The objective was to determine the immunological response of Litopenaeus vannamei using mannan-oligosaccharides (T1), garlic (T2) and a compound from plant extracts (T3) after being infected with Vibrio parahaemolyticus. Two bioassays were performed with duration of six (1x106 CFU) and fourteen (3x106 CFU) days in the Marine Sciences and Limnology Research Center, University of Costa Rica in 2011. Research used 120 shrimp for each experimental period, extracted from farms located in the Nicoya Peninsula. Weight gain and feed conversion were evaluated after product’s dosage. After infection with V. parahaemolyticus, hemograms, coagulation, hemolymph’s bacteriology and accumulated mortality analysis were made. Immunological parameters showed no statistical difference (P>0,05) between treatments in any period, however after six days, T1 showed 41,07 x105 hemocytes/ml, coagulation of 34,40 s and 4,44x103 CFU/ml. Throughout the fourteen day period, T2 obtained the best values (55,76x105 hemocytes/ ml, coagulation of 34,20 s and 15,4x103 CFU/ml). After 10 hours of inoculation, accumulated mortality presented fewer deaths in T1 and T3 (76,2%) at six days, whereas at 14 days was (93,2%) for T1. Weight gain and feed conver-sion presented statistical differences (P<0,05) only for the bioassay which lasted six days, where T2 showed a 54,3 g biomass, an increase in weight gain of 19,3% and feed conversion of 1,4.

Keywords: Shrimp immunology, immunostimulation, hemograms, bacterial infection, feed conversion.