Infectious muscle necrosis etiology in the Pacific White Shrimp (Penaeus vannamei) cultured in Ecuador

In recent years, some Ecuadorian shrimp farmers have reported several cases of muscle necrosis in P. vannamei grow out cultures, associated to low mortalities at harvest. This abnormal condition is characterized by focal to extensive necrotic areas in striated tail muscle tissues, displaying a white, opaque appearance. Furthermore, histological analysis from suspected samples with macroscopic lesions revealed a loss of sarcomeric structure accompanied by coagulative muscle necrosis along with hemocytic infiltration. Two viruses, not reported in Ecuador, are described as etiological agents of muscle necrosis in P. vannamei: infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV). In this study, the etiology of the muscle necrosis cases found in the Pacific white shrimp P. vannamei cultured in Ecuador was examined. This muscle necrosis was hypothesized to be caused by an infectious agent. Three sequential challenge tests, using diseased P. vannamei with macroscopic lesions (opaque, whitish discolorations in the abdominal muscles) as starting material (crude inoculum), were carried out. Essentially, histological examination confirmed that most of the challenged shrimp had lesions in skeletal muscle, including multifocal necrosis, fibrocytic inflammation and phagocytosis. In the first trial, 7 challenged shrimp (out of 10) presented multifocal necrosis and hemocytic infiltration in the skeletal muscle. In the second trial, 14 challenged shrimp (out of 20) exhibited coagulative muscle necrosis and hemocytic infiltration. In the third trial, 48 challenged shrimp (out of 69) showed muscle necrosis and hemocytic infiltration. There were no mortalities throughout the all four-week trials. All challenged shrimp tested by RT-PCR assay were negative for IMNV. In our experimental procedures, muscle necrosis could be consistently reproduced through three sequential trials, confirming that the disease has an infectious etiology. Our results suggest that the etiological agent of this disease could be a new infectious agent or a different strain of IMNV.(AU)

Fine Structure Analysis of White Spot Syndrome Virus of Shrimp

White Spot Syndrome Virus (WSSV) was isolated from diseased shrimps presenting with clinical signs of WSSV infection. The seed virus was identified as WSSV by PCR, and used to inoculate to specific pathogen free (SPF) P. vannamei bloodstocks. WSSV was purified as described by Huang from infected gills from inoculated animals that were homogenized in a blender and partially purified by differential centrifugation. The final purification was carried out using density gradient in 10-40% NaBr and purified virus used for morphological analysis using transmission electron microscopy. A negative staining method using 2% PTA was used for purified virus, and electron staining with lead citrate and uranylacetate was used for ultra thin sections of infected tissues. This analysis determined that i) the spikes of intact virions could be clearly identified on the virion surface and on a partially broken envelopes, ii) the nucleocapsid structures were similar to those previously reported, even though the so called "ring" structure described previously was different and, iii) negative staining of purified WSSV fractions identified nucleosome like structures.(AU)

Fine structure analysis of White Spot Syndrome Virus of shrimp

White Spot Syndrome Virus (WSSV) was isolated from diseased shrimps presenting with clinical signs of WSSV infection. The seed virus was identified as WSSV by PCR, and used to inoculate to specific pathogen free (SPF) P. vannamei bloodstocks. WSSV was purified as described by Huang from infected gills from inoculated animals that were homogenized in a blender and partially purified by differential centrifugation. The final purification was carried out using density gradient in 10-40% NaBr and purified virus used for morphological analysis using transmission electron microscopy. A negative staining method using 2% PTA was used for purified virus, and electron staining with lead citrate and uranylacetate was used for ultra thin sections of infected tissues. This analysis determined that i) the spikes of intact virions could be clearly identified on the virion surface and on a partially broken envelopes, ii) the nucleocapsid structures were similar to those previously reported, even though the so called "ring" structure described previously was different and, iii) negative staining of purified WSSV fractions identified nucleosome like structures. 

Infectious muscle necrosis etiology in the Pacific white shrimp (Penaeus vannamei) cultured in Ecuador

In recent years, some Ecuadorian shrimp farmers have reported several cases of muscle necrosis in P. vannamei grow out cultures, associated to low mortalities at harvest. This abnormal condition is characterized by focal to extensive necrotic areas in striated tail muscle tissues, displaying a white, opaque appearance. Furthermore, histological analysis from suspected samples with macroscopic lesions revealed a loss of sarcomeric structure accompanied by coagulative muscle necrosis along with hemocytic infiltration. Two viruses, not reported in Ecuador, are described as etiological agents of muscle necrosis in P. vannamei: infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV). In this study, the etiology of the muscle necrosis cases found in the Pacific white shrimp P. vannamei cultured in Ecuador was examined. This muscle necrosis was hypothesized to be caused by an infectious agent. Three sequential challenge tests, using diseased P. vannamei with macroscopic lesions (opaque, whitish discolorations in the abdominal muscles) as starting material (crude inoculum), were carried out. Essentially, histological examination confirmed that most of the challenged shrimp had lesions in skeletal muscle, including multifocal necrosis, fibrocytic inflammation and phagocytosis. In the first trial, 7 challenged shrimp (out of 10) presented multifocal necrosis and hemocyt