Immune response to nematode larvae in the liver and pancreas of minnow, Phoxinus phoxinus (L.)

Ultrastructure investigations of the liver and pancreas of minnow, Phoxinus phoxinus (L.), infected with encysted Raphidascaris acus larvae, in 10 fish containing 1-15 individuals, showed extensive tissue damage. Larvae were encapsulated by epithelioid granulomata, and these cells showed typical epithelial features such as tonofilaments and desmosomes. Often they co-occurred with neutrophils and macrophages at the site of infection and direct contact among immune cells was common. In the infected organs, both mast cells and rodlet cells were present in larger numbers (P< 0.01) in comparison to uninfected liver and pancreas. These cells were also observed in hepatic sinusoids. In parenchyma of infected pancreas, rodlet cells and mast cells were close to the fibres. In many instances, mast and rodlet cells were at the periphery of the infected liver and pancreas, beneath the mesentery, partially or entirely surrounded by mesothelial cells. In both organs, slight degranulation of mast cells was noticed near the nematode larva. Our data suggest that mast cells, rodlet cells, neutrophils, and macrophages of liver and pancreas of P. phoxinus are actively involved in host immune response to infection by nematodes.