Empirical assessment of a threshold model for sylvatic plague

Abstract

Plague surveillance programmes established in Kazakhstan, Central Asia, during the previous century, have generated large plague archives that have been used to parameterize an abundance threshold model for sylvatic plague in great gerbil (Rhombomys opimus) populations. Here, we assess the model using additional data from the same archives. Throughout the focus, population levels above the threshold were a necessary condition for an epizootic to occur. However, there were large numbers of occasions when an epizootic was not observed even though great gerbils were, and had been, abundant. We examine six hypotheses that could explain the resulting false positive predictions, namely (i) including end-of-outbreak data erroneously lowers the estimated threshold, (ii) too few gerbils were tested, (iii) plague becomes locally extinct, (iv) the abundance of fleas was too low, (v) the climate was unfavourable, and (vi) a high proportion of gerbils were resistant. Of these, separate thresholds, fleas and climate received some support but accounted for few false positives and can be disregarded as serious omissions from the model. Small sample size and local extinction received strong support and can account for most of the false positives. Host resistance received no support here but should be subject to more direct experimental testing.

A curve of thresholds governs plague epizootics in Central Asia

“…The resulting data reveal clear epizootics in the gerbils, interspersed by periods of 2–5 years in which plague is not detected in either fleas or rodents anywhere in the area (see Supporting Information, Appendix S1). The detectable presence of plague is well predicted by a threshold level of past‐occupancy (1–2 years previously) (Davis et al. 2004, 2007a,b; Samia et al.…”

Interannual Variability of Human Plague Occurrence in the Western United States Explained by Tropical and North Pacific Ocean Climate Variability

“…9 Climate factors are known to regulate both host and vector abundances. [11][12][13][14] Small mammal population dynamics are controlled by both intrinsic and extrinsic factors, where the latter includes contemporaneous and past local temperature and precipitation. 11,[14][15][16][17][18][19][20] Fleas spend the majority of their life cycle in the environment and thus, are affected by surrounding conditions, notably by temperature and humidity changes.…”

“…[11][12][13][14] Small mammal population dynamics are controlled by both intrinsic and extrinsic factors, where the latter includes contemporaneous and past local temperature and precipitation. 11,[14][15][16][17][18][19][20] Fleas spend the majority of their life cycle in the environment and thus, are affected by surrounding conditions, notably by temperature and humidity changes. [21][22][23] During plague epizootics, the abundance of these hosts and the prevalence of Y. pestis in them, as well as the numbers of active fleas, 24 are high.…”

Emergence, spread, persistence and fade-out of sylvatic plague in Kazakhstan

“…The high spatial resolution and the long study period of the resulting dataset make the types of analyses we do here, possible. Previous studies based on the Pre-Balkhash dataset include Davis et al [10,11], who studied predictive gerbil abundance thresholds for plague, and Stenseth et al [12] and Samia et al [13], who studied climatic influences on plague occurrence. All these studies (but the second) concentrated on data from up to four selected areas of 40 Â 40 km 2 where plague monitoring had been most regular.…”