WSSV and YHV remain to be among the most lethal viral pathogens of shrimps, especially in P. monodon and P. vannamei. Prevention of these diseases is best achieved by using SPF broodstock (esp. for WSSV) which is also the main reason for record shrimp production.
For YHV, the mode of transmission is still unknown but covering the ponds with mosquito nets somehow prevents YSV infection, suggesting an insect vector. The covers also prevent infection via infected shrimps dropped by birds. TSV and IHHNV in P. vannamei is no longer a problem, showing no effect on cultivated shrimps since 1998 (for TSV), and the high percentages of shrimps having viral inserts in their genomes (for IHHNV).
The most recent threat is IMN which is currently affecting P. vannamei in Indonesia. The possibility of spreading the disease to neighboring P. vannamei producing countries is high, considering the transport (both legal and illegal) of P. vannamei broodstock and postlarvae from one country to another. Other diseases that should be monitored in P. vannamei are:
- Penaeus vannamei nodavirus (PvNV) which is not yet reported in the region; the disease was first seen in Belize with disease signs indistinguishable from IMNV;
- MrNV which was shown to infect P. monodon, P. japonicus and P. indicus experimentally; some P. vannamei samples from China, Vietnam and Indonesia showed strongly positive for MrNV by RT-PCR and immunohistochemistry.
- ASDD which is associated with a retrovirus-like agent (ASDV), but the clinical sign (deformed body) seem to be stress-induced; not lethal but deformities significantly affect the commercial value of the harvested shrimps.
For P. monodon, aside from WSSV and YHV, MSGS is another major problem which is associated with LSNV (necessary but insufficient cause of the disease). It is also found that small shrimps from MSGS affected ponds show retinopathy while large shrimps did not, indicating that another factor might be involved in the disease.
Recent findings on viral inserts showed that not all shrimps found positive for viral diseases using routine molecular techniques (e.g. PCR) or diagnostic kits contain the complete genome (infective unit) of the causative virus. False positive diagnosis might result in discarding uninfected shrimps or shrimps with protective inserts. This finding is especially important for shrimp breeding programs with aims to select for disease tolerance.
Discussion
TSV can be considered not a problem anymore, as there was no report of further transmission to other species.
IMN should be considered a high priority because of the threat it currently pose in spreading the disease from Indonesia to other neighboring Penaeus vannamei -producing countries. Necessary preparedness is needed in these countries in any event of IMN outbreaks.
Viral inserts in shrimp genome (which usually produce positive results using routine PCR diagnosis) is a novel and most recent findings and maybe the reason why some shrimps found positive for viruses are not showing any signs of the disease.
Shrimps with viral inserts cannot transmit the disease from one shrimp to another (since it is incomplete). The inserts, however, can be passed from one generation to another.
Recommendations
Recognising the high risk and threats posed by IMNV to other P. vannamei-producing countries in the region, AG strongly recommended that a meeting on IMNV Awareness and Preparedness be organized focusing on understanding the current situation of IMN in Indonesia (spread and economic impact), assisting them with containment plans, and to prepare a draft emergency response and contingency plan for selected P. vannamei producing countries in dealing with IMN. NACA and AAHRI offered technical assistance in this regard, while FAO expressed its interest in organizing/funding the meeting.
AG also suggested NACA to prepare a one-page Advisory Report on IMNV documenting the disease characteristics, pathogen, spread of the disease within Indonesia and its economic impact in P. vannamei industry for wider dissemination in the region.
With the novel findings on viral inserts, AG recommended that those involved in surveillance and disease diagnosis should be informed to be more cautious in interpreting the presence or absence of viral infections (e.g. IHHNV) in shrimps using the routine molecular diagnostic techniques (e.g. PCR, RT-PCR).
The AG recommended that a brief write up on viral inserts and their implications for diagnosis, surveillance, and seed and broodstock screening programs be developed by NACA with technical assistance from the Crustacean disease expert and widely disseminated in the region.
