Lernaea, an ectoparasite, can infect most freshwater fish species (especially the members of cyprinidaea family) and the damage and the economic impacts associated with this species have been widely recognized by numerous epizootics to farmed fish, especially fingerling fish, which may be killed by only a few parasites. The present study investigated the possible causes of selective Lernaea attack in different fish species. The study was conducted in five trials. During the first trial Lernaea susceptibility and infestation were observed in indigenous major and Chinese carps. Trials were conducted in 4 earthen ponds with two ponds per experimental group. Fishes in the both groups were fed isonitrogenous diet containing 40% crude protein. Fishes were identified and examined for the presence of Lernaea species. L. cyprinacea was the species observed during the whole study period. The results showed that C. catla is the most susceptible fish to L. cyprinacea infestation and its appropriate control for conservation of this precious and declining fish is of extreme importance. Thunder is biologically safe as it is biodegradable and degenerated after 36 h without causing any negative effect on the water quality parameters and other water flora and fauna. Treatment of L. cyprinace with „Thunder‟ (0.10-0.25ppm) (Dichlorvos - 2,2-dichlorovinyl dimethyl phosphate or DDVP) gave promising results without adverse effect on fish. Though, thunder worked well in control but its heavy infestation really weakened fish incapacitating its feeding and competing abilities with its counterparts. In the 2nd experiment, comparison of whole fish body bio-chemical profile was observed on healthy and infected Major (L. rohita, C. mrigala and C. catla) and Chinese carps (C. idellus, H. molitrix and C. carpio). There were two-groups, one group without any treatment served as “control group” and the second group with application of DDVP termed as “treatment group”. All the ponds were randomly stocked with 6 locally culturable fish species. Fishes from both groups were dealt uniformly except administration of regular applications of DDVP (0.25ppm) in treatment group. Water physico-chemical parameters, such as temperature, salinity, dissolved oxygen etc. were recorded on fortnightly basis. Samples of healthy and infected fishes were collected for proximate composition and minerals and phosphorous analysis. The results showed that dry matter, fat and crude protein percentages were significantly decreased in lernaeid fishes. Moisture and ash contents of fishes increased in infected fishes as compared to healthy and treated fish. A slightly lower level of protein (12.65±0.49%) and fat (7.30±0.28%) were observed in C. catla than rest of the species. The protein was the highest (26.00±4.24%) in L. rohita while the fats were the highest (10.55±0.92%) in C. mrigala and C. carpio. Mineral profile, howver, was not much different, so it is not possible to suggest that level and type of nutrients are solely responsible for L. cyprinacea attack. Mineral composition of infected fish indicated that minerals balance upsets during disease condition. Pathogenicity is a complex of so many factors, which encompass environmental, biological, and physiological aspects so still lot remains to be explored before suggesting any final recommendation that which factor is more active and critical in inviting and attracting this parasite. In the 3rd experiment, various blood indices were compared among Chinese and Indian major carps for their resistance against lernaeaosis and probable role of blood and its components in reception of L. cyprinacea. Major and Chinese carps free from L. cyprinacea with an average weight of 830 ± 316 gm each, were collected. Blood parameters, red blood cells (RBC, 10 -6/μL), white blood cells (WBC, 10-3/μL) and platelets (10-3/μL) were counted by placing sample on haemocytometer grids. Differential leucocyte count (DLC), red blood indices (mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), packed cell volume (PCV) and mean hemoglobin concentration in the cell (MCHC) and blood chemistry (IgM (g/dl), Hb (g/dl), total protein (g/dl), albumin (g/dl), globulin (g/dl) and erythrocytes sedimentation rate (ESR) were determined. for males and females of experimental fishes. The results revealed that females of each species have relatively higher values for blood indices as compared to males. C. carpio has maximum number of granulocytes that may protect against the parasitic attack. C. catla has lowest values for the immunoglobulin Ig M as compared to the species which showed less susceptibility. In the 4th trial, healthy fishes of major and Chinese carps were collected from rearing ponds. Each fish weighed 830 ± 316 g on the average. For mucus collection, fishes were bathed in Potassium permanganate (KMnO4) solution (8.0 ppm) to remove microbial or fungal infection/infestation. Standard curve was drawn from various but consecutive dilutions of bovine serum albumin (BSA) solution and protein concentrations in different samples were calculated. Electrophoresis was carried out with slight modifications. 15% separating and 4% staking buffer were used to run the SDS- PAGE under constant voltage of 120. Fermentas PageRulerTM protein ladder was used as the standard marker for non reducing protein. The gel was stained with PageBlueTM (Fermentas) stain for identification of protein bands for molecular weight determination. Lectin activity and Alkaline Phosphatase test were determined. The increase in optical density (OD) was measured continuously for 2 to 3 hours at 405 nm using a micro plate reader. The results showed that lectin activity was the highest in C. idellus (109) indicative of low resistance while it was the lowest (21) in H. molitrix. Alkaline phosphatase level was the highest in C. catla, C. idella was the second highest and was the lowest in C. carpio. Protein concentrations were the highest in C. idella (3.29 ± 0.13 mg/ml) and C. catla (3.02 ± 0.57 mg/ml) while it was the lowest in C. carpio (1.80 ± 0.09 mg/ml). C. catla contained the highest molecular weight proteins (100 kDa) while C. carpio has one unique protein band of 14.13 kDa not present in any other species in current setup. In the 5th experiment, the L. cyprinacea were observed in the month of June to August. Lernaeied infestation was observed in all experimental fishes except in C. carpio. Fishes were treated with Thunder (DDVP- an organophasphate) to eliminate the parasite. Parasite free major and Chinese carps were collected with an average weight of 830 ± 316 g each were used for studies of whole-body amino acid composition. Total amino acid composition was determined by o-phthaldialdehyde (OPA) method using an Agilent chromatograph, reverse phase high performance liquid chromatography (HPLC). The results showed that the essential amino acids (arginine, lysine, phenylalanine, tyrosine, histidine and leucine) play a major role in the immune system. It is revealed that amino acids will widely become cost-effective neutraceuticals for improving health and preventing infectious disease in animals. C. carpio have no infestation due to increased numbers as well as higher concentration of certain essential amino acids when compared to other species of major and Chinese carps. C. catla, C. idella and H. molitrix have the lowest number and concentration of essential amino acids and more susceptible to L. cyprinacea attack. Finally it was concluded from the entire study that C. carpio may have high resistance for the L. cyprinacea as compared to the other experimental fishes. It possesses higher values for Ig M- immunoglobulin as compared to C. catla that indicated its high immunity against the parasite. Similarly C. carpio has maximum number of granulocytes (WBCs, esinophils, basophils and lymphocytes) that may support the fish against the parasitic attack. Polyacrylamide Gel Electrophoresis (SDS-PAGE) analysis of mucus revealed that C. carpio has one unique protein band of 14.13 kDa not present in any other species in current setup. This protein band may indicate the presence of lysozyme enzyme that actively participates against the invading pathogen. Essential and non-essential amino acids concentrations were also higher in the C. carpio that play a vital role in immunity especially arginine, lysine, phenylalanine, tyrosine, histidine, leucine, glutamic acid and aspartic acids." xml:lang="en_US