Cartilage proliferation in response to metacercarial infections of fish gills

The pathology observed in a metacercarial (species not yet identified) infection of pond-raised, aquarium fish is described. In natural infections, there was extensive proliferation of cartilage resulting in loss of respiratory epithelium and occasionally in fusion of primary lamellae. Grossly, the most consistent sign of infection was flared opercula with protruding gills. Although haemorrhage was produced in early infections as the cercariae migrated, it was not macroscopically evident. A few heavily infected fish did have grossly observable signs of haemorrhage but this was not a consistent finding. Cercariae were collected from snails (Pleurocera sp.) and fish were experimentally infected to study the response. Over a 6-day-period, a progression from fibroblast to chondroblast occurred, indicating that piscine fibroblasts are able to differentiate into chondroblasts. This differentiation may be dependent on environmental conditions or may be stimulated by the parasite itself.