Baculoviral and baculoviruslike viruses

The first baculovirus described from shrimp was baculovirus penaei (BP) in Penaeus duorarum from Florida (Couch, 1974a,b). J.-R. Bonami, L.D. Bruce, B.T. Poulos and D.V. Lightner (unpublished, cited in Bruce et al., 1993) propose changing BP to 'PvSNPV' ('the most characterised variant, from Penaeus vannamei, in the singly enveloped nuclear polyhedrosis virus group'), in compliance with the guidelines for virus nomenclature designated by the International Committee on Taxonomy of Viruses. For the sake of consistency with previous descriptions, however, the acronym BP is used here.

Baculovirus penaei is reported from 12 species of Penaeus (Lightner et al., 1992b), including P. aztecus, P. vannamei, P. setiferus, P. stylirostris, P. schmitti, P. penicillatus, P. brasiliensis, P. paulensis, P. subtilis, P. marginatus and P. monodon, ranging from the Gulf of Mexico, south to Brazil and in the Pacific from Peru to Hawaii. Of these species, P. vannamei, P. aztecus, P. duorarum and P. marginatus are most severely affected, with serious mortalities in commercial hatcheries at larval, postlarval and early juvenile stages. Occlusion bodies have also been found in up to 80% of wild pink (P. duorarum) and brown (P. aztecus) shrimp; however, no mass mortalities in wild populations have been attributed to BP (Couch, 1978; Brock and Lightner, 1990).

Clinical signs of BP infections are non-specific: gill and surface fouling, decreased growth and anorexia. Severe infections result in necrosis and loss of hepatopancreatic and midgut epithelia. Transmission occurs at all life stages, but patent infections (detectable occlusion bodies) may not develop in older shrimps. Pyramid-shaped occlusion bodies principally occur in the nuclei of the hepatopancreas and midgut epithelia (Fig. 20.5a) and are released in the faeces. Up to six occlusion bodies, measuring 0.5-20 mm across the base, can be found in a single nucleus and may contain rod-shaped virions, measuring 55-75 nm x 300 nm (Table 20.1; Fig. 20.5b) (Couch, 1974a).

A second baculoviral-like infection of shrimps is found in cultured shrimp throughout Asia, the Middle East, Italy, South America and the southern USA (Lightner and Redman, 1981; Lightner et al., 1992b; Natividad and Lightner,

Fig. 20.5. (Opposite) (a) Baculovirus penaei (BP) occlusion bodies in the nucleus of hypertrophied hepatopancreas cells of Penaeus aztecus. Photograph courtesy of D.V. Lightner. TEM, x 5400. (b) Occlusion body of BP containing rod-shaped virions (arrows). Note virions also lying around the edge of the occlusion body. Photograph courtesy of D.V. Lightner. TEM, x 61,200.

1992). Although originally described as 'P. monodon-type' baculovirus (MBV), infections have since been detected in a wide range of penaeid species (Lightner and Redman, 1992), and a similar baculovirus in Penaeus plebejus in Australia ('plebejus baculovirus' (PBV) (Lester et al., 1987) is also considered to be an MBV-type virus (Doubrovsky et al., 1988; Lightner et al., 1992b). The diverse host and geographical range of MBV-type baculoviruses strongly suggests a group of morphologically related strains, rather than a single species. Recently, Mari et al. (1993) proposed renaming MBV as PmSNPV ('the most characterized variant, from P. monodon, in the singly enveloped nuclear polyhedrosis virus group'); however, for sake of consistency, MBV is used here.

Penaeus monodon-type baculovirus is most pathogenic to P. monodon larvae, with lesser effects in P. merguiensis and P. semisulcatus larvae (Johnson and Lightner, 1988); however, intense infections may also occur in apparently healthy shrimp. Determination of MBV pathogenicity is further complicated by multiaetiology infections and senility (Lightner et al., 1992b). Clinical signs include lethargy, anorexia, stunted growth, dark discoloration and surface fouling - for example, with Leucothrix mucor (bacteria) or Zoothamnium spp. (ciliates). Acute MBV infections cause loss of hepatopancreas and midgut epithelia, although this may also be associated with secondary Vibrio infections (Lightner et al., 1983a). Adult and juvenile stages appear to be more resistant than larvae, and stress (overcrowding, nutrient build-up, transportation and handling) is believed to be a significant exacerbating factor (Brock and Lightner, 1990).

Penaeus monodon-type baculovirus is distinguished from BP by size and shape of the occlusion body. The occlusion bodies of the former are <0.1 mm to nearly 20 mm in diameter and roughly spherical (Lightner et al., 1992b) (Fig. 20.6). Virion and nucleocapsid sizes of BP and MBV overlap (~75 nm in diameter and 300 nm in length). Johnson and Lightner (1988) provide detailed ultrastructural descriptions of both types of baculovirus.

A third nuclear polyhedrosis baculovirus was isolated from Penaeus orientalis suffering mass mortalities in Korea (Lee and Kim, 1994). Virions replicated in a Spodoptera frugiperda (Lepidoptera: Noctuidae) cell line and this is the first case of successful reproduction of an aquatic virion on an invertebrate cell line. The virus produced tetragonal or hexagonal polyhedra, which differ in shape from BP and MBV, but overlap in size (3 mm in width). Johnson and Lightner (1988) cite: 'Gouranton (1972) pointed out, the ability of granulosis viruses ... to form occlusion bodies differs according to the cell-type infected.' Thus, the difference between the tetragonal-hexagonal polyhedra of P. orientalis and the subspherical polyhedra of MBV may not be distinct and requires further investigation. Penaeus orientalis virions (30-40 nm in diameter and 200-400 nm in length) overlap BP and MBV in size.

A fourth baculovirus is non-occluded (type C) and associated with midgut-gland necrosis (baculovirus midgut-gland necrosis (BMN)) (Sano et al., 1981). It affects hatchery-cultured Kuruma shrimps (Penaeus japonicus) from southern Japan and Korea and has been a chronic problem since 1971 (Sano et al., 1984). Penaeus monodon, P. chinensis (= orientalis) and P. semisulcatus have also been infected experimentally.

Baculo Penaei Ultrastructure Hepatopancreas

Fig. 20.6. (a) Monodon baculovirus (MBV) occlusion bodies in the nucleus of a hypertrophied hepatopancreas cell in Penaeus monodon. Photograph courtesy of D.V. Lightner. TEM, x 5940. (b) MBV virions (arrows) lying within and around the occlusion body of an infected hepatopancreas nucleus. Photograph courtesy of D.V. Lightner. TEM, x 33,333.

Fig. 20.6. (a) Monodon baculovirus (MBV) occlusion bodies in the nucleus of a hypertrophied hepatopancreas cell in Penaeus monodon. Photograph courtesy of D.V. Lightner. TEM, x 5940. (b) MBV virions (arrows) lying within and around the occlusion body of an infected hepatopancreas nucleus. Photograph courtesy of D.V. Lightner. TEM, x 33,333.

Baculovirus midgut-gland necrosis infects the hepatopancreas of larval and postlarval shrimp, causing up to 98% mortality in PL-10 (10-day-old postlarvae) (Sano et al., 1981). Early clinical signs are a 'white turbid' or 'cloudy'

hepatopancreas. Severely infected shrimp are inactive, float to the surface and are distinguished by a white midgut line (Sano et al., 1984). Early names for the disease reflect these clinical signs: 'midgut gland cloudy disease', 'white turbid liver disease' and 'white turbidity disease'.

Infected hepatopancreas tissues may contain Gram-negative bacteria, but no occlusion bodies. As with BP and MBV, BMN causes marginated, reduced, nuclear chromatin and nucleolar dissociation. Normal nuclei (approximately 10 mm diameter), are easily distinguished from infected nuclei (20-30 mm diameter), using routine histology. Host-cell hypertrophy, marginated chromatin and refractive chromatin are relatively accurate for presumptive diagnosis of BMN at patent, moderate or late stages of development (in conjunction with clinical signs). Diagnosis can be confirmed ultrastructurally or by indirect fluorescent antibody, which can detect infections as early as 18-24 h postexposure (prepatent period) (Sano et al., 1984). Baculovirus midgut-gland necrosis virions are rod-shaped and 72 nm x 310 nm and they closely resemble non-occluded MBV. No clinical infections have been detected in wild shrimp (Brock and Lightner, 1990).

A fifth baculovirus-like virus, found in giant tiger shrimp (P. monodon) suffering from 'yellow-head disease' (YHD) (Chantanachookin et al., 1993), has caused acute epizootics in PL-20 and subadult shrimps in Thailand since 1990 (Boonyaratpalin et al., 1993). Early clinical signs include erratic swimming, floating near the surface and yellow discoloration of the cephalothorax and gills. Cumulative mortalities can reach 100% within 3-5 days following the appearance of clinical signs.

Spherical or irregular-shaped, Feulgen-positive inclusion bodies occur in the haemocytes, haemopoietic tissues, lymphoid organ, gills, cuticle and fixed phagocytes of the heart (Fig. 20.7a). Epidermal necrosis may be evident in the gills and cuticle of the body. Virogenesis occurs in the cytoplasm and the virions (40-50 nm x 150-200 nm) have a single envelope (Fig. 20.7b-d). Occasional intranuclear replication has also been noted, with virions budding through the nuclear membrane (Boonyaratpalin et al., 1993). Although originally described as a non-occluded baculovirus (Boonyaratpalin et al., 1993), granulosis-type morphology has recently been observed (Chantanachookin et al., 1993). Granulosis viruses (GV), or type B baculoviruses, form ellipsoid or round-ended, rod-shaped, occlusion bodies ('granules' or 'capsules'), which contain one (rarely two) virions (Singleton and Sainsbury, 1991).

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