Is systemic Vibriosis a predisposing factor for white spot disease in shrimp, Penaeus monodon in grow-out ponds?

Abstract

Results of preliminary observations carried out and recorded by some grow out farm managers indicated that the cultured shrimp stocks predisposed to vibriosis are easily susceptible to white spot viral disease. Therefore the present study was carried out in May, 2005 when most of the shrimp grow out ponds at Arachchikattuwa area were affected with white spot disease to see whether there is any relationship between systemic vibriosis and white spot disease. A farm with 5 earthen ponds at Arachchikattuwa area was selected and a commercially available probiotic was used to suppress the Vibrio populations in 3 ponds; other 2 ponds did not receive the probiotic . Weekly random samples of shrimp were observed from each pond and haemolymph was inoculated on TCBS agar and cephalothorax of these shrimp were preserved in Davison Afa for histological preparations. Shrimp in each pond were observed for gross clinical signs of white spot disease. Heamolymph samples obtained from the shrimp of the 2 ponds that did not receive the probiotic treatment gradually increased and reached 480 ± 73 cfu/ml of haemolymph and such shrimp had multifocal melanized and/ or non-melanized haemocytic nodules with septic centers which is the principal diagnostic feature of systemic vibriosis. The stocks of shrimp in these 2 ponds exhibited gross clinical signs of white spot disease within 10-12 days (from the date they had Vibro count of 480±73 cfu/ml of haemolymph) and emergency harvest was carried out. In contrast, haemolymph of shrimp was free of Vibrio in the ponds that received the probiotic treatment and the shrimp did not develop systemic vibriosis and farm manager could complete the normal production cycle. Results of the present study suggest that systemic vibriosis is a predisposing factor for the occurrence of white spot disease in shrimp, P.monodon in grow out ponds.