Histopathological analyses of native silver catfishRhamdia quelen(Quoy and Gaimard, 1824) immunized against and challenged with live theronts ofIchthyophthirius multifiliis

Abstract

As an alternative to treating with chemicals, immunization using Ichthyophthirius multifiliis as a vaccine has been studied in fishes that were often affected with white spot diseases also to understand the possible changes to the tissue caused by the vaccine. The focus of this study was the analysis of the influence of immunization via intraperitoneal injection (i.p.) and via immersion bath (im.b.) on the histopathology of R. quelen after being challenged with live theronts of I. multifiliis distributed in: control (non-immunized and non-challenged); non-immunized and challenged with 12,000 theronts/fish; non-immunized and challenged with 22,000 theronts/fish; immunized and challenged with 12,000 theronts/fish; immunized and challenged with 22,000 theronts/fish. Water quality was measured in each assay, with 300 fingerlings distributed among 15 tanks with 20 fish in each of three replicates. Six days after challenge, samples for histopathological and parasitological analyses were collected. In both i.p. and im.b. fish the prevalence of I. multifillis in the gills was higher in the non-immunized fish (33.33% and 27.77%, respectively). Melanomacrophages were present in 53% of the samples of i.p. non-immunized fish. Fish im.b. immunized and challenged showed more atrophied areas in the hepatocytes. Higher numbers of melanomacrophages in the i.p. non-immunized fish kidneys were observed compared to control. The results showed no difference in the gill lesions of either immunized or non-immunized fish compared to control. Histological alterations in the organs of silver catfish were considered light, except in the liver that presented significant atrophy and hypertrophy of hepatocytes after immunization via i.p.