Parvoviruslike viruses

A parvovirus-like infection of hepatopancreas epithelia ('hepatopancreatic parvo-like virus' (HPV)) was found in four penaeid species (P. orientalis, P. merguiensis, P. semisulcatus and P. monodon) from culture facilities in Asia (Lightner and Redman, 1985). Similar viruses have since been reported in P esculentis, P. indicus, P. penicillatus and P. vannamei from Asia, Australia and Africa. The spread of HPV was traced from Kenya to Israel and from Asia to Brazil in P. penicillatus, as well as to Hawaii with P. chinensis (= orientalis) (where the infected stock were destroyed). Also, HPV-like lesions have been reported from P. vannamei in Ecuador (Brock and Lightner, 1990; Lightner et al., 1992b).

Clinical signs of HPV infection are poor growth, surface fouling, atrophy of the hepatopancreas and opacity of the abdominal musculature (Lightner and Redman, 1985). As with reovirus-like infections, these signs are non-specific and frequently complicated by multiple infections (Lightner et al, 1992b). Mortalities are higher in juveniles than in younger stages, with 100% cumulative mortality in P. merguiensis within 4-6 weeks of the appearance of clinical signs (Lightner and Redman, 1985). In contrast, a heavy infection was detected in an apparently healthy P. esculentis (Paynter et al., 1985, cited in Lightner, 1992b).

Infected hepatopancreatic-tubule epithelia show displaced chromatin and hypertrophy due to Feulgen-positive intranuclear inclusions (Fig. 20.8a). In larval P. orientalis, these inclusions occur predominantly in the distal tubules of the hepatopancreas, as well as the anterior midgut. Infected tissues are atrophied but show minimal necrosis or haemocyte infiltration. The intranuclear inclusions consist of granular virogenic stromata and isometric HPV virions, 22-24 nm in diameter (Lightner and Redman, 1985) (Fig. 20.8b). Particle size, Feulgen-positive histochemistry and intranuclear replication from virogenic stromata containing microfibrils are all features of previously described Parvoviridae (Francki et al., 1991).

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