Theories and mechanisms of environmental factors that cause allergies in dogs: veterinarian involvement in lifestyle choices to support long-term well-being

Over the last few decades, the occurrence of chronic inflammatory and allergic disorders has increased exponentially in people and pets. Changes in our environment impact both human and animal health, as One Health recognizes. Several theories have been proposed, and various mechanisms have been considered. The first was the "hygiene hypothesis," which focused on the observation that allergies became more common once hygienic conditions improved. Although the concept that allergies happen because living conditions are too clean has been mostly disproven, parts of it are included in the "old friends hypothesis," which emphasizes the importance of exposure to harmless bacteria through food and the environment that have evolved with us. A further refinement of this idea is incorporated in the "biodiversity hypothesis," which emphasizes the importance of diverse environmental, food, and microbial exposures in educating the immune system and promoting tolerance. The most recent hypothesis is the "epithelial barrier hypothesis," which emphasizes the role of pollutants and chemicals that chronically disrupt epithelia, whether in the skin, gut, or respiratory tract, promoting low-grade inflammation and increasing the risk of allergic sensitization. Each of these theories has focused on the role of urbanization in promoting allergies, and each theory has some supporting evidence. Numerous epidemiological studies, addressed in the companion Currents in One Health by Marsella, JAVMA, forthcoming 2025, demonstrated that urban life increases the risk for allergies in people and dogs. The purpose of this review is to present the current knowledge in dogs and raise awareness regarding these topics.