Investigation into jaundice in farmed catfish (Pangasianodon hypophthalmus, Sauvage) in the Mekong Delta, Vietnam

Abstract

Disease outbreaks continue to be a major problem in the sustainable development of the aquaculture industry. Clinical outbreaks can negatively impact on the welfare of the fish and the economic gain derived from this industry. Jaundice observed as a yellow colouration in the abdominal skin, sclera of the eyes and fin bases is a significant health problem affecting the Vietnamese freshwater catfish industry. This study was designed to investigate jaundice of farmed catfish, Pangasianodon hypophthalmus using several complementary approaches. These included clinical investigations and identification of potential aetiological agents as well as epidemiological analyses to identify farm-based risk factors for this economically devastating condition occurring in the catfish farms of the Mekong Delta.

The results of this survey demonstrated that the jaundice was not linked to a single geographical location as affected fish were found widely distributed throughout the five main production areas. Nor was any association found between any weight groupings, feed type or feeding regime applied in the affected farms. The highest prevalence occurred between June to October and fish mortalities ranged from 1 to 10% in the study sites. The duration of this condition was significantly correlated (P < 0.05) to mortality but not to total farm area, depth of pond, stocking density, or amount of water exchanged. The number of fish ponds affected was not as high in the large-scale farms compared to the small-scale farms.

The results from the clinical description study showed that the affected fish were suffering a form of jaundice or icterus. Histological examination revealed a number of serious pathologies in the affected fish. Spleenomegaly was associated with the loss of cell structure and connective tissue and the haematopoietic tissue had large areas of necrosis. In the liver, histological changes consisted of vasculitis and multifocal to diffuse hepatocellular necrosis. The presence of haemosiderin was observed in melano-macrophage centres in the spleen and kidney of jaundiced fish. No single pathogen was identified in the jaundiced fish.

Myxosporean infection was found in both apparently normal fish and jaundiced fish. However, there was a definite tendency for jaundiced fish to be more heavily infected. Histopathological examination found several changes that could not be ascribed to specific aetiological factors and presume that both groups (jaundiced alone and myxosporean-affected jaundiced fish) have similar lesions. The results of this study would suggest that the parasite identified as M. pangasii was not a primary pathogen associated with the haemolytic jaundice. Neither were the gills myxosporeans associated with the haemolytic jaundice and they may be considered more as a nuisance rather than as primary pathogens in farmed P. hypophthalmus in the Mekong Delta, Vietnam.

Univariate analysis of the whole dataset showed several variables were significantly associated with the haemolytic jaundice. However, none of the variables achieved lasting statistical relevance in multivariable models.

In conclusion, this study identified a haemolytic jaundice condition affecting farmed P. hypophthalmus in Vietnam, but no single aetiological agent or farm based risk factor was identified with this condition. Several priority areas for further work were identified and include a prospective, longitudinal cohort study to identify further the risk factors associated with the clinical jaundice condition.