Ligula intestinalis(L.) (Cestoda: Pseudophyllidea): plerocercoid‐induced changes in the spleen and pronephros of roach,Rutilus rutilus(L.), and gudgeon,Gobio gobio(L.)

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Abstract
The effects of the plerocercoid ofLigula intestinalis(L.) (Cestoda: Pseudophyllidea) on the major lymphoid organs, the spleen and pronephros, of roach,Rutilus rutilus(L.), and gudgeon,Gobio gobio(L.), are described. The spleen of ligulosed roach showed a significant decrease in weight. Differential cell counts suggested this was due to a reduction in erythrocytes, despite significant increases in macrophages and vacuolated granulocytes. The spleen of gudgeon, which consisted almost entirely of erythrocytes, showed a slight reduction in weight in ligulosed fish. In both roach and gudgeon this decrease was independent of parasite burden. Differential cell counts of the pronephros from ligulosed roach revealed a significant decrease in neutrophils and increase in vacuolated granulocytes. In the pronephros of gudgeon, however, cell counts were unaffected by ligulosis. Ultrastructural observations included an apparent disintegration of vacuolated granulocytes and increased pinocytic activity in specialized endothelial cells in the spleens of ligulosed roach. Also, melano‐macrophage centres and melano‐macrophages increased in the spleen of ligulosed roach and gudgeon, respectively. The marked changes in spleen weight and differential cell counts in ligulosed roach and lack of such changes in ligulosed gudgeon correlate with the differential response to the parasite by these two fish species.