Observation of Two Viruses in Cultured Giant Freshwater Prawns (Macrobrachium rosenbergii de Man) in Thailand

Abstract

Two viruses in the hepatopancreas (HP) of Macrobrachium rosenbergii postlavae (PL) and adult prawns are described. Investigations were carried out by light microscopic examination of HP tissue squashes stained with 0.05 % malachite green and histological sections stained with hematoxylin and eosin (H&E), as well as by transmission electron microscopy (TEM). Histologically, the first pathogen was a baculo-like virus (BLV), recognized by the presence of enlarged nuclei with marginated nucleoli and eosinophilic inclusions that completely filled the nucleus of the HP tubular epithelium. In contrast, the malachite green-stained HP squashes did not show any marginated nucleoli. TEM of the infected HPs revealed the presence of BLV particles of approximately 250-300 nm in enlarged nuclei. The second virus was a hepatopancreatic parvo-like virus (HPLV). Spherical intranuclear inclusions were easily observed in the HP tubular epithelium in both fresh malachite green-stained tissue squashes and H&E- stained histological sections. TEM of infected HPs revealed intranuclear inclusions containing small virus-like particles of approximately 20-24 nm in diameter. PL (n=200) with percentage infection rates of 20% with the BLV, 50% with the HPLV and 10% with dual infection were stocked into earthen ponds at a density of 60 000 PL/1600 m². After rearing for 60 d, the prawns were transferred into earthen ponds at a density of 10 000 prawns/1600 m2. Sixty days later, (120-day-old) prawns were transferred into earthen ponds at a density of 5000 prawns/ 1600 m² and reared until harvesting at day 180. The prawns were sampled and divided into small and large-sized prawns at 60, 120, 150 and 180 d for histological studies to determine the viral infections. Results showed that at 120 d, the BLV infection in the small-sized prawns (12.5%) was higher than that of the large-sized prawns (5%). Dual infection with both viruses was 5.0% in small-sized prawns and 2.5% in large-sized prawns. However, at day 150, the BLV pathogen was not observed in either size group. In contrast, the HPLV infection was found in prawns throughout the culture period. The percentage infection rates in small-sized prawns at days 120, 150 and 180 were 22.5, 15.0 and 12.5%, respectively, similar to the infection rates for large-sized prawns of 20.0, 12.5 and 12.5%, respectively. The percentage survival rate and yield from both ponds at harvest were 67-68% and 197-199 kg/1600 m², respectively. These results indicated that the infection rates of these two viruses did not increase during the culture period and that the virus infections might have a negative effect on production.