The infestation of Trypanosoma sp. (Kinetoplastida) in the hybrid catfish and some fresh water fish was studied showing such parasitic infestation was specific only in catfish. The virulence as determined by LD50 for 5 days was 2.28x1010 cell in a fish sample. The parasitic infestation caused hematological changes by the reduction of red as well as white blood cells. The reductions were highly significant as compared to the healthy sample (p<0.05) as noted by the red and white blood cell count which dropped from 2.14±0.48x106 to 1.62±0.27x106 cells/ml and from 1.45±3.76x105 to 2.42±0.78x104 cells/ml blood in the infested samples, respectively. Similar trend was noted for hemoglobin and hematocrit which dropped significantly (p<0.05). The hemoglobin in healthy fish is 7.075±0.929g/100g, which dropped to 6.268±0.697g/100g in the samples with infestation. The percentage of hematocrit in healthy sample is 25.275±3.318%, which dropped to 21.722±3.068% in the samples with infestation. The reverse trend was recognized for serum protein and leukocrit which increased in the samples with Trypanosoma sp. infestation. The density gradient centrifugation technique was employed in the isolation of parasites in which 50% Percoll solution in 0.85% final preparation of saline solution was capable of removing Trypanosoma sp. from the blood. The study of antibody levels in serum showed that the infested hybrid catfish could develop the antibody which reached a peak 14 days after the infestation. Trypanosoma sp. was unable to cause histological changes in the tissues of gill, liver, kidney, spleen, stomach and intestine. Minor inflammations were observed, even the cases that large number of parasites were found in the tissues, blood streams and sinuses. Marked reductions were recorded for mature red blood cells while there were the formation of immature red blood and phagocytotic cells at higher rates as compared to the healthy individual.
