`Syndrome 93' in New Caledonian outdoor rearing ponds of Penaeus stylirostris: history and description of three major outbreaks

Abstract

Farming of Penaeus stylirostris (Mexican strain) in New Caledonia is a developing industry conducted in flow-through semi-intensive system, at 20 to 40 animals/m 2 with a theoretical extrapolated yield of 4.2 t/ha/yr. In 1995, approximately 878 t of shrimps were produced from 352 ha of ponds. From the 1980s to 1992, production increased progressively (538 t in 1990, 648 t in 1991 and 734 t in 1992). The increase was the result of improved techniques and greater production surface area. In 1993, production decreased (621 t in 1993), then increased again in 1994 and 1995 due to increased grow out surface only. Low yields and survival rates in 1993, 1994 and 1995 were associated with abnormal mortalities. This pathology, given the name `Syndrome 93', appeared during a period of drought and was more pronounced in winter (mid-May to mid-August). The mortalities were chronic or acute. High pond biomass and sudden water temperature drop seemed to have an impact on the development of the disease. Moribund shrimps were weak, sensitive to stress and swam near the water surface. Shrimps were soft with dark and dull exoskeleton and whitish muscles. Their hemolymph clotted slowly. The hemolymph of diseased shrimps was generally infected with one predominant strain of bacteria, Vibrio sp. The hemolymph of presumed healthy shrimp was sometimes sterile or contained variable but generally limited numbers of different bacteria of the genera Vibrio, Pseudomonas, Flavobacterium and Cytophaga. Moribund shrimps exhibited classical anatomopathologic septicemic vibriosis lesions. However, numerous pycnotic hemocytes and atypical basophilic granulations were also observed in most tissues, especially the lymphoid organ, heart, connective tissue of the stomach and cuticular epithelium. These basophilic granulations seemed to correspond partly to nuclear debris and partly to intracytoplasmic inclusions which are similar to those described in penaeid viral infections (`yellow head disease', `Taura syndrome') indicating that pathogens other than Vibrio sp. could be involved.