Epidemiology of PRRSV on Pig Farms in Costa Rica

Abstract

Taken together, the findings of this thesis demonstrate that PRRSV in Costa Rica is a multifactorial problem in which viral diversity, epidemiological patterns, farm management, and economic pressures interact to sustain endemicity. While each chapter addresses a different aspect of the disease, together they explain why PRRSV persists and identify practical strategies for its control. At the molecular level, the detection of multiple introductions and the co-circulation of different PRRSV-2 lineages reveal that the virus has entered Costa Rica on several occasions, resulting in substantial genetic diversity. This diversity complicates vaccine matching, reduces cross-protection, and contributes to the persistence of infection. Since the vaccine currently registered in Costa Rica is based on the historical strain ATCC VR-2332, molecular surveillance remains essential to monitor viral evolution, detect emerging variants, and guide vaccination strategies. The epidemiological study showed that nearly half of commercial pig farms were PRRSV-seropositive, although prevalence varied considerably among ecozones. Higher infection rates were associated with regions characterized by greater pig density, intensive animal movement, and proximity to slaughterhouses, demonstrating that geography and production systems strongly influence viral transmission. Backyard and smallholder farms further complicate disease control because of limited biosecurity, shared breeding animals, and the movement of clinically or subclinically infected pigs facilitate viral spread. These findings emphasize that effective PRRSV control requires approaches adapted to tropical production systems rather than relying solely on strategies developed for temperate regions. The farm-level biosecurity analysis identified several management practices that promote virus transmission. Controlled exposure emerged as one of the strongest risk factors, demonstrating that the intentional introduction of field virus increases rather than reduces infection. Other important risk factors included unrestricted workforce movement, inadequate visitor and vehicle sanitation, and insufficient access control. Machine learning approaches, including Random Forest and LASSO, confirmed that human-mediated transmission plays a dominant role in PRRSV spread while providing an objective method for ranking the relative importance of individual risk factors. The stochastic economic model translated these biological and management factors into measurable financial consequences. Estimated annual losses ranged from approximately USD 142,000 under low-impact scenarios to more than USD 500,000 under severe outbreaks for a medium-sized farrow-to-finish farm. Even moderate deficiencies in biosecurity substantially reduced farm profitability, demonstrating that investments in prevention are economically justified in addition to being epidemiologically beneficial. Integrating the findings from all chapters reveals a coherent chain of events: repeated viral introductions generate genetically diverse PRRSV populations; geographic and production characteristics shape disease distribution; inadequate biosecurity allows viral persistence and transmission; and these factors ultimately result in considerable economic losses. Accordingly, priority interventions include strengthening import surveillance, eliminating controlled exposure, improving visitor and vehicle sanitation, regulating workforce movements, and implementing continuous molecular surveillance to support evidence-based vaccination and control programs. Overall, this thesis addresses the principal knowledge gaps identified for PRRSV in Costa Rica by characterizing viral genetic diversity, determining national prevalence and major risk factors, quantifying production and economic losses, and identifying practical, science-based control strategies. Beyond Costa Rica, these findings contribute to the understanding of PRRSV epidemiology in tropical production systems, where environmental conditions, farming structures, and resource limitations differ substantially from those in temperate regions.