Viral diseases due to Monodon

Viral diseases due to Monodon Baculovirus (MBV) and Hepatopancreas Parvovirus (HPV) in hatchery reared larvae have been reported in India [6,36,24]. Although MBV is relatively well tolerated by P. monodon[5] it has been implicated in mass mortalities in shrimp cultured at high densities [10]. Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) is one of the major viral pathogens of penaeid shrimps worldwide [16,7] and has been found to be widely distributed in wild and cultured P. monodon in east and SE Asia [37]. Even though IHHNV infection does not cause mortality in P. vannamei and P. monodon; it results in a disease called Runt Deformity Syndrome in both species [3,35] and hence causes substantial economic losses [43]. In India, there has been a considerable increase in the culture of P. monodon due to its taste, market demand both national and international markets and because of the recent introduction of Litopenaeus vannamei, it has become an issue of concern and more detailed studies are essential to understand the diversity of viral pathogens in hatcheries. Viral pathogens appear to exert the most significant constraints on the growth and survival of calcium channel blockers under culture conditions. Expansion of the aquaculture industry, and the increasingly globalized trade in brood stock, larvae and commodity products arising from shrimp farming led to trans-boundary movements the viral pathogens and emergence of several significant disease conditions in the country. In light of this problem, new approaches are urgently required to enhance yield by improving brood stock and larval sourcing, and promoting best management practices to control disease problems. Methods for prevention of viral infection in shrimp ponds are possibly through avoidance by screening the PL׳s before stocking shrimp in the pond. The diagnosis of viral pathogens and the rapid enforcement of biosecurity measures to avert the subsequent spread of pathogens is central to the prevention of epidemics. The objective of the present study was to determine the viral pathogens in post-larvae of P. monodon from hatcheries in India by Polymerase Chain Reaction to understand the viral diversity in P. monodon hatcheries to enforce biosecurity measures.
Materials and methods
Results Of 104 post-larvae samples tested, 4.8% samples were uninfected. All the four viruses were not detected in any samples. Three viruses HPV/MBV/IHHNV and HPV/IHHNV/WSSV were detected in 4.8% samples. Moreover, 13 (12.5%) of samples were infected only with WSSV. 79 (76%) were positive for IHHNV, 65 (62.5%) were positive for HPV and 13 (12.5%) were positive for MBV (Table 2). Dual to triple infection was present in 60.6% of the total post-larvae tested. Out of the 51 double positives 50 (98%) included either HPV or IHHNV infection. HPV or IHHNV was present in 11 (100%) post-larval samples found positive for triple viral infection. A total of 99/104 (95.2%) post-larval samples were positive for HPV and IHHNV alone or in combination with other viruses. HPV or IHHNV was present alone in 36 (34.6%) of the total post-larval samples tested. Out of the 99 samples infected with virus, 79 (79.8%) of the samples had IHHNV alone or in combination with other viruses while HPV was present in 65 (65.6%) of the samples tested. With respect to multiple and single viral infections detected in the 95 hatchery samples by PCR (Table 2), 26 (27%) were positive for all 3 viruses (HPV, MBV and WSSV), 1 (1%) was positive for HPV and MBV, 3 (3%) were positive for HPV and WSSV, 21 (22%) were positive for MBV and WSSV, and 2 (2%) were positive for HPV only. MBV alone was detected in 8 samples (8%) and WSSV alone in 26 (27%). No viruses were detected in only 8 (8%) of the 95 samples. PCR analysis for multiple and single viral infections in 107 samples submitted by farmers (Table 3) revealed that 31 (29%) were positive for all 3 viruses (HPV, MBV and WSSV), 1 (1%) was positive for HPV and MBV, 1 (1%) was positive for HPV and WSSV, and 20 (19%) were positive for WSSV and MBV. MBV alone was detected in 10 samples (9%), and WSSV alone in 27 samples (25%), while no viruses were detected in 17 samples (16%). These results indicate not only a high prevalence of all the 3 viruses in P. monodon PL in India, but also a high prevalence of concurrent viral infections that usually include HPV. A significant number of samples (22% of hatchery samples and 19% of farmer samples) also showed dual infections of MBV and WSSV, as reported earlier by Otta et al. [29]. The presence of concurrent viral infections causing mortality in P. monodon PL has also been recorded by Manivannan et al. [24]. Thus, it is concluded that screening of PL for all these viruses is essential before stocking of culture ponds. The presence of multiple viruses may impact the health of P. monodon PL. The observation that infection with HPV alone occurs in only a small percentage (2%) of P. monodon PL may be significant. Similarly, infection with MBV alone was observed in only 8% of hatchery samples and 9% of samples submitted by farmers, while infection with WSSV alone was found in 27% of hatchery samples and 25% of samples submitted by farmers (Tables 2 and 3). The results suggest that HPV and MBV are mostly found as components of multiple viral infections, while WSSV is more frequently found alone.