Background: Infection with the cardiopulmonary nematode Angiostrongylus vasorum may cause severe disease in dogs, therefore prophylactic treatments are necessary to prevent infection in dogs at risk. A clinical field study was conducted to demonstrate the efficacy and safety of an oral combination of sarolaner, moxidectin and pyrantel (Simparica Trio ® ) for the prevention of A. vasorum infection in dogs (prevention study). A survey study was conducted concurrently to determine the infection pressure in the same areas. Methods: Prevention and survey studies were both conducted at the same veterinary clinics in endemic hot spots for A. vasorum in Denmark and Italy. The prevention study was a randomized, placebo controlled, double masked study where 622 client-owned dogs were treated and tested at 30 days intervals for 10 months. In the survey study 1628 dogs that were at risk of infection and/or were suspected to be infected were tested by fecal and/or serological methods, and the percent of dogs positive for A. vasorum was calculated. Results: In the prevention study, there were no adverse events related to treatment with Simparica Trio ® . Two placebo-treated animals became infected with A. vasorum during the 10-month study period, while none of the dogs in the combination product-treated group became infected. In the survey study, 12.2% of the study dogs were found positive to A. vasorum, indicating high exposure to the parasite during the period of the prevention study. Conclusions: Monthly oral treatment with the combination of sarolaner, moxidectin and pyrantel (Simparica Trio ® ) was 100% effective in the prevention of natural infection with A. vasorum in dogs in highly endemic areas. In endemic areas, A. vasorum occurrence in dogs at risk is considerable.[Figure not available: see fulltext.]