The ecological niche and seasonality of Theileria equi in Portugal

Equine piroplasmosis, caused by the tick-borne protozoan Theileria equi, is an evolving concern in Europe. In recent years, it has been associated with substantial economic costs and with transmission showing signs of pushing into previously free or marginal northern regions. While piroplasmosis is endemic in Portugal, not much is known about the interplay of seasonally varying and spatial abiotic drivers that may modulate infection. In this study, we leveraged unique field-surveillance data based on equine PCR testing together with publicly available data to unravel and characterize the spatio-temporal dynamics and ecological niche of Theileria equi. Infection probability was estimated to increase with local bovine abundance, temperature and the presence of planosol soils, and to decrease with local equine density due to an intraspecific dilution effect of vector exposure. At an individual level, the highest infection probabilities were estimated among female and pasture-kept equids. We estimate strong seasonal signatures in local and national infection probability, maximized in August and minimized in February, and a spatially heterogeneous ecological niche landscape. Far northern and north-eastern regions combined low infection probability both in- and off-season with prolonged off-seasons, whereas centre and north-western regions combined high probability both in- and off-season with brief off-seasons. The former plausibly function as unsuitable reservoirs, and the latter as suitable year-round reservoirs. Our results provide a first quantitative, risk-stratified basis for regional and individual targeted Theileria equi surveillance, awareness and control in Portugal.