Process evaluation of integrated West Nile virus surveillance in northern Italy: an example of a One Health approach in public health policy

West Nile virus (WNV) is endemic in the Po valley area in northern Italy. Regional health authorities have implemented integrated WNV surveillance following a One Health approach, based on collaboration between human, animal and environmental health institutions. We evaluated this integrated WNV surveillance system in Emilia-Romagna, Lombardy and Piedmont regions by means of a process evaluation. We examined the system's implementation fidelity, dose delivered and received, reach, and we identified strengths and weaknesses in the system. Qualitative and semi-quantitative data were obtained from three regional focus groups. Data were discussed in a follow up focus group, where participants suggested recommendations for improving the surveillance system. Inter-institutional and interdisciplinary integration and the creation of a 'community of practice' were identified as key elements for effective surveillance. We identified differences in the degree of interdisciplinary integration in the three regions, likely due to different epidemiological situations and years of experience in surveillance implementation. Greater collaboration and sharing of information, public engagement and economic assessments of the integrated surveillance approach would facilitate its social recognition and guarantee its sustainability through dedicated funding. We demonstrate that a transdisciplinary research approach based on process evaluation has value for designing and fine-tuning integrated health surveillance systems.