Vol. II, No. 2 ISDCI ECTOTHERM SYMPOSIUM 439

ABSTRACTS OF POSTERS DEALING WITH FISH IMMUNOLOGY

MORPHOLOGICAL CHANGES IN THE LYMPHOID ORGANS OF SAPROLEGNIA- INFECTED BROWN TROUT, SALMO TRUTTA FARIO

F. Alvarez, B.E. Razquin, A. Villena and A. Zapata I

Depar tamento de Morfologia Microscopica, Facul tad de Biologia, Universidad de Leon, and 1Depar tamento de Morfologia Microscopica, Facul tad de Biologia, Universidad Complutense, Madrid, Spain.

Infect ions by Saprolegnia diclina have recent ly caused a high morta l i ty in wild populations of brown trout, Salmo t ru t t a fario, in Leon (Spain). Due to the seasonal incidence of the disease and the demons t ra ted e f fec t s of some environmental pa ramete r s on the immune system of the ec to therms , we have compara t ive ly studied the s tatus of the lymphoid organs of heal thy (without external signs of disease) and infected trouts at d i f ferent months of the year . Heal thy and infected brown trouts were caught weekly from February 1984 to April 1985 in several r ivers of Leon. The trout , whose age was determined by scal imetry , were brought to the laboratory, anesthet ized and weighed, and their thymuses, spleens, pronephroi and mesonephroi were removed and processed for light and e lec t ron microscopy.

The grea tes t number of infected trout were caught from January to April (the spawning period) and in September. No important d i f ferences occurred between the lymphoid organs of males and females in both heal thy and ill trout. Decreased numbers of c irculat ing lymphocytes occurred in the ill t rout in all considered months. By contrast , the amount of lymphoid tissue in heal thy t rout showed a gradual increase from winter to spring. Infec ted t rout did not show.this increase, however, containing in any month, lesser amounts of lymphoid tissue than found in heal thy fishes of winter. Thymi of infected t routs displayed a few thymocytes , increased connect ive tissue, enlarged epithelial cells and a reduced outer region. Fur thermore , splenic and renal lymphoid tissues appeared devoid of lymphocytes and had remarkable vascular modificat ions. A phagocyt ic re t icular sheath was lacking in the splenic ellipsoids of ill t rout while large areas were filled with pycnot ic cells. In any case, the most striking fea ture in the spleen and kidney of infected t routs was the existence of hyper t rophic lining sinusoidal cells containing phagocytosed material , possibly of a fungal nature. The significance of our results will be discussed from the perspect ive of the relationships between the s tatus of the immune system and the evolution of the disease.