High and dry: Environmental conditions drive opportunities for disease transmission in desert bighorn sheep

Heterogeneity in host contact patterns can impact a pathogen's ability to invade and persist. Climate and landscape variation likely shape animal contact ecology across biological scales, but their impact on the dynamics of directly transmitted pathogens remains poorly understood.

We investigate the impacts of environmental conditions on host contacts and disease dynamics by integrating long‐term GPS and geospatial environmental data into network analyses and disease dynamic models. By analysing data from eight replicate populations of desert bighorn sheep ( Ovis canadensis nelsoni ) in the Mojave Desert, we assess (i) how variation in environmental conditions across time and the landscape drives synchrony or variation in contact ecology, (ii) the relative importance of individual traits vs. population‐level environmental conditions in structuring contacts and (iii) their consequences for disease outbreak size and persistence.

Population‐level environmental conditions explained the most variation in contact patterns, driving the timing and strength of contacts. Among the environmental factors studied, normalised difference vegetation index (NDVI) had the most significant impact on contact strength, with greener habitats associated with more frequent contacts.

Temperature also played a role in sex‐specific mixing, with male–male contacts being more frequent than female–female or male–female contacts during the cooler months but not the warmer months.

We simulated infectious disease spread in the system using our predicted networks. The model‐predicted basic reproduction number ( R ₀ ), outbreak size and outbreak duration were highest in large populations with high NDVI and when the first infection occurred during months with high NDVI, indicating that environmental conditions may influence disease dynamics at the population level.

Synthesis and applications . By using probabilistic models to predict contacts as a function of spatially and temporally varying environmental conditions, we show that population‐level conditions at the time of disease introduction can structure opportunities for transmission within each population. Differences in disease outcomes may therefore be driven by population‐specific variation in environmental conditions that shape contact patterns. As climate change continues to alter vegetation productivity and temperature regimes, monitoring NDVI and seasonal temperature patterns could inform the timing of disease surveillance and guide targeted conservation interventions for populations at greatest risk.