Impact of yellow head virus outbreaks in the whiteleg shrimp,Penaeus vannamei (Boone), in Thailand

Yellow head virus (YHV) is known as a major pathogen in the black tiger shrimp, Penaeus (Penaeus) monodon. It can also cause serious mortality in farmed whiteleg shrimp, Penaeus (Litopenaeus) vannamei. However, there is no published information on the economic and/or production impact of the disease in P. vannamei. Shrimp with gross signs of YHV disease (faded body colour and 60–70% mortality) were observed in 20 study farms rearing P. vannamei in the central part of Thailand from the end of 2007 through early 2008. The estimated economic loss for these farms according to the Thai Animal Aquaculture Association was approximately US$3 million. Detailed sequence analysis of RT‐PCR amplicons from shrimp in all the study ponds revealed the presence of YHV Type 1b (YHV‐1b) alone (characterized by a 162‐bp deletion in the ORF3 region encoding the structural gene for gp116) and the absence of YHV Type 1a (YHV‐1a), the original YHV type reported from Thailand. Despite the large 162‐bp deletion (= 54 deduced amino acids) in the gp116 structural gene, histopathology of YHV‐1b infections was identical to that of YHV‐1a infections, and electron microscopy revealed that YHV‐1b virions were morphologically indistinguishable from those previously reported for YHV‐1a. In addition, an existing commercial RT‐PCR detection kit and an immunochromatographic test strip for the detection of YHV were proven to have been valid tests for both YHV‐1b and YHV‐1a. The source of the virus for these outbreaks was unlikely to have been the post‐larvae used to stock the ponds, as they were derived from domesticated specific pathogen‐free stocks free of YHV. Thus, it is possible that they originated from an unknown, natural reservoir.     

Nucleotide sequence variations of the major structural proteins (VP15, VP19, VP26 and VP28) of white spot syndrome virus (WSSV), a pathogen of cultured Litopenaeus vannamei in Mexico

White spot syndrome virus (WSSV) was first reported in farmed Litopenaeus vannamei stocks in Sinaloa and Sonora, Mexico during 1999 and continues to cause severe shrimp losses. WSSV genes encoding nucleocapsid (VP26 and VP15) and envelope proteins (VP19 and VP28) of a Mexican isolate were cloned in the pMosBlue vector. The nucleotide sequences of these genes were compared with WSSV isolates in GenBank. VP15 is highly conserved, and VP26 showed 99% homology to a Chinese isolate. The VP28 fragment demonstrated 100% homology to the majority of the isolates analysed (UniProt accession no. Q91CB7), differing from two Indian WSSV and one Chinese WSSV isolates by two non-conserved and one conserved replacements, respectively. Because of their highly conserved nature, these three structural proteins are good candidates for the development of antibody-based WSSV diagnostic tools or for the production of recombinant protein vaccines to stimulate the quasi-immune response of shrimp. In contrast, VP19 of the Mexican isolate was distinguishable from almost all isolates tested, including an American strain of WSSV (US98/South Carolina, GenBank accession no. AAP14086). Although homology was found with isolates from Taiwan (GenBank accession no. AAL89341) and India (GenBank accession no. AAW67477), VP19 may have application as a genetic marker.