for Disease Control and Prevention) have now linked these patterns of climate variability with ebbs and flow in annual plague incidence from 1950 through 2005. “The underlying mechanism could involve changes in precipitation and temperatures that impact both hosts and vectors,” the researchers write in the September American Journal of Tropical Medicine and Hygiene.

Stenseth and his coworkers described the general idea a couple years ago in a paper that linked cyclical changes in human plague with climate anomalies driven by PDOs. In the new paper, they find that adding ENSO data leads to even better prediction of the likelihood of up ticks in plague cases.

More moisture fuels the growth of food for rodents, and wetter weather often encourages mice and rats to infest homes — ferrying any fleas with them. Indeed, ENSO-linked anomalous moisture was the hypothesis tendered to explain a deadly rodent-borne outbreak of another virus in the usually arid Southwest, something I wrote about 15 years ago.

Then as now, confirming such hypotheses would require blood or flea-sampling data to see whether increased rodent densities indeed fostered elevated populations of plague-carryng fleas, Stenseth’s group observes.

But the idea certainly makes ecological sense. Greater snowfalls in winters during ENSO and PDO cycles can build up strong water reservoirs that parcel out their life-giving moisture during the spring and summer months. In recent years, the mountainous West “has been losing snow in favor of relatively more rain and earlier snowmelt across a broad region,” the scientists note. This has led to warmer springs and summers and drier soils.

Such changes pose problems for fleas. Their survival diminishes as the temperature climbs and humidity drops. And above-ground conditions may not offer a good gauge of the environment that fleas experience. In rodent burrows, “humidity depends more on past rainfall and soil type than on the humidity of air outside,” the scientists note. Summer soil moisture tends to reflect snow accumulations the winter before and the date of spring snow melts (with earlier ones leading to drier soils).

As the globe warms, fleas are likely to find the West increasing arid and unwelcoming. So plague cases should remain low or even fall, the researchers conclude. Except, that is, when short-term climate fluctuations bring cooler, wetter weather. Or, they add, if people “in places like New Mexico” continue to expand home construction into wildlands — a domain where rodents reign.