Epidemiology of a primary pneumonic plague in Kantoshu, Manchuria, from 1910 to 1911: statistical analysis of individual records collected by the Japanese Empire

Background Among the potential uses of Yersinia pesos, intentional release of its aerosolized form, causing person-to-person transmission, is thought to be the most threatening. With the current rarity of pneumonic plague epidemics, our epidemiological knowledge remains insufficient for detailed characterization of effective control measures. Methods Temporal patterns and key biological parameters of a pneumonic plague epidemic in Manchuria from 1910 to 1911 were analysed based on historical records collected by Kanto Totokufu, the administration of the Japanese Empire in Manchuria at that time. The serial intervals were fitted to gamma distribution using the maximum likelihood method, and time-delay distributions from onset-to-admission, admission-to-death, and onset-to-death were investigated. Results Whereas a total of 228 cases were diagnosed with pneumonic plague in areas under direct control of the Japanese Empire, 4781 cases were also recorded in surrounding areas. Although the epidemic grew exponentially in the early phase, the average doubling time steadily increased reflecting successful control efforts. The estimated mean serial interval (and standard deviation) was 5.7 (3.6) days. All cases with known dates of onset were admitted to hospital within 4 days after onset, and the mean time from onset to admission was 1.1 (0.4) days. Conclusions The increase in doubling time demonstrates the efficient and rapid countermeasures employed. Since the short interval from onset to death implies the importance of rapid responses, the challenge in confronting a future bioterrorist attack is to implement rapid and appropriate integration of control measures both at the individual and community level to prevent further transmissions as well as lower case fatality.