We have developed an index based on macroscopic criteria to easily assess the health state of the eel swimbladder as a result of infection by the nematode Anguillicola crassus. In the sampling area (brackish lagoons of the French Mediterranean coast), 92% of the host sample (1251 eels covering all size classes) showed pathological signs of infection. The general trend was for increasing damage as eel size increased, thus suggesting accumulation of pathological effects. We also revealed a non-linear relationship between the swimbladder index and the abundance of living worms. In particular, we showed that severely damaged swimbladders harboured very few living nematodes. We argue that the swimbladder degenerative index more closely reflects the parasite pressure than does classic parasite count. We found seasonal variation in the swimbladder index, with maximum damage occurring in July and thereafter a trend for healthy individuals. We discuss the possibility that the seasonal decrease in the swimbladder index could reflect the death of the more severely affected individuals during the warmest months.