Abstract
Several hatcheries in Latin America reported mortality of zoea stage 2 Penaeus vannamei. In fresh mounts, round structures resembling lipid droplets were observed, reminiscent of a disease called "Las Bolitas Syndrome" first identified in 1987. Closer examination under routine histopathology revealed the presence of detached cells and tissue in the digestive tract, whereas unaffected tanks displayed a typical intestinal content containing algae cells. Polymerase Chain Reaction of diseased and healthy batches of larvae for 22 shrimp pathogens revealed similar test results. The larvae were negative for nineteen pathogens, including AHPND. The detection of Vibrio spp. in both samples of affected zoea 3 (Z3) was the principal difference. Histology of affected zoeas were characterized by tissue degeneration in the hepatopancreas forming spheres that eventually moved into the upper gut, midgut and midgut caeca - a pathology known as Bolitas syndrome (BS). Microbiological analysis showed Vibrio spp. at ≤ 105 CFU zoea/g, ≈ 2 orders of magnitude higher than healthy zoea. Isolation of bacteria from healthy and BS affected zoea onto TCBS and CHROMagar™ and consequentially identified by API 20 E revelated six strains of V. alginolyticus. Though fresh mounts resembled the general description for “Las Bolitas Syndrome”, the histopathology differed from the original description. The intestine contained sloughed cells; the lateral lobes constituting the developing hepatopancreas in Z3 could be differentiated by their colour, with sloughed cells inside the peritrophic membrane. PCR and microbiological analyses verified that the origin of Las Bolitas Syndrome is bacterial in nature, with Vibrio playing a significant role.