Spatial Patterns of Hantavirus Pulmonary Syndrome in California and Nevada

BACKGROUND:  Hantavirus pulmonary syndrome (HPS) is a rare but serious viral disease in humans. Most HPS cases in the United States are caused by Sin Nombre virus (SNV), a species of hantavirus most commonly associated with deer mice (Peromyscus maniculatus). Transmission typically occurs by inhalation of aerosolized rodent urine or feces. Though deer mice are prevalent across North America, most HPS cases occur in the western United States. The objective of this study was to evaluate the spatial pattern of HPS cases relative to key environmental variables in two endemic western states. 

METHODS: Information of confirmed HPS cases in California and Nevada was extracted from the national HPS surveillance maintained by CDC’s Viral Special Pathogens Branch. Cases were assigned to the ZIP Code where exposure most likely occurred. ZIP Codes were assessed for spatial clustering using the Global and Local Moran’s I statistical tests. Hot-spots of HPS occurrence were evaluated using the Getis Gi*(d) function in ArcMap. Remotely sensed data of elevation, precipitation, and temperature were used to create a linear regression model of environmental differences between cases across the study region, with a focus in the Sierra Nevada Mountains.  

RESULTS: From 1995, when HPS became nationally notifiable, through 2012, 75 HPS cases with complete exposure information were reported from the study area. Global and local spatial clustering tests of HPS cases within ZIP codes were statistically significant (p-value<0.05). Hot-spots were identified around the central Sierra Nevada Mountain range and in central Nevada. Case exposures were associated with areas of higher elevation, cooler temperature, and drier climates.  Of the 213 ZIP codes included in the Sierra Nevada Mountain range model, 112 fell on the western slope and 102 on the east.  Cases were most frequently identified in ZIP codes occurring east of peak elevation (30 HPS cases as compared to 5 on the west). 

CONCLUSIONS: Significant differences in case distribution across ZIP Codes and environmental variables disprove the assumption that populations at risk for HPS are evenly distributed in California and Nevada. Further, assessment of environmental differences within and outside identified hotspots yielded common characteristics of higher elevation, cooler temperatures, and drier climates in ZIP Codes of HPS occurrence. With continued research, spatial descriptive statistics, detailed hantavirus ecological studies, and HPS epidemiological data may permit development of more sensitive and accurate models predicting human health risk.