Dr. Murai Éva szerk.: Parasitologia Hungarica 16. (Budapest, 1983)

symptoms. In a progressed state of illness, however, disturbances in swimming and enlargement of the abdomen could be observed. On dissection, besides swim bladder lesions, swelling of the kidney, haemorrhages on the peritoneum, pseudomembrane formation in the tissues surrounding the swim bladder, thickening of the swim bladder wall, accumulation of tissue debris and fluid in the lumen of the swim bladder were found. In the initial and acute stages of inflammation, and for the most part also in cases of chronic inflammation, pathognomic lesions were observed in the anterior sac of the swim bladder. Description of the swim bladder of healthy common carp fry on the basis of literary data and our own experiences The swim bladder of common carp fry, similarly to that of other members of Cyprinidae, consists of two sacs. In fry the posterior sac is undeveloped (Fig. 1). Tissue structure of the anterior sac (Fig. 2): External layer, tunica externa: This layer consists of the thin adventitia and of the external longitudinal and internal circular fibres of elastic connective tissue. Internal layer, tunica interna: This also consists of several layers. It is attached to the tunica externa by a loose connective tissue rich in blood vessels and containing numerous lipoid cells, in addition to elastic fibres. Towards the lumen, this layer joins a bundle of smooth muscle which, however, does not extend to the full cross-section of the swim bladder. This layer is followed by the lamina propria made up of a hyperaemic, double layer of connective tissue abundantly interwoven by capillaries. The "rete mirabile" having a role in gas formation and made up of a network of numerous capillaries occupies a cranioventral position in the loose connective tissue. The lumen is lined with a layer of cuboidal epithelium. The tissue structure of the posterior sac is similar to that of the anterior one, but its external layer consists solely of adventitia, i.e. the double connective tissue layer is lacking. Its composition is as follows: adventitia (external layer) and tunica interna which contains many muscle fibres and less elastic fibres than that of the anterior sac. The posterior sac is connected with the anterior intestine by the ductus pneumaticus, the tissue structure of which is similar to that of the posterior sac but which is lined with columnar epithelium on the inside. Pathomorphological examination of the swim bladder of diseased fish In both acute and subacute inflammation, changes of the anterior sac are predominant. In the acute stage, only the tunica interna shows lesions upon removing the external layer. In cranioventral areas of the tunica interna, initially only a smoke-like opacification and capillary dilatation, then haemorrhages can be observed (Fig. 3). In sections prepared from such swim bladders, only the loose connective tissue of the anterior sac shows changes, namely small diapedeses around blood vessels filled with erythrocytes, and in other areas perivascularly and in the walls of dilated, empty vessels, numerous acinose protozoon forms 17 to 30 um in size can be observed (Figs 4 and 5). The epithelial layer was intact. Occasionally, haemorrhages were present also in the tunica externa. In preparations made of pathologically more severe, acute cases when already diffuse opacity was observed macroscopically on the anterior sac of the swim bladder, protozoa were demonstrated: not only in the loose connective tissue but also among the fibres of lamina propria and, furthermore, also in the epithelium (Fig. 6). This stage of the pathological process was characterized by manifold widening of the loose connective tissue which was oedematous and showed a mild infiltration by eosinophil cells. In this stage of inflammation, the proliferation of epithelial cells resulted in the formation of protuberances in the epithelium.