Zika epidemic in Colombia and environmental and sociodemographic contributors: an application of a space-time Markov switching model

Abstract

Zika, a viral disease transmitted to humans by the bite of infected Aedes mosquitoes, emerged in the Americas in 2015, causing large-scale epidemics. Colombia alone reported 72,031 Zika cases between 31/May/2015 and 01/October/2016. We used national surveillance data from 1,121 municipalities over 70 epidemiological weeks to identify sociodemographic and environmental factors associated with Zika's emergence, re-emergence, persistence, and transmission intensity in Colombia. We fitted a zero-state Markov-switching spatio-temporal model under the Bayesian framework, assuming Zika switched between periods of presence and absence according to spatially and temporally varying probabilities of emergence/re-emergence (from absence to presence) and persistence (from presence to presence). These probabilities were assumed to follow a series of mixed multiple logistic regressions. When Zika was present, assuming that the cases follow a negative binomial distribution, we estimated the transmission intensity rate. Our results indicate that Zika emerged/re-emerged sooner and that transmission was more intense in municipalities which were more densely populated, with lower altitude and/or less vegetation cover. Higher weekly temperatures and less weekly-accumulated rain were also associated with Zika emergence. Zika cases persisted for longer in more densely populated areas and with a higher number of cases reported in the previous week. Overall, population density, elevation, and temperature were identified as the main contributors of the first Zika epidemic in Colombia. The estimated probability of Zika presence increased weeks before case reporting, suggesting undetected circulation in the early stages. These results offer insights into priority areas for public health interventions against emerging and re-emerging Aedes-borne diseases.