Tilapia is an important farming species of the Vietnamese aquaculture industry and has been designated by the government and the Ministry of Agriculture as a key farming species, with favorable conditions to expand farming scale and increase output to serve processing and export. In 2018, tilapia exports reached 7,900 tons and reached 15.3 million USD, in 2019, tilapia exports reached more than 8,000 tons with a turnover of 16 million USD. With continuous development, the farming area in 2020 is expected to increase to 33.00 hectares and 1.5 million m3 of cage farming. The Ministry of Agriculture and Rural Development has set a target of increasing tilapia output from 225,000 tons (2017) to 400,000 tons by 2030, at which point the farming area can increase to 40,000 hectares and 1.8 million m3 of cage farming. The Directorate of Fisheries also aims to develop tilapia into a key product line with high competitiveness, forming large raw material areas in reservoirs, concentrated farming areas, especially cage farming on rivers and lakes, and intensive farming in the deltas, in which tilapia is the main aquaculture species in the North (Directorate of Fisheries, 2020).

Although it is an easy-to-raise and disease-resistant species, with the increasing development of the aquaculture industry, increasing intensification, along with environmental pollution, and increasingly strong trade of fish between countries, aquatic diseases are becoming more and more complicated, in which tilapia is one of the species with the most diseases in freshwater aquaculture today. The disease is highly contagious, through many routes between countries that share the same aquaculture species. In particular, if the fish are infected with A. hydrophila in the herd and not treated promptly, the mortality rate in the herd can be up to 40-60%, if infected, the mortality rate can be up to 80%. Although the disease is causing a lot of damage to tilapia farmed in Vietnam, up to now, in-depth studies on biological characteristics, virulence, antibiotic resistance and treatment regimens are still quite modest. Therefore, disease treatment is difficult and causes great economic losses to farmers. With the disease caused by Edwardsiella ictaluri, this is one of the dangerous pathogens in tilapia reported in the  US  in 2012 and first discovered in Northern Vietnam in 2016. When fish are sick, internal organs are necrotic, so in addition to affecting productivity due to reduced survival rate, it also affects meat quality and product design when consumed domestically and especially for export.

Based on the increasingly complicated practical situation of tilapia diseases, in which hemorrhagic and visceral necrosis caused by Aeromonas hydrophila and Edwardsiella ictaluri bacteria are occurring, causing huge economic losses to farmers, timely research is needed to provide to farmers and aquaculture managers. Therefore,  Dr. Truong Dinh Hoai  and his research team at the Agricultural Academy conducted the project  “Research on diseases in tilapia caused by Edwardsiella ictaluri and Aeromonas hydrophila bacteria”  with the aim of providing disease information, a database of pathogen characteristics, some epidemiological characteristics and antibiotic resistance status to provide information to support  the work of preventing diseases caused by Edwardsiella ictaluri and Aeromonas hydrophila bacteria on tilapia effectively, minimizing economic losses to farmers and helping tilapia farming develop sustainably.

After  a period of research, the topic has obtained the following results:

1) Conducted investigation and completed data analysis on farming, management, disease prevention and disease situation caused by A. hydrophila and E. ictaluri bacteria from 270 farming households in the Northern region.

2) A large number of diseased samples (776 tilapia samples suspected of being infected with A. hydrophila, 479 tilapia samples suspected of being infected with E. ictaluri phi collected from diseased ponds/cages) were collected during the investigation and random samples throughout the study. Preliminary assessment of disease characteristics and clinical signs.

3) A total of 187 suspected A. hydrophila strains and 128 E. ictaluri strains were isolated and cultured purely on Rimler-Shotts (RS) and TSA media.  After  random selection, analysis, sequencing of specific genes, and PCR identification, 34 A. hydrophila strains and 31 E. ictaluri strains were selected for in-depth studies, of which the 16S-rRNA, gyrB, and rpoB genes of 12 bacterial strains were registered in the GenBank.  After  the PCR identification results, the clinical characteristics of positive samples were compiled and compared with suspected samples and pathological characteristics of the disease were determined to develop a disease card.

4) The main microscopic lesions on 30 tilapia infected with A. hydrophila bacteria and 30 tilapia samples infected with E. ictaluri with typical visceral white spots were specifically analyzed and the main microscopic pathological characteristics of each disease were drawn.

5) The LD50 values ​​of 06 A. hydrophila strains and 06 E. ictaluri strains were determined through infection experiments, in which the E. ictaluri strains circulating in the North are highly virulent.

6) The biochemical and virulence characteristics of the bacterial strains have been tested and evaluated as stable through 10 generations of subculture, and the environmental factors suitable for bacterial growth and development have been  successfully determined . In particular, A. hydrophila can survive and develop at a temperature range of 15-40°C, salinity 0-65‰; pH 5-9.5, but the suitable range is 25-30°C, salinity 0-40‰, pH 7-8. E. ictaluri bacteria can grow at a temperature range of 15-40°C, salinity 0-25‰, pH 5-9; it grows best at a temperature of 25-30°C, salinity 0-5‰, pH 7-8.

The full text of the research report (code 20208/2022) can be found at the  National Center for Science and Technology Information .

Source: D.TV (NASATI)