Angiostrongylus vasorum is a cardiopulmonary nematode increasingly found in dogs and foxes in many European countries. ELISAs for the detection of circulating A. vasorum antigen or specific antibodies in dogs were evaluated for foxes. Blood from 75 farmed Danish foxes experimentally infected with A. vasorum and from 215 dissected wild Swiss foxes were evaluated for detection of circulating A. vasorum antigen and specific antibodies. Antigen detection had 91.2% sensitivity and 89.4% specificity, and antibody detection 42.2% and 92.0%, respectively. The experimentally infected foxes became antigen positive 5 to 10 weeks post inoculation (wpi) and remained positive up to 22 wpi. The antibody responses in the same foxes were highly variable, irrespectively of further challenge inoculations, reaching seropositivity 5 to 7 wpi and followed by a decrease in over half of the animals despite persistent infections. Infected foxes may develop a variable and non-protective immunity which contributes to long term survival of A. vasorum that could explain its efficient spread within the fox population. Furthermore, the effect of EDTA/heat treatment on A. vasorum antigen detection in dog sera was assessed with 205 sera before and after treatment. An improvement of 34.6% was observed in treated samples between 7 and 10 wpi, while detection was impaired between 3 and 5 wpi. The same procedures applied to 74 samples tested with a rapid assay did not substantially improve antigen detection.