Pathogenesis of a Thai strain of white spot syndrome virus (WSSV) in juvenile, specific pathogen-free Litopenaeus vannamei

Abstract

White spot syndrome virus (WSSV) causes disease and mortality in cultured and wild shrimp. A standardized WSSV oral inoculation procedure was used in specific pathogen-free (SPF) Litopenaeus vannamei (also called Penaeus vannamei) to determine the primary sites of replication (portal of entry), to analyze the viral spread and to propose the cause of death. Shrimp were inoculated orally with a low (10 1.5 shrimp infectious dose 50% endpoint [SID 50 ]) or a high (10 4 SID 50 ) dose. Per dose, 6 shrimp were collected at 0, 6, 12, 18, 24, 36, 48 and 60 h post inoculation (hpi). WSSVinfected cells were located in tissues by immunohistochemistry and in hemolymph by indirect immunofluorescence. Cell-free hemolymph was examined for WSSV DNA using 1-step PCR. Tissues and cell-free hemolymph were first positive at 18 hpi (low dose) or at 12 hpi (high dose). With the 2 doses, primary replication was found in cells of the foregut and gills. The antennal gland was an additional primary replication site at the high dose. WSSV-infected cells were found in the hemolymph starting from 36 hpi. At 60 hpi, the percentage of WSSV-infected cells was 36 for the epithelial cells of the foregut and 27 for the epithelial cells of the integument; the number of WSSV-infected cells per mm 2 was 98 for the gills, 26 for the antennal gland, 78 for the hematopoietic tissue and 49 for the lymphoid organ. Areas of necrosis were observed in infected tissues starting from 48 hpi (low dose) or 36 hpi (high dose). Since the foregut, gills, antennal gland and integument are essential for the maintenance of shrimp homeostasis, it is likely that WSSV infection leads to death due to their dysfunction

Pathological changes in Fenneropenaeus indicus experimentally infected with white spot virus and virus morphogenesis

“…Therefore, it could be reasonably inferred that collapse of the supportive connective tissue was responsible for the profound pathological changes of hepatopancreatic tubules; nevertheless, they themselves were not susceptible to the virus. This result agrees with the studies done in P. monodon Wang et al, 1999) and L. vannamei (Escobedo-Bonilla et al, 2007) which revealed virus replication in the myoepithelial cells of the hepatopancreatic sheath and cells of the connective tissues but not in the hepatopancreatic tubular epithelial cells.…”

“…Virus replication at different stages of morphogenesis was observed in this tissue even when animals were moribund. Previous studies showed that connective tissue of the nervous system were alone infected by WSV (Karunasagar et al, 1997;Sudha et al, 1998;Wang et al, 1999;Escobedo-Bonilla et al, 2007). It is therefore concluded that both neurosecretory and connective tissue of the nervous system are equally susceptible to WSV infection.…”

“…The foregut cuticular epithelium was one of the target tissues in which the viral infection could be demonstrated as hypertrophied nuclei Mohan et al, 1998). In L. vannamei, foregut epithelium is found to be the primary site of replication post infection (Escobedo-Bonilla et al, 2007). The underlying muscular layer of the foregut showed multifocal necrosis.…”

“…2f). In L. vennamei higher proportion of hypertrophied cells showed hydropic degeneration, and some areas of focal necrosis after 48 h post infection, and gill epithelium was found to be the primary site for viral replication (Escobedo-Bonilla et al, 2007). Normally when the animals get infected they come to the surface of water and move towards the periphery of the pond apparently to get more oxygen, a behavioral trait considered as an outward manifestation of the impairment of breathing capacity.…”

“…At least 18 cultured and/or wild penaeid shrimp (Park et al, 1998), eight caridean species (Pramod-Kiran et al, 2002), seven species of lobster (Rajendran et al, 1999), seven species of crayfish (Edgerton, 2004), 38 crab species (Yoganandhan et al, 2003) six non-decapod crustacean species (Hossain et al, 2001), members of the phyla Chaetognata and Rotifera (Yan et al, 2004), polychaete worms (Supak et al, 2005) and some aquatic insect larva (Ramírez-Douriet et al, 2005) have been found susceptible to the virus. Histopathological observations during WSV infection in brief is available regarding shrimp species such as Penaes monodon (Durand et al, 1997;Wang et al, 1999Wang et al, , 2000Mishra and Shekhar, 2005) Litopenaeus vannamei and Marsupenaeus japonicus (Lu et al, 1997;Lightner et al, 1998;Escobedo-Bonilla et al, 2007;Perez et al, 2005;Pantoja and Lightner, 2003), species of Crabs (Kanchanaphum et al, 1998;Kou et al, 1998) and lobster (Rajendran et al, 1999;Wang et al, 1998;Jiravanichpaisal et al, 2001).…”