Climatic predictors of the intra- and inter-annual distributions of plague cases in New Mexico based on 29 years of animal-based surveillance data

Abstract

Within the United States, the majority of human plague cases are reported from New Mexico. We describe climatic factors involved in intra- and inter-annual plague dynamics using animal-based surveillance data from that state. Unlike the clear seasonal pattern observed at lower elevations, cases occur randomly throughout the year at higher elevations. Increasing elevation corresponded with delayed mean time in case presentation. Using local meteorological data (previous year mean annual precipitation, total degrees over 27 degrees C 3 years before and maximum winter temperatures 4 years before) we built a time-series model predicting annual case load that explained 75% of the variance in pet cases between years. Moreover, we found a significant correlation with observed annual human cases and predicted pet cases. Because covariates were time-lagged by at least 1 year, intensity of case loads can be predicted in advance of a plague season. Understanding associations between environmental and meteorological factors can be useful for anticipating future disease trends.

Figure 1.

Pet cases reported to the New Mexico Department of Health. The shaded areas denote the risk area defined by Eisen and others.

Figure 2.

Total number of pet cases occurring by calendar week and 30 yr average monthly maximum/minimum temperature and precipitation data for the elevation quartiles (a: 1,238–1,846 m; b: > 1,846–2,022 m; c: > 2,022–2,115 m; d: > 2,115–2,945 m). The x axis is week of the year for the pet plague cases and month of the year for temperature and precipitation. The horizontal line in the temperature graph is drawn at 27°C.

Figure 3.

The final model explained 75% of the inter-annual variance in pet plague cases. The model was tested by singly dropping each year and running the model (Leave-One-Out). The mean R² of this method was 0.747. The plotted observed vs. predicted cases show a good fit of pet case peak and trough, with limited discrepancy with respect to amplitude. Because of the reliance of the model on previous years' meteorological data, the predictions begin in 1984.

Figure 4.

Predicted pet and the actual human plague cases since 1991. The bars represent the total number of human plague cases reported from New Mexico to the CDC (1991 through 2007). The line represents the model's predicted number of pet cases. We limited the data to post 1990 for this comparison to capture the increased surveillance that was significant in the model. The correlation between predicted pet cases and human cases is 0.520 (P = 0.032).