Tissue and cell types infected by Ecytonucleospora hepatopenaei (EHP).

Ecytonucleospora hepatopenaei (EHP) is a microsporidian pathogen causing significant losses in shrimp aquaculture worldwide. The hepatopancreas is recognized as the primary target tissue, but the broader tissue and cell tropism of EHP and its ability to infect other components of the digestive system or non-digestive tissues remain unclear, especially when infections are intense and the host physiology is compromised. This study aimed to comprehensively investigate the histopathology of EHP infections in severely infected Penaeus vannamei to determine its tissue and cell tropism and assess the potential for systemic infection. The severity of infection was graded based on hepatopancreatic lesions. Histopathology showed that EHP spores were distributed in the digestive system of heavily infected shrimp, but not in non-digestive tissues such as gills, heart, gonad, nerves or skeletal muscle. EHP only infected the epithelial cells of the hepatopancreas, midgut, and midgut caeca, which lack the protective chitin layers. While the epithelial cells of the esophagus, stomach and hindgut were unaffected due to the protection of the inner chitinous layer, despite the presence of large numbers of EHP spores in these regions. Histopathology and ultrastructural pathology demonstrated that the R (reserve), F (fibrillar), B (blister), E (embryonic) and M (small midget) cells of the hepatopancreas were infected. These findings indicate that EHP does not cause systemic infection and has a strict cell tropism for the epithelium in the shrimp host.