Comparison of traditional and molecular detection methods for baculovirus penaei infections in larval penaeus vannamei

Penaeid shrimp Penaeus vannamei were experimentally infected at a mysis II stage with the penaeid shrimp virus Baculovirus penaei (BP). The larvae were sampled at 0, 8, 12, 18, 24, 48, and 72 h postinfection and were examined for evidence of infection by three different methods: wet mounts, histology, and nucleic acid probes. Wet-mount squashes of the hepatopancreas were prepared and examined for BP tetrahedral occlusion bodies (TOBs) or larvae were fixed in Davidson's fixative and processed for routine histology, then either stained with hematoxylin and eosin (H&E) or reacted with nonradioactive BP gene probes by in situ hybridization. Sections stained with H&E or reacted with labeled gene probes were examined for signs of BP infection, such as cell cytopathology, the presence of TOBs, or the presence of a purple precipitate (which indicates BP-specific nucleic acid in the tissue). Occlusion bodies were initially observed in the wet-mount squash of the hepatopancreas of one shrimp at 18 h postinfection, and advanced infections were commonly seen at 48 h post-infection. The first definitive positive by H&E histology was detected at 24 h postinfection, whereas the probes detected infections at 12 h postinfection. The results indicate that the BP gene probes have the capability of detecting a BP infection before TOBs are readily observable in wet-mount squashes of the hepatopancreas and before the formation of TOBs in infected tissue.