Contamination of Fish Feed with Pathogenic Organisms: Implications on Fish Diseases in Aquaculture Systems

Abstract

Tracing contamination to its ultimate source is considered difficult as multiple factors affect stable microbial community in culture water.  Feed, covering over 60 percent of production cost although less considered, could represent a significant source of fish diseases, the  major constraint in aquaculture production. The present study based on the sensitivity of quantitative Polymerase Chain Reaction (qPCR),  and sequencing analysis was undertaken to determine the microbial load in fish feed and its potential to cause disease  outbreaks in aquaculture farms in Ghana. From the general screening of a total of 30 feed samples, 90% were positive for bacterial  contamination, 70% for fungal contaminations, with 40 % showing multiple contaminations with both bacterial and fungal pathogens  Three important bacteria pathogens (Streptococcus iniae, Streptococcus agalactiae and Staphylococcus aureus) implicated as major  causes of fish diseases occurred at prevalence rates of 89 %, 60 % and 60 % respectively in the fish feed and with average concentrations(Ct values) of 28.39, 40.78 and 34.26. Additionally, sequencing analysis based on the 16S rDNA gene of bacteria and at less than 96% identity,  revealed the presence of three different genus of bacteria, and six different species, representing a profile of over 50 different strains of  bacteria from the Sphingomonas genus. A positive correlation (0.667) existed between the prevalence rate of bacteria species in fish feed  and that in diseased fish (P<0.05). Also, the prevalence rate of bacteria isolated from the gut of fish (26.09%) were significantly (P<0.05)  similar to that from diseased portions (26.89%). Although mode of transmission of most infections are known to be largely through the  water in which the fish are submerged, prevalence rate of S. iniae (32 %) and S. agalactiea (24%) were significantly(p<0.05) lower in water  samples than that in the feed (89%, 60%) respectivelyIn addition, Staphylococcus aureus which was not isolated from the culture water samples at all, occurred at a prevalence rate as high as 60 % in fish-feed. This thus confirms contamination of fish-feed as a significant  pathway for entry of pathogens. Evaluation of microbial quality of fish feed as a routine practice on farms should therefore be enforced.