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This paper develops a large-scale Bayesian spatio-temporal binomial regression model for the purpose of investigating regional trends in antibody prevalence to Borrelia burgdorferi, the causative agent of Lyme disease. The proposed model uses Gaussian predictive processes to estimate the spatially varying trends and a conditional autoregressive model to account for spatio-temporal dependence. Careful consideration is made to develop a novel framework that is scalable to large spatio-temporal data. The proposed model is used to analyze approximately 16 million Borrelia burgdorferi test results collected on dogs located throughout the conterminous United States over a sixty month period. This analysis identifies several regions of increasing canine risk. Specifically, this analysis reveals evidence that Lyme disease is getting worse in some endemic regions and that it could potentially be spreading to other non-endemic areas. Further, given the zoonotic nature of this vector-borne disease, this analysis could potentially reveal areas of increasing human risk., Comment: 19 pages without figures. All figures are available as ancillary files

Gaussian Markov random fields (GMRFs) are popular for modeling dependence in large areal datasets due to their ease of interpretation and computational convenience afforded by the sparse precision matrices needed for random variable generation. Typically in Bayesian computation, GMRFs are updated jointly in a block Gibbs sampler or componentwise in a single-site sampler via the full conditional distributions. The former approach can speed convergence by updating correlated variables all at once, while the latter avoids solving large matrices. We consider a sampling approach in which the underlying graph can be cut so that conditionally independent sites are updated simultaneously. This algorithm allows a practitioner to parallelize updates of subsets of locations or to take advantage of `vectorized' calculations in a high-level language such as R. Through both simulated and real data, we demonstrate computational savings that can be achieved versus both single-site and block updating, regardless of whether the data are on a regular or an irregular lattice. The approach provides a good compromise between statistical and computational efficiency and is accessible to statisticians without expertise in numerical analysis or advanced computing., Comment: Revised introduction and expanded numerical examples to include Rcpp and parallel implementation. Supplementary material available from the authors. 38 pages, 8 figures