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Biosecurity Briefing
Plague Bacteria Able to Survive in Soil; More Study Needed to Determine Biodefense, Public Health Implications By Crystal Franco, June 27, 2008 In the June 2008 issue of the journal Emerging Infectious Diseases, a study was published that may help “shed light” on the “long standing mysteries of how [the plague bacteria] Yersinia pestis persists during interepizootic periods [periods of time between disease outbreaks among animals].” The study demonstrated that Y. pestis, which is most commonly transmitted via direct contact, respiratory droplets, and flea bites, is able to survive “for at least 24 days in contaminated soil under natural conditions.”1 Until now, there has been no “clear understanding” of how Y. pestis bacteria persist during the quiescent periods between epizootic outbreaks. Also unclear is why plague is endemic in the western United States, but not in the eastern U.S. This study suggests that persistence of the bacteria in soil may be the answer.1 The study was conducted as part of an “environmental investigation” of a fatal case of plague in a wild mountain lion in Grand Canyon National Park, Arizona in 2007. Investigators tested the “viability of Y. pestis in naturally contaminated soil” from underneath the dead animal and confirmed via direct fluorescent antibody testing and culture isolation and lysis that the soil contained plague bacteria. Three weeks following the determined date of death of the mountain lion, investigators returned to the same location (pinpointed by global positioning satellite and photographs) to collect follow-up soil samples.1 The soil was analyzed at the Bacterial Diseases Branch of the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado. Four soil samples were suspended in saline solution and injected into laboratory mice to determine the presence of infectious Y. pestis bacteria. One of the 4 mice became ill after 12 hours following injection with the infected soil. Liver and spleen samples cultured from the mouse tested positive for the presence of Y. pestis. One of the remaining 3 mice became ill after 7 days following injection. However, Y. pestis cultures were not isolated from that mouse or the other 2 remaining healthy mice.1 Study investigators also conducted a gel electrophoresis test to compare the Y. pestis sample isolated from the soil to the sample that was isolated from the mountain lion. The results for the two isolates were indistinguishable, indicating that the “Y. pestis in the soil originated through contamination by this animal.”1 The study authors recognize that “these results are preliminary and do not address 1) the maximum time that the plague bacteria can persist in soil under natural conditions, 2) possible mechanisms by which the bacteria are able to persist in the soil, or 3) whether the contaminated soil is infectious to susceptible hosts that might come into contact with the soil.” The authors suggest that more study is needed and “from a public health or bioterrorism preparedness perspective, answers to these questions are necessary for evidence-based recommendations on bioremediation after a natural or intentional contamination of soil by Y. pestis.”1 References
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