Angiostrongylus vasorum is a cardiovascular nematode increasingly found in dogs and foxes in endemic foci throughout Europe. The present study evaluates ELISAs for detection of circulating antigens and specific antibodies against A. vasorum in foxes. Blood and worm burdens (WBs) from carcasses of 215 Swiss wild red foxes (Vulpes vulpes) and from 75 farmed foxes of different age groups experimentally inoculated once or repeatedly with infective doses of 50, 100 or 200 third-stage larvae were obtained. Antigen detection in the naturally infected Swiss foxes had 91·2% sensitivity and 89·4% specificity, whereas the corresponding figures for antibody detection were 42·2 and 92·0%. The experimentally infected foxes became positive for circulating antigens 5-10 weeks post-inoculation (wpi) and remained highly positive up to 22 wpi, irrespectively of further challenge inoculation. The antibody responses in the same foxes were highly variable: high optical density (OD) values were reached 5-7 wpi in all animals, followed by a decrease in over half of the animals despite accumulating and consequently high WBs resulting in persistent infections. After each challenge, a slight increase of OD values was observed 7 weeks later. We hypothesize that infected foxes develop a variable and non-protective immunity. Such parasite tolerance allows long-term survival of A. vasorum in the animals, and may explain why the parasite appears to spread rapidly within a fox population, an epidemiological dynamic that is evident in many parts of Europe where A. vasorum has been found over the last decades.