Penaeus monodon-type baculovirus ( MBV ) is an occluded baculo-like virus that contains double-stranded DNA as its nucleic acid type ( 20, 22, 24 ) . MBV from P. monodon was designated PmSNPV ( for singly enveloped nuclear polyhedrosis virus from P. monodon) in accordance with the guidelines for virus nomenclature published by the International Committee on Taxomony of Viruses ( ICTV )( 28 ) , but it appears as Penaeus monodon NPV, or PemoNPV, in the Seventh Report of the ICTV ( 37 ) . Although PemoNPV may be the most correct name for the virus, the term MBV will be used to designate this virus ( and its closely strains ) in this Aquatic Manual .
MBV is considered to be a potentially serious pathogen in the larval, postlarval, and early juvenile stages of its host penaeid shrimps and prawns. MBV has a wide geographical distribution and diverse host range, and multiple strains of the virus are likely to exist ( 1, 2, 5, 6, 8-21, 25, 29, 30, 34, 40 ) . MBV infections are characterised by the presence of prominent, spherical intranuclear occlusion bodies in affected epithelial cells of the hepatopancreas and midgut, or free within lysed cell debris in the faeces ( 5, 20 ) . Crowding or environmental stress may enhance the pathogenicity and increase the prevalence of MBV in its hosts ( 2, 13, 20, 24, 31 ) . Transmission and infection by MBV is exclusively by the oral route in which cannibalism and faecal-oral contamination are the principal mechanisms of transmission ( 17, 20, 31, 32, 35 ) .
MBV-type baculoviruses are widely distributed in cultured and wild penaeid shrimp and prawns in the Eastern Hemisphere and have been observed or reported from Australia, East Asia, South-East Asia, India, East Africa, the Middle East, from many of the Indo-Pacific countries ( 1, 2, 5, 6, 8-15, 18-24, 26, 27, 29, 30, 34, 40, 41 ) . MBV-type baculoviruses have been found in cultured ( but not in wild ) shrimp and prawns only at sites in the Mediterranean and in West Africa ( 20 ) . In most of these examples, the affected stocks were either introduced P. monodon or the facility had a recent history of P. monodon introduction. MBV has been found in introduced stocks of P. monodon in the Western Hemisphere. MBV has been observed in introduced P. monodon in the Pacific in Tahiti and Hawaii, and in the Americas in a number of shrimp-farming sites in North America, South America, and the Caribbean. Despite the simultaneous culture in a number of farms, and the consequent direct exposure of certain Western Hemisphere penaeids to MBV-infected P. monodon , MBV did not produce significant infections in native species, nor has it become established in shrimp farms or in wild stocks ( 20, 23 ) .
Several surveillance methods are available for use in certification of the MBV-infection status of shrimp and prawn stocks. The simplest method is based on the microscopic demonstration of the characteristic occlusion bodies produced by the virus. With direct microscopy, characteristic occlusion bodies ( clusters of refractive spheres for MBV ) 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 nonlethal method to detect carriers. Histology of fixed specimens may also be used for surveillance. Routine histology provides a positive diagnosis of MBV infection when characteristic occlusion bodies are demonstrated in hypertrophied nuclei of mucosal epithelial cells of the hepatopancreas or midgut. Molecular methods for MBV are also available and provide a satisfactory method for surveillance applications. An antibody-based enzyme-linked immunosorbent assay for diagnosis of MBV has been reported ( 16 ) , but the method used polyclonal antibodies and the antibodies are not commercially available. Gene probes applicable to in-situ hybridisation assays and polymerase chain reaction methods are also available for MBV ( 3, 7, 16, 20, 26, 28, 38, 39 ) . A nested PCR method for MBV may provide the most sensitive method available for screening and surveillance applications ( 3 ) .
Because MBV 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 by thoroughly washing nauplii or eggs with formalin, iodophores, and clean sea water ( 9 ) .