Yosemite Plague Cases Lead to Insight on Detection, Protection, and Prevention


A research team examines two Yosemite plague diagnoses to determine effectiveness of procedures for the detection, protection, and prevention of Yersinia pestis.

Although only about seven human plague cases are reported each year in the United States in a “normal” year, western states reported more than double that number of cases in 2015. Of those cases, two were traced back to patients who visited Yosemite National Park; however, the cases were ultimately determined to be unrelated to each other. A team of researchers from the California Department of Public Health (CDPH), the Centers for Disease Control and Prevention (CDC), and the National Park Service, evaluated regional response to the two diagnoses—which occurred in quick succession—to determine how effective procedures were in identifying potential sites for plague-causing Yersinia pestis and educating the public on potential health risks.

According to the CDC, the plague is most common in the southwestern states—particularly New Mexico, Arizona, and Colorado—and most likely to infect humans when it is already amplified among rodent hosts, such as the California ground squirrel, chipmunks, and deer mice. These species have high mortality rates and “release” infected fleas frequently when they die; they also carry fleas that are prone to biting humans in addition to other animals. Thus, when the initial patient in the study, a 14-year-old male who had recently returned from a camping trip at Crane Flat campground at Yosemite, presented with a septicemic plague infection, it seemed likely that a transmission from fleas-to-rodents-to-humans had occurred. When a second case of plague was diagnosed shortly thereafter in an 18-year-old female from Georgia who had spent a week in Oakhurst and had visited multiple sites in Yosemite Valley and the surrounding area, it became clear that at least one site of Y. pestis transmission was likely active in the national park.

After the diagnoses, the patients provided lists of all the sites that they had visited. They also answered questions about their activities and what they had observed in the area. Both were accompanied by family members who did not become ill, and neither reported seeing any dead rodents. Neither patient reported touching any rodents, although one reported feeding them.

Preliminary evaluation of the sites visited by the patients enabled scientists to rule out several sites up front due to altitude (plague activity and transmission in the region has generally been restricted to locations at elevations >1,200 meters according to the CDPH) and a lack of historical evidence of Y. pestis activity in the area. All locations were given a visual risk assessment to identify indicators such as abandoned rodent burrows, lower-than-expected diversity and population size of rodents, and the presence of rodent carcasses. The team noted that at prioritized sites, rodent burrows were “swabbed” with flannel swatches to collect fleas, rodent trapping and testing was carried out on both rodents and their fleas, and rodent carcasses were collected and tested.

The CDHP, CDC, and National Park Services were able to identify Crane Flat campgrounds as the likely site of infection for patient 1, while they speculated that patient 2 likely picked up the infection during her visit to Glacier Point. (The groups could not conclusively determine the latter due to the fact that no evidence of active infection of rodents or fleas at Glacier Point was found at the time of testing.) The only site that the two patients had both visited, Yosemite Valley, showed no sign of Y. pestis transmission. Extended environmental investigations did yield evidence of Y. pestis transmission in several other areas in Yosemite and the surrounding region, including in the Sierra Nevada Mountains, but no additional cases were reported at that time.

The park quickly took action in the wake of the first diagnosis, rapidly expanding its environmental investigations, informing the public via a variety of media outlets, and immediately closing potentially “active” sites in addition to treating the areas with pesticides to eliminate infected fleas. “Increased educational efforts informing the public how to reduce their exposure to the cause of this potentially fatal disease contributed to the early diagnosis of patient 2,” the team reported, noting that there was also a spike in reports of finding dead rodents in the park after the public-education push; that rise in reports led to detection of Y. pestis transmission at additional locations. The team appeared satisfied that “rapid interagency investigation and public health response…probably reduced the risk for plague among Yosemite visitors and staff.”

National media coverage of the infections also likely raised awareness in the medical community as well as the general population, which could have contributed to the spike in plague diagnoses during 2015.

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