FEATURE
system, encapsulation, nodule formation, phagocytosis) defense responses in shrimp. Experimental animals fed with immunostimulants before or during challenge with bacteria or viruses showed reduced mortality compared to untreated controls.
Antivirals
Low water temperature is also effective to inhibit virus replication in species living in temperate or cold water. In shrimp M. japonicus, water temperature at 15 ºC showed better WSSV inhibition than 33 ºC
(EMS) or acute hepatopancreatic necrosis syndrome (AHPNS), is caused by a singular type of Vibrio parahaemolyticus. This bacterium contains an extrachromosomic plasmid (pVPA31) encoding two toxins (PirA/PirB), related to Photorhabdus luminiscens that are responsible for shrimp mortality. EMS can produce high mortalities to affected shrimp early after pond stocking (10 - 40 days). Surviving shrimp might undergo stunting. EMS was first reported in 2009 in China and since then, it has caused massive mortalities to farmed shrimp in Asian countries such as Vietnam (2011), Thailand (2012) and Malaysia (2012). In 2013, EMS was also recorded in Mexico where it caused severe production losses (up to 80 percent of total production in Sinaloa, Sonora and Nayarit). Several strategies have been developed and tested under experimental conditions to tackle the negative impact of infectious diseases caused by viruses or bacteria. These include:
Substances from plants, algae or even synthetic, have been tested in shrimp with variable results. A diet containing Spirulina platensis showed no antiviral effect but only slightly delayed mortality in WSSV-challenged shrimp. In contrast, an extract from Indian Cynodon dactylon supplemented to feed (2 percent), showed 100 percent protection upon a per os WSSV infection. An antiviral (bis [2-methylheptyl] phthalate) extracted from the Indian plant Pongamia pinnata fed before and during a per os WSSV challenge showed a dosedependent reduction of mortality (60 to 20 percent). A synthetic antiviral (cidofovir) showed higher efficacy than the Spirulinasupplemented diet to reduce and delay mortality of treated shrimp. Nonetheless, cidofovir did not prevent WSSV infection (figure 2).
Induction of a “quasi-immune” response and virus neutralisation
This strategy is based on the rationale that some shrimp surviving a virus outbreak may become resistant to a subsequent pathogen infection. Therefore it indicates a sort of “memory” in these animals. Several studies evaluated the protective effect of inactive viral particles or recombinant viral envelope proteins administered to shrimp to prime their innate defense system. Results showed reduced mortality of treated animals. In addition, monoclonal or polyclonal antibodies directed against viral envelope proteins Immunostimulants have been used to inactivate viral particles through virus Substances (peptidoglycans, β-glucans or lipopolysaccharides) neutralisation assays. extracted from cell walls of bacteria (Bacillus sp.), fungi Three concentrations (10-1, 10-2 and 10-3) of a WSSV stock each (Saccharomyces cerevisiae, Schizophyllum commune), algae (Sargassum polycystum) or herbs, which activate humoral mixed with an equal volume of a purified monoclonal antibody (antibacterial activity, agglutinins, cytokine-like factors, against WSSV VP28 showed a dose-dependent neutralisation modulators, clotting factors) and cellular (prophenoloxidase effect. Shrimp inoculated only with WSSV showed 100 percent mortality at seven days post inoculation (dpi). Shrimp treated with neutralised virus concentrations 10-1 and 10-2 showed a slight delay in time to mortality (100 percent at 11 dpi). Animals treated with the 10-3 neutralised concentration showed 20 percent mortality at 25 dpi. Recombinant subunit peptides displayed 20 – 40 percent shrimp mortality depending on time of WSSV challenge [3 to 21 days post treatment (dpt), respectively]. Other experiments reported mortalities between 48 percent with VP292 to 30 – 5 percent with VP28 as these recombinant peptides were Figure 4 - Mortality curve from shrimp treated with dsRNA against WSSV administered twice during the experiments. genes vp26 or vp28 using a high WSSV dose. RNAi against WSSV vp28 or vp26 effectively reduced WSSV infection and shrimp mortality compared to an unrelated dsRNA (LacZ) and controls. From Mejía-Ruíz et al., (2011) J Inv. Pathol. 107: 65-68.
DNA vaccines
These tools are plasmids encoding WSSV envelope proteins (VP15, VP28, VP35 and VP281). Shrimp
28 | January | February 2016 - International Aquafeed