Bovine respiratory syncytial virus (BRSV) infection is an important cause of outbreaks of respiratory disease among calves. This virus commonly induces mild to severe respiratory signs but, in a substantial proportion of cases, is also reported to be associated with paroxystic respiratory distress syndrome (PRDS). The pathogenesis of this 'malignant' clinical form has not been fully elucidated. The present study aimed at determining whether mast cell (MC) degranulation plays a role in the physiopathologic cascade leading to the PRDS. Paired serum samples were taken in herds during outbreaks of severe respiratory diseases (acute sera) and 3 weeks after (convalescent sera). Based on seroconversion to BRSV and clinical picture, 67 pairs of sera were selected from calves with a BRSV-associated PRDS for circulating MC tryptase determination. A MC metachromatic score was measured in post-mortem lungs from animals died from a BRSV-associated PRDS (principals) and compared with reference scores obtained from healthy lungs (controls). Levels of tryptase were significantly higher in acute sera (26.6 +/- 12.4 microg/l) compared to convalescent sera (8.4 +/- 7.8 microg/l; P<0.001). Metachromatic scores yielded significantly different results between controls and principals (P<0.01), demonstrating a significant disappearance of metachromatic granules from lung MCs in principals. Taken together, these data demonstrate the presence of an extensive MC degranulation in BRSV-associated PRDS.