Investigation of the bacterial disease outbreaks in farm Tilapia and Catfish in Indonesia

Abstract

Infectious diseases remain the major challenge affecting the sustainability of aquaculture production due to mortalities and morbidities which caused significant economic losses. A comprehensive approach has not been widely adopted to understand the disease status in Indonesia. Therefore, a field study was conducted in 41 farms producing tilapia and catfish which were representative of the range of production systems identified in East Nusa Tenggara Indonesia. A semi structured questionnaire was designed to describe the current status of the farming systems, level of disease outbreaks and health management strategies adopted in this area. A disease diagnostic approach was adopted to understand the role of infectious diseases related to the outbreaks. Bacterial isolates and tissue samples were collected for bacterial identification and histopathology to confirm the aetiology. A combination of traditional identification methods and molecular methods were performed for bacterial identification and characterization. The results from the field study identified that the farming systems in the area of study were categorized as in the early stage of intensification, where good health and disease management and biosecurity practice were not in place within the visited farms. From the questionnaire provided, a total of 73% (n=30) of participating farms reported that they experienced high mortality where, 34% (n=13) of farmers identified that the diseases were the main cause of the mortality, where the farmers recognised fish disease from the presentation of external clinical signs. The gross presentation of moribund fish samples showed a minimum either one or a combined clinical sign of the disease such as loss of appetite, sluggish movement, and swimming near the water surface, with the complete absence of the reflex, redness, and other external signs. The abnormalities of internal clinical signs such as the enlargement and change colour of fish organs. The examination of histopathological from the tissue samples from fish with clinical signs presented several changes of the tissue including vacuolation, degenerative changed in glomerular, necrosis, infiltration of inflammatory cells, and the presence of excessive number of MMC. The Gram stain of the tissue samples was also showing the figure of rod shapes bacterial colonization. The vast majority of isolates were Gram-negative bacteria and dominated by motile Aeromonas species including A. veronii, A. hydrophila, and A. caviae, which then 40 representative samples were confirmed by 16s rRNA and the two housekeeping genes rpoD and gyrB. A total of 12 virulence genes were detected with 95% (n=38) of all Aeromonas species positive for cytotoxic enterotoxin (act) gene. Whereas the aerolysin (aerA) gene was detected in 55% (n=22) of the strains. The aerA and act genes which is considered as one of the most important genes related to MAS disease, in the present study 53% (n=21) of the tested strains contained of the combination act+aerA genes. For the AMR, 39% (n=16) of the strains were resistant to a minimum 1 of antibiotics tested where oxytetracycline (30μg) was the most prevalent with 35% (n=14) of the Aeromonas strains where 13/14 of the same strains were also had tetE gene. All of the 6 Aeromonas strains selected for experimental challenge in vivo caused infection which led to the significant mortality of wax moth larvae compared with the unexposed controls. The strains of Aeromonas with different virulence profiles were administered to the larvae at the same concentration and incubation temperature but produced varied survival to the infected larvae for both tested A. veronii and A, hydrophila strains (1x105 CFU/ml). the strains categorized as high virulence caused all larvae dead since the second day post infection whereas the strains with low virulence caused 47% of larvae dead at the end of the experiment. These findings provide critical information on the role of motile Aeromonas in the occurrence of the disease outbreaks and pathogenic capacity of the motile Aeromonas species associated with infectious disease outbreaks in fish farming systems in Indonesia.