Deciphering the socio-environmental factors associated with realized heartworm transmission risk in dogs from Portugal and Spain

Abstract

Dirofilaria immitis is a vector-borne zoonotic parasite that is expanding throughout Europe, with the Iberian Peninsula and its archipelagos acting as important endemic areas. Althoug climatic suitability models are available, few studies have analyzed the specific socio-environmental factors associated with Realized Transmission Risk (RTR) in areas with confirmed canine heartworm infection. This study analyzed 3,670 antigen-positive dogs from Spain and Portugal (2023–2024) to characterize these factors. An Ecological Niche Model (ENM) for Culex pipiens, integrated with parasite development thresholds, was used to estimate the RTR at the exact geolocation of each infected animal. Multiple linear regression was then applied to assess the association of RTR with geographic, climatic, and host-related variables. The geographic region emerged as the strongest predictor of infection risk, with the insular territories (Balearic and Canary Islands) showing the highest risk coefficients relative to the peninsular baseline. Among climatic classifications, the hot semi-arid climate was significantly associated with increased risk, suggesting that transmission may also become relevant in more arid settings with anthropogenic water sources. Dogs kept indoors also showed a significantly higher risk coefficient than the outdoor/indoor reference category, indicating that indoor housing may noy fully prevent exposure under certain conditions, possibly due to vector endophily and stable domestic microclimates. In addition, RTR was significantly higher in 2024, with marked regional heterogeneity, including a notable increase in northern regions such as Castile and León. Overall, these findings suggest that heartworm transmission is shaped by the interaction of geographic, climatic, and anthropogenic factors rather than occurring randomly. From a One Health perspective, the results support the need for geographically adapted prevention and surveillance strategies, particularly in insular, semi-arid, and emerging inland areas.