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Background. Primary pneumonic plague (PPP) caused by Yersinia pestis is the most threatening clinical form of plague. An outbreak was reported in July 2009 in Qinghai Province, China. Methods. This outbreak was investigated by clinical, epidemiological, bacteriological, and immunological methods. Multilocus variable number tandem repeat analysis (MLVA) was used to track the source of the outbreak. Results. The index case, a patient with PPP, contaminated 11 close contacts. All the 12 cases, including the index patient, experienced sudden onset of fever, headache, and productive coughing with bloody sputum. Three of them died. Nevertheless, another 61 direct and 256 indirect contacts were not infected during the 2-week quarantine. Antibodies to F1 antigen were detected in 9 survival cases, with a 4-fold increase in titers in serum samples collected at different periods. Seven strains of Y. pestis were isolated from dogs and patients. Field investigation and MLVA of the isolated strains revealed that this outbreak was started by a deceased dog. Conclusion. Dogs are believed to be an indicator animal for plague surveillance, but their association with PPP is rare. Our results provide evidence for this possibility, which suggests the public health significance of dogs as a source of plague. [ABSTRACT FROM AUTHOR]