Baculovirus penaei (BP) and Penaeus monodon -type baculovirus (MBV) are occluded baculoviruses that contain double-stranded DNA as their nucleic acid type. MBV from P. monodon has been 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 Nomenclature of Viruses, and BP has been designated PvSNPV (for the most characterised geographical strain of BP from P. vannamei )(3, 16,17, 21, 22, 28, 30, 35, 36). Although PmSNPV and PvSNPV may be the most correct names for these groups of viruses, the terms MBV and BP will be used to designate these groups of viruses in this Manual.
BP and MBV are considered to be potentially serious pathogens in the larval, postlarval, and early juvenile stages of host shrimps. These viruses possess a wide geographical distribution and diverse host range, and multiple strains of both viruses have been documented. Infections by both viruses are characterised by the presence of prominent, 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 (26, 29) Crowding, chemical stress, or environmental stress may enhance the pathogenicity and increase the prevalence of MBV or BP in their hosts. Infection by BP and MBV are exclusively by the oral route in which cannibalism and faecal - oral contamination are the principal mechanisms of transmission (22, 26, 38, 39).
The geographical distribution of BP is limited to the Western Hemisphere. 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. BP has a widespread distribution in cultured and wild penaeid shrimps in North and South America (6, 10, 19, 23, 26, 29, 32).
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 Africa, the Middle East, from many of the Indo-Pacific countries, and from south and east Asia (1, 11-15, 18, 20, 24, 27, 28, 31, 32, 37, 38, 41, 43). MBV-type baculoviruses have been found in cultured shrimp and prawns only at sites in the Mediterranean and in West Africa. 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 (26, 29, 34).
Several surveillance methods are available for use in certification of the BP or 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 viruses. With direct microscopy, characteristic occlusion bodies (tetrahedral for BP and clusters of 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 BP or MBV infection when characteristic occlusion bodies are demonstrated in hypertrophied nuclei of mucosal epithelial cells of the hepatopancreas or midgut. Molecular methods for BP and MBV are also available and provide a satisfactory method for surveillance applications. Gene probes applicable to in situ hybridisation assays and polymerase chain reaction methods are available for both viruses (26, 29).
Because MBV and BP are 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.