Nghiên cứu đồng nhiễm virus đốm trắng và Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), caused by specific strains of the bacterium Vibrio parahaemolyticus (VpAHPND), has been reported in shrimp cultured in Southeast Asia and the Americas since 2009. The clinical signs of the disease include atrophied hepatopancreas (HP), an empty gastrointestinal tract, a milky appearance of the stomach and lethargy. This bacterial disease has caused up to 100 percent mortality in populations of farmed Pacific white shrimp (Penaeus vannamei) and black tiger shrimp (P. monodon), and collective economic losses due to AHPND and other shrimp diseases are estimated at $23.6 billion globally with a further loss of $7 billion in feed sales.

In some cases, VpAHPND has been reported in co-infection with the spore-forming microsporidian Enterocytozoon hepatopenaei (EHP). Although EHP does not appear to cause mortality, EHP-infected animals would be more vulnerable to AHPND, which may cause substantial mortality in shrimp ponds. And EHP-infected shrimp have an increased susceptibility to VpAHPND, suggesting that shrimp weakened by EHP would succumb to secondary pathogens, such as VpAHPND.

For several years, white spot syndrome virus (WSSV) has spread worldwide throughout much of Asia, the Americas, Europe, Africa, Middle East and Australia. In some cases, WSSV has been detected in healthy animals without clinical symptoms or mortalities, indicating that infection by WSSV does not necessarily result in high mortality, and secondary infection by opportunistic pathogens, such as Vibrio spp., would expedite the onset of the disease.

In 2017, P. vannamei mortality rates of up to 60 percent were reported in the Philippines. The shrimp exhibited clinical signs of AHPND but were confirmed by PCR to be positive for both VpAHPND and WSSV. From this case, we hypothesized that WSSV infection without clinical symptoms would allow VpAHPND to cause faster and higher mortality of pond cultured shrimp than those infected with VpAHPND alone.

This article – adapted and summarized from the original – reports on a study designed to support this hypothesis by simulating the co-infection with WSSV and VpAHPND in juvenile shrimp P. vannamei under laboratory conditions. As a result, we demonstrated that shrimp initially exposed to WSSV were easily infected by VpAHPND, but hardly recovered from the disease according to histopathology examination. In addition, using qPCR assay and immunohistochemistry [IHC; the most common application of immunostaining, involving the process of selectively identifying antigens (proteins) in cells of a tissue section using the binding of antibodies specifically to antigens in biological tissues] examination, we determined that WSSV infection could be accelerated by the secondary VpAHPND infection. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (grant number NRF-2018R1C1B5086350).

Infections with multiple pathogens (co-infection) are common in practical shrimp cultures. Several cases of shrimp vibriosis with WSSV infection have been reported with various penaeid species in different countries. There has been no prior detailed analysis on the roles of these pathogens in shrimp co-infected with WSSV and Vibrio. In shrimp farms, the WSSV infection without symptoms or mortality probably causes shrimp weakening and might trigger the VpAHPND infection.

To the best of our knowledge, our study is the first to examine the co-infection of WSSV and VpAHPND under laboratory conditions. In ponds, shrimp that were infected with WSSV (or other pathogens) did not always show clinical signs or mortalities. In those cases, even WSSV-positive shrimp without any clinical symptoms may cause higher and faster mortality than their disease-free counterparts, and the shrimp would hardly recover from the disease following a secondary infection, such as VpAHPND.

Also, the secondary infection would provoke WSSV infection or virus replication, as confirmed in the present study. Therefore, shrimp farmers should pay attention to the management of multiple infections (WSSV or other pathogens) in their ponds to prevent further losses in shrimp production.