Experimental infection of some penaeid shrimps and crabs by yellow head virus (YHV)

Two species of penaeid shrimps, Metapenaeus affinis and Metapenaeus brevicornis, as well as 16 species of crabs were collected around Penaeus monodon farming areas. Some of the shrimps and crabs in each species were artificially infected with yellow head virus (YHV) by injection with diluted haemolymph of YHV-infected P. monodon, and some of the shrimps were fed with muscle from P. monodon infected with YHV. The YHV infection was monitored by immunohistochemistry using monoclonal antibodies specific to gp116, gp64 (envelope), and 20 kD (capsid) proteins of YHV and by RT-PCR. YHV infection was neither observed in all shrimp and crab species obtained from natural habitats nor in all crab species injected with YHV. In both species of shrimps, all shrimp individuals were infected with YHV at day 3 after injection, and most of M. affinis died within 10 days following injection. In contrast, some of the YHV-infected M. brevicornis survived until day 30 following injection and the degree of infection dramatically diminished. In the feeding infection experiment, some of M. affinis were infected with YHV at day 3 while none of the M. brevicornis was infected. Immunohistochemical studies of infected tissues revealed strong immunoreactivity with monoclonal antibodies against gp64 and 20 kD proteins, but displayed light immunoreactivity or absence with the monoclonal antibody against gp116 protein. This evidence suggested that the resistance against YHV infection might involve an ability of the host to suppress gp116 protein production. In summary, both penaeid shrimp species are potential carriers of YHV as they survived for a long period after YHV infection. All crabs are not carriers of YHV since the YHV can not proliferate in crab tissues. Natural infection with other viruses such as white spot syndrome virus (WSSV) was found in three crab species, and occlusion bodies potentially caused by an unknown virus were observed in the hepatopancreatic tissues of two grapsid crab species.