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Larvae (15 to 21 d post hatch, dph) of the Asian sea bass Lates calcarifer (Bloch) suffered heavy mortalities (60 to 90%) during the hatchery-rearing phase. Darkened and moribund larvae showed no evidence of bacterial or parasitic infections. Tissue sections of brain and spinal cord showed clear necrotic vacuolation. Electron microscopy revealed membrane-bound viral particles in the cytoplasm of the nerve cells. The viral particles measured 28 to 30 nm in diameter. Primer sets, designed for the amplification of the RNA2 segment of the piscine nodavirus coat protein gene, were used in the RT-PCR analysis of moribund larvae of 20 and 21 dph which produced the amplified product of 430 bp. The clinical manifestations, pathology and electron microscopy observations supported by the RT-PCR analysis suggest that the nerve necrosis was due to nodavirus infection in the larvae. This is the first report of piscine nodavirus infection from the Indian sub-continent. KEY WORDS: Nodavirus · Lates calcarifer · Larvae · RT-PCR · Histopathology · Nervous necrosis virus Resale or republication not permitted without written consent of the publisherDis Aquat Org 63: [113][114][115][116][117][118] 2005 geographical locations have been reported to be susceptible to the virus (Glazebrook et al. 1990, Yoshikoshi & Inoue 1990, Mori et al. 1992, Muroga 1995, Comps et al. 1996, Munday & Nakai 1997, Castric et al. 2001. The disease has also been reported in freshwater aquarium fish from Singapore (Hegde et al. 2003).Histology and RT-PCR are the common methods used for diagnosis of the virus in fish (Mori et al. 1991, Munday et al. 1992, Nishizawa et al. 1994, Grotmol et al. 1995, Lai et al. 2001, Johansen et al. 2002, Hegde et al. 2003. Diagnosis of piscine nodavirus using RT-PCR for the T4 region (427 bp) of SJNNV coat protein gene (RNA2) has been widely used and many workers have reported positive amplification for the presence of the nodavirus in fish species from different geographical locations (Nishizawa et al. 1994, Castric et al. 2001, Curtis et al. 2001, Lai et al. 2001. Primer sequences of Nishizawa et al. (1994) have been recommended for the diagnosis of the piscine nodaviruses (OIE 1997).A batch of Asian sea bass larvae produced in the fish hatchery facilities of the Central Institute of Brackishwater Aquaculture were observed to experience sudden and unexplained mortality after 15 d of larval rearing. Subsequent investigations revealed that the cause of the disease was piscine nodavirus. The present investigation represents the first report of VNN from the Indian sub-continent and extends the known range of this disease. MATERIALS AND METHODSFish. Broodstock procured from the coastal waters of Chennai (Madras), acclimatized and maintained in cement tanks are being used for the purposes of seed production. Since 1997 more than 15 batches of Asian sea bass larvae have been produced in the institute facility. Though the hatchlings were sampled from 0 d post hatch (dph) for routine histology, the larvae from 15 to 2...

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A probable vertical mode of piscine nodavirus transmission is reported in the present investigation based on a case of nodavirus associated larval mortalities in hatchery produced Asian sea bass. Polyclonal rabbit anti-SJNNV antibodies (SGWak97) detected the viral antigens in the tissue sections from the eggs and the larvae at different time intervals from À 1 to 42 days post hatch (dph). Immunopositive ovarian connective tissue associated with the oocytes along with the progressive localization of the viral antigens in the brain, spinal cord, liver, stomach and dermal musculature during the larval development indicates a probable vertical transmission of nodavirus in the Asian sea bass. The surviving larger larvae, from the batch suffering mass mortalities, produced very intense immunofluorescent positivity in the liver, stomach and dermal musculature. Results of this investigation demonstrating a possibility of vertical transmission of the nodavirus emphasize the need for screening of eggs and larvae for evolving suitable preventive and prophylactic health management strategies. D

Probiotics are gaining worldwide acceptance as means to improve growth and health of fish. However, the type of probiotic, dose, duration and mode of the application, age and size of the fish are among the factors involved in determining the efficacy of the applied probiotic. In this study, two trials were conducted to evaluate the effect of feeding three probiotics on growth, feed conversion (FCR) and gross yield (GY) in juvenile (28.3 g) and adult (93.4 g) tilapia compared to a control probioticfree diet (CON). In the first trial, an autochthonous Bacillus subtilis isolated from the gut of cultured tilapia (TPI), a commercial Lactobacillus acidophilus probiotic (COM1) and a combination of both (MIX1) were tested. In the second trial, the same treatments were tested in adults (101.8 g) only with the replacement of COM1 with another commercial probiotic (COM2) consisting of four bacterial species. Results showed that in juveniles, the MIX1 treatment had the highest (P < 0.05) mean weight (MWT), daily growth rate (DGR), specific growth rate (SGR), GY and best FCR, followed by the TPI and the COM1 treatments. Growth in adults was not affected by probiotics and was depressed by COM1. In the second trial, the COM2 treatment had the highest (P < 0.05) MWT, DGR, SGR, GY and best FCR, followed by the MIX2 and the TPI treatments. These results indicate the positive effect of the probiotics on growth and the synergetic effect of mixing more than two bacteria. The TPI had the highest gut colonization and retention after 15 days of withdrawing the probiotics followed by the MIX1 and MIX2 treatments. The COM1 and COM2 showed no proliferation. Generally, probiotics improved the immunity with the TPI causing the highest improvement. Tilapia fed with the TPI, MIX1 and MIX2 bacteria, then challenged by injecting the LD 50 dose (10 4.86 cfu per fish) of the pathogenic Proteus vulgaris bacterium had a higher relative percentage of survival than the COM1, and COM2 treatments. Results obtained in this study indicate the higher potential of the autochthonous probiotic bacteria and the mixture of bacteria in enhancing growth, immunity and disease resistance in tilapia than the commercial products and a tendency of fish size discrepancy in the response to probiotics.