Identifying the spatiotemporal clusters of plague occurrences in China during the Third Pandemic.

Plague, a devastating infectious disease caused by Yersinia pestis, has killed millions of people in the past and is still active in the natural foci of the world today. Understanding the spatiotemporal patterns of plague outbreaks in history is critically important, as it may help to facilitate prevention and control of potential future outbreaks. In this study, we explored spatiotemporal clusters of human plague occurrences in China using a machine-learning clustering method and reconstructed the potential transmission pattern during the Third Pandemic (1772-1964). We succeeded in identifying 6 clusters in the space domain (2D) and 13 clusters in the spatiotemporal domain (3D). Our results suggest that there were several temporal outbreaks and transmissions of plague in different spatial clusters. Together with the spatiotemporal nearest neighbor approach (ST-NNA), this method could allow us to have a clearer look at the spatiotemporal patterns of plague.

Human plague system associated with rodent diversity and other environmental factors.

Plague remains a threat to public health and is considered as a re-emerging infectious disease today. Rodents play an important role as major hosts in plague persistence and driving plague outbreaks in natural foci; however, few studies have tested the association between host diversity in ecosystems and human plague risk. Here we use zero-inflated generalized additive models to examine the association of species richness with human plague presence (where plague outbreaks could occur) and intensity (the average number of annual human cases when they occurred) in China during the Third Pandemic. We also account for transportation network density, annual precipitation levels and human population size. We found rodent species richness, particularly of rodent plague hosts, is positively associated with the presence of human plague. Further investigation shows that species richness of both wild and commensal rodent plague hosts are positively correlated with the presence, but only the latter correlated with the intensity. Our results indicated a positive relationship between rodent diversity and human plague, which may provide suggestions for the plague surveillance system.

Wet climate and transportation routes accelerate spread of human plague.

Currently, large-scale transmissions of infectious diseases are becoming more closely associated with accelerated globalization and climate change, but quantitative analyses are still rare. By using an extensive dataset consisting of date and location of cases for the third plague pandemic from 1772 to 1964 in China and a novel method (nearest neighbour approach) which deals with both short- and long-distance transmissions, we found the presence of major roads, rivers and coastline accelerated the spread of plague and shaped the transmission patterns. We found that plague spread velocity was positively associated with wet conditions (measured by an index of drought and flood events) in China, probably due to flood-driven transmission by people or rodents. Our study provides new insights on transmission patterns and possible mechanisms behind variability in transmission speed, with implications for prevention and control measures. The methodology may also be applicable to studies of disease dynamics or species movement in other systems.

[Ecological-geographic landscapes of natural plague foci in China VIII. Typing of natural plague foci].

Since plague is an important natural focus zoonosis, the typing of natural plague foci becomes one of the elements in understanding the nature and developing related prevention program of the disease. Natural foci of plague are composed by four fundamental parts which include Eco-geographical landscape (natural plague foci), hosts, vectors and pathogens (Yersinia pestis) that comprehensively interact through the large temporal scale of evolution. Human activities have had great impact on the foci of natural plague. Based on the published serial research papers, we tried to integrate the knowledge of each factor in natural plague foci and focusing on theoretical aspects, so as to strengthen the prevention and surveillance programs of plague to be extrapolated to other zoonosis.

[Ecological-geographic landscapes of natural plague foci in China VI. Biological characteristics of natural vectors of Yesinia pestis].

OBJECTIVE: To characterize the biological characteristics of natural vectors of Yersinia (Y.) pestis in China. METHODS: Species and genera of natural vectors of Y. pestis in China and their faunal distribution were characterized with modern insect taxonomic techniques. The ecological roles of natural vectors of Y. pestis in natural plague foci were determined according to insect ecological experiments. RESULTS: There were 63 species of natural vectors of Y. pestis including 28 major reservoirs and 35 secondary ones. CONCLUSION: The biology characteristics of major vectors on Y. pestis and their roles in natural plague foci were defined.

[Eco-geographic landscapes of natural plague foci in China v. biological characteristics of major natural reservoirs of Yesinia pestis].

OBJECTIVE: To study the biological characteristics of natural reservoirs of Yesinia (Y.) pestis in China. METHODS: Species and genera of natural reservoirs of Y. pestis in China and their faunal distribution were characterized with modern taxonomic techniques. The ecological roles of natural reservoirs of Y. pestis in the natural plague foci were determined according to the animal ecological experiments. RESULTS: There were 86 species of natural reservoirs of Y. pestis including 14 major reservoirs and 72 kinds of the secondary or occasional reservoirs. CONCLUSION: The biological characteristics of major reservoirs of Y. pestis and their roles in natural plague foci were defined.

Identification of Chinese plague foci from long-term epidemiological data.

Carrying out statistical analysis over an extensive dataset of human plague reports in Chinese villages from 1772 to 1964, we identified plague endemic territories in China (i.e., plague foci). Analyses rely on (i) a clustering method that groups time series based on their time-frequency resemblances and (ii) an ecological niche model that helps identify plague suitable territories characterized by value ranges for a set of predefined environmental variables. Results from both statistical tools indicate the existence of two disconnected plague territories corresponding to Northern and Southern China. Altogether, at least four well defined independent foci are identified. Their contours compare favorably with field observations. Potential and limitations of inferring plague foci and dynamics using epidemiological data is discussed.

[Ecological-geographic landscapes of natural plague foci in China VII. Typing of natural plague foci].

OBJECTIVE: To group and characterize natural plague foci in China. METHODS: A novel two-class typing method as well as a three-indication nomenclature method were established to group and characterize the natural plague foci, on the basis of eco-geographical landscapes of plague foci, genetics of Yersinia pestis, zoology of rodent reservoirs and the entomology of flea vectors. RESULTS: A total of 12 distinct natural plague foci (including 19 subtypes) as well as their biological features were characterized. CONCLUSION: Natural plague foci in China were grouped and characterized in this study.

Nonlinear effect of climate on plague during the third pandemic in China.

Over the years, plague has caused a large number of deaths worldwide and subsequently changed history, not the least during the period of the Black Death. Of the three plague pandemics, the third is believed to have originated in China. Using the spatial and temporal human plague records in China from 1850 to 1964, we investigated the association of human plague intensity (plague cases per year) with proxy data on climate condition (specifically an index for dryness/wetness). Our modeling analysis demonstrates that the responses of plague intensity to dry/wet conditions were different in northern and southern China. In northern China, plague intensity generally increased when wetness increased, for both the current and the previous year, except for low intensity during extremely wet conditions in the current year (reflecting a dome-shaped response to current-year dryness/wetness). In southern China, plague intensity generally decreased when wetness increased, except for high intensity during extremely wet conditions of the current year. These opposite effects are likely related to the different climates and rodent communities in the two parts of China: In northern China (arid climate), rodents are expected to respond positively to high precipitation, whereas in southern China (humid climate), high precipitation is likely to have a negative effect. Our results suggest that associations between human plague intensity and precipitation are nonlinear: positive in dry conditions, but negative in wet conditions.

[Eco-geographic landscapes of natural plague foci in China I. Eco-geographic landscapes of natural plague foci].

OBJECTIVE: To study the eco-geographic landscapes of natural plague foci, in China. METHODS: According to the surveillance records on plague epidemics and the eco-geographic landscapes of natural plague foci based on the county level, the criterion for classifying the ecological geographic zone of Chinese natural plague foci was established. RESULTS: 12 types and 19 subtypes of eco-geographic landscapes on Chinese natural plague foci were identified. CONCLUSION: Scientific basis for Chinese natural plague foci classification was provided.