Passive surveillance for shrimp pathogens in Penaeus vannamei submitted from 3 Regions of Latin America

Multiple PCR analyzes were performed using 19 different primer sets to open and broaden the search spectrum for shrimp pathogens. In addition, multiple primer pairs for 10 pathogens were compared to see if there were differences in selectivity or sensitivity among them. Some pathogens that did not present histological lesions were detected. The most important outcome was that selection of appropriate primers was the most critical factor in obtaining reliable results. We found high variability in results among primers and we learned it was prudent to seasonally assess among them for the best set selection. It is important to understand that a PCR positive test result alone does not confirm the presence of a viable pathogen or a disease state. Nor, as might be expected, does it mean that the positive PCR test results will be necessarily accompanied by histological lesions characteristic of the targeted pathogen. However, the use of appropriately selected primers sets can reveal whether there is an evolution in the result spectrum over time and if some pathogens disappear or reappear or new ones emerge. In general, most shrimp presented coinfections that consisted of the presence of WzSV8, DHPV, chronic midgut inflammation and tubule distension/epithelial atrophy consistent with Pir A/B toxicity. Also included were RLB/NHPB, microsporidia, striated muscle necrosis, gregarines in the hindgut caecum (gametocyte stage, and not associated with tegumental glands but glands that line the mouth and anus) and encysted, presumed nematode larvae. WzSV8 was newly discovered in gonads. Histological changes and the presence of spheroids in the lymphoid organ were considered as healthy host responses of often unidentified cause.