Current growth in aquaculture production is parallel with the increasing number of disease outbreaks, which negatively affect the production, profitability, and sustainability of the global aquaculture industry. Vibriosis is among the most common diseases leading to massive mortality of cultured shrimp, fish, and shellfish in Asia. High incidence of vibriosis can occur in hatchery and grow-out facilities, but juveniles are more susceptible to the disease. Various factors, particularly the source of fish, environmental factors (including water quality and farm management), and the virulence factors of Vibrio, influence the occurrence of the disease . Affected fish show weariness, with necrosis of skin and appendages, leading to body malformation, slow growth, internal organ liquefaction, blindness, muscle opacity, and mortality. A combination of control measures, particularly a disease-free source of fish, biosecurity of the farm, improved water quality, and other preventive measures (e.g., vaccination) might be able to control the infection. Although some control measures are expensive and less practical, vaccination is effective, relatively cheap, and easily implemented. In this review, the latest knowledge on the pathogenesis and control of vibriosis, including vaccination, is discussed. 4INA-SALWANY ET AL.

This paper reports a case of natural co-infection of Tilapia Lake Virus (TiLV) and Aeromonas veronii in a Malaysian red hybrid tilapia (Oreochromis niloticus × O. mossambicus) farm. In May 2017, a tilapia farm operator reported a mass mortality among cultured red hybrid tilapia juveniles, approximately 45 days after introduction into earthen ponds. Affected fish showed lethargy, loss of appetite, swim near the pond edge, pale and isolated from schooling group. There were skin redness and haemorrhages, particularly at the operculum area and at the base of dorsal, caudal and anal fins. Histopathological examinations revealed swollen hepatocytes, haemorrhagic spleens and perivascular cuffing consisted of mononuclear cells in the brains. PCR and sequence analyses confirmed the presence of TiLV and A. veronii in the diseased fish. Phylogenetic tree revealed that Malaysian's TiLV strain was more closely related with the virus isolated in Israel than in Egypt or Thailand, while Malaysian's A. veronii strain 5L was closely related with strains from China. TiLV infection is a significant threat to global tilapia industry. However, synergistic co-infection of TiLV and other bacterial might aggravate the problem.

Vibriosis in cultured marine fishes: a review ABSTRCT For more than a century, vibriosis affects various species of economically important cultured marine fishes around the globe. The knowledge of this bacterial disease on many species of cultured fish is still lacking, but progressing. This review focuses on updated fundamental knowledge related to vibriosis including the history, taxonomy, and various epidemiological aspects such as socio-economy, clinical signs, pathological changes, diagnosis, pathogenesis, transmission, risk factors and control measures of vibriosis. This review revealed a rising prevalence of vibriosis in aquaculture, concomitant with the rapid development of this industry worldwide. Yet, information on Vibrio infection in cultured fish, particularly on the Vibrio of non-medical importance, the influence of their virulence toxins to host cells, effects of global warming and the socioeconomic impacts are still scarce, and need more profound studies. Moreover, comprehensive epidemiological information on vibriosis are quite limited in many Asian countries with tropical climate, limiting the progression in control and prevention aspects of the disease.

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BackgroundStreptococcosis is an important disease of tilapia throughout the world. In Malaysia, streptococcosis outbreak was commonly reported during the 3-month period of high water temperature between April and July. This study describes the duration of protection following single and double booster dose regimes against streptococcosis in tilapia using a feed-based vaccine containing formalin-killed Streptococcus agalactiae. A total of 510 tilapias of 120 ± 10 g were selected and divided into 3 groups. Fish of Group 1 were vaccinated at weeks 0 and 2 (single booster group) while fish of Group 2 were vaccinated at weeks 0, 2 and 6 (double booster group) with a feed-based vaccine against streptococcosis. Fish of Group 3 was not vaccinated. Serum samples were collected weekly to determine the antibody level while samples of eye, brain and kidney were collected for bacterial isolation. At week 10, all fish were challenged with live S. agalactiae and the survival rate was determined.ResultsBoth vaccinated groups showed significant (p < 0.05) increase in the antibody levels following the first booster dose, which lasted until week 6. Group 2 showed consistent high level of antibody following the second booster dose at week 6 and remained high until week 12. Challenge trial at week 10 resulted in 45 %, 70 % and 0 % rate of survival for Groups 1, 2 and 3, respectively.ConclusionDouble booster regime is most suitable to be applied for feed-based vaccination against streptococcosis prior to the start of the hot season.

Attempts were made to identify the association between water quality parameters and the presence of Streptococcus agalactiae in cage cultured red hybrid tilapia, Oreochromis niloticus × O. mossambicus. Fish from commercial floating net cage‐culture systems in a river and lake were randomly sampled over a 24‐month period. Swabs from the brains, eyes and kidneys were streaked directly onto blood agar to isolate S. agalactiae. Water temperature, dissolved oxygen, pH, clarity, ammonia, nitrite, sulfide, rate of water flow and depth of water at sampling sites were measured at the same time of fish sampling. The prevalence of fish that were cultured positive to S. agalactiae was significantly higher in lake compared with river. The length and weight of the infected fish were between 9 and 33 cm, and between 20 and 760 g respectively. There was a significant and positive strong correlation between the presence of S. agalactiae and fish mortalities in lake. All water quality parameters showed significant differences between river and lake. However, only water temperature, clarity and pH of lake and the ammonia, temperature and dissolved oxygen in river showed significant correlation with the presence of S. agalactiae in the cultured fish. It was concluded that several unfavourable water quality in the fish farm influencing the presence of S. agalactiae in cultured red hybrid tilapia.

A high death rate among red hybrid tilapias was observed in a farm in Selangor, Malaysia, in January 2020. The affected fish appeared lethargic, isolated from schooling group, showed loss of appetite, red and haemorrhagic skin, exophthalmia and enlarged gall bladders. Histopathological assessment revealed deformation of kidney tubules, and severe congestion with infiltrations of inflammatory cells in the brains and kidneys. Syncytial cells and intracytoplasmic inclusion bodies were occasionally observed in the liver and brain sections. Tilapia Lake Virus (TiLV), Aeromonas hydrophila and Streptococcus agalactiae were identified in the affected fish, either through isolation or through PCR and sequencing analysis. The phylogenetic tree analysis revealed that the TiLV strain in this study was closely related to the previously reported Malaysian strain that was isolated in 2019. On the other hand, A. hydrophila and S. agalactiae were closer to Algerian and Brazilian strains, respectively. The multiple antibiotic resistance index for A. hydrophila and S. agalactiae was 0.50 and 0.25, respectively. Co-infections of virus and bacteria in cultured tilapia is a new threat for the tilapia industry.

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