Histopathology and cellular response in Enteromyxum leei (Myxozoa) infections of Diplodus puntazzo (Teleostei)

Enteromyxum leei is an intestinal parasite responsible for serious outbreaks in Mediterranean sharpsnout sea bream Diplodus puntazzo. E. leei infection was experimentally transmitted to healthy D. puntazzo (R) by cohabitation with infected donor fish. Haematological changes and histopathological damage were evaluated in relation to the course of infection. The prevalence of infection in R fish was 100% from day 10 post-exposure (p.e.) onwards, and the infection intensity and histopathological damage increased progressively. Different developmental stages were found in the infected intestines, including proliferative (stages 1–3) and sporogonic (stages 4 and 5) stages. Intestinal damage consisted of vacuolation, necrosis, detachment and sloughing of epithelium, and was correlated with the progression of the infection and with the development of the parasite. Sporogonic stages appeared from day 20 p.e. onwards. Initially, D. puntazzo seems to counteract the infection through the increase in leucocyte numbers, respiratory burst activity, haematopoietic activity and MMC. Two types of eosinophilic granular cells (EGC1 and EGC2) were detected in the intestinal epithelium and lamina propria. EGC1 numbers decreased with the progression of infection, whereas an increase in EGC2 occurred, mainly in the lamina propria. The involvement of the cellular immunity in the response of D. puntazzo to E. leei was demonstrated. The depletion of this response at a certain point of the infection could contribute to the high virulence of this myxozoan in this fish species.