Baculovirus penaei( BP ) is an occluded baculo-like virus that contains double-stranded DNA as its nucleic acid type ( 2, 3, 11, 12, 16, 25, 32 ) . Although BP was not listed with the baculoviruses in the Seventh Report of International Committee on Taxomony of Viruses ( ICTV )( 34 ) , it had been cited and recognised as a possible member of the nuclear polyhedrosis genus of the Baculoviridae in the Fifth Report of the ICTV ( 14 ) . Furthermore, Penaeus monodon -type baculovirus ( MBV ) , a related nuclear polyhedrosis virus of Eastern Hemisphere penaeid shrimp, was listed as a possible member of this group in the Seventh Report of the ICTV ( 34 ) . According to ICTV guidelines ( 14, 26, 34 ) , BP was designated PvSNPV ( for singly enveloped nuclear polyhedrosis virus from P. vannamei, the most characterised geographical strain of BP from P. vannamei)( 2 ) . Although PvSNPV may be the most correct name for BP, the term BP will be used to designate this virus ( and its closely strains ) in this Manual .
BP is considered to be a potentially serious pathogen in the larval, postlarval, and early juvenile stages of host shrimps. BP possesses a wide geographical distribution and diverse host range, and multiple strains of the virus have been documented ( 6, 10, 13, 17-19, 21, 29-31 ) . Infections by BP are characterised by the presence of prominent, tetrahedral intranuclear occlusion bodies, which are referred to as polyhedral occlusion bodies or polyhedral inclusion bodies, in affected epithelial cells of the hepatopancreas and midgut, or free within lysed cell debris in the faeces ( 3-5, 21, 22 ) . Crowding, chemical stress, or environmental stress may enhance the pathogenicity and increase the prevalence of BP in its hosts ( 11 ) . Infection by BP is exclusively by the oral route in which cannibalism and faecal-oral contamination are the principal mechanisms of transmission ( 15, 16, 21, 22, 27, 29 ) .
BP has a widespread distribution in cultured and wild penaeid shrimps in North and South America, but its geographical distribution is limited to the Western Hemisphere ( 2-5, 10, 20-24, 27, 35 ) . However, within the Americas and Hawaii, multiple geographical strains of BP exist, and some of these may be distinguished by the size of the virion or by molecular methods ( 9, 13, 21, 24 ) .
Several surveillance methods are available for use in certification of the BP infection status of shrimp stocks. The simplest method is based on the microscopic demonstration of the characteristic occlusion bodies produced by the virus. With direct microscopy, characteristic tetrahedral occlusion bodies are demonstrated in wet-mounts of whole larvae, of excised portions of the hepatopancreas from postlarvae or older shrimp, or of faeces from large juvenile to adult shrimp. Wet-mount examination of the faeces of adult broodstock for characteristic occlusion bodies may be used as a simple nonlethal method to detect carriers with moderate or heavier infections ( 3-5, 20, 21 ) . However, the wet-mount method may not detect developing, low-grade, or low prevalence BP infections. Histology of fixed specimens may also be used for surveillance ( 1, 21 ) . Routine histology provides a positive diagnosis of BP infection when characteristic tetrahedral occlusion bodies are demonstrated in hypertrophied nuclei of mucosal epithelial cells of the hepatopancreas or midgut ( 3-5, 20, 21 ) . Molecular detection methods for BP, ( gene probes applicable to in-situ hybridisation assays and polymerase chain reaction methods ) are also available and provide a satisfactory method for surveillance applications ( 3, 7-9, 13, 21, 22, 35 ) .
Because BP is transmitted from adults to their offspring by faecal contamination of the spawned eggs, prevention of infection in hatcheries may be achieved by taking steps to eliminate faecal contamination of spawned eggs and larvae thorough the use of disinfectants, drying, and other routine sanitation methods with tanks and equipment ( 18 ) , and by direct decontamination of spawns by washing nauplii or eggs with formalin, iodophores, and clean sea water ( 28 ) .