Abstract:
Fish diseases including bacterial diseases cause significant loss in aquaculture. Antibiotic
treatments are expensive and lead to antimicrobial resistance. The objective of this study was to
identify the potential of a low cost herb which enhance fish immunity using Carica papaya leaf
powder as a feed additive on zebrafish infected with bacteria and determined the effect on
immunity. Ninety healthy fish were divided into three groups with three replicates (30 fish per
group) exposed to Aeromonas hydrophila in water for a week followed by feeding trial. Three
groups were fed with commercial (control), antibiotic and papaya leaf incorporated fish feed with
2% of body weight per day. Differential white blood cell (WBC) count was taken to identify the
effects on disease response. Antibiotic susceptibility test was performed to identify, antibiotic
resistance of bacteria and antibacterial activity of aqueous papaya leaf extract. Growth parameters
and growth performances of experimental fish were calculated. WBC counts were measured before
exposure to bacteria, one week after infectious challenge, one week after feed trial. One way
ANOVA followed Tukey Pairwise comparison test used for data analysis. Fish exposed to A.
hydrophila showed haemorrhages on the base of pectoral fin, fin rot and body discoloration. As per
the results, monocyte count was decreased and lymphocyte count was increased in all three tanks
after infection but one week after receiving the antibiotic and papaya feed, lymphocyte counts were
decreased significantly compared to the control, while monocyte count and neutrophil counts were
increased compared to the control. Accordingly, initial increase of lymphocyte count was observed
due to immune response against bacteria as they involved in antigen-antibody reactions. Lowered
monocyte count was observed due to recruit them for wound healing. Lowered lymphocyte count
was observed due to the reduction of pathogens in response to antibiotic and papaya feed.
Neutrophils play a role in phagocytizing bacteria, increased in papaya fed fish indicated a positive
immune response. The results indicated that papaya leaf incorporated feed increased the immune
response of zebrafish against A. hydrophila, which can be used as a low cost, herbal feed additive
in ornamental fish industry.
Keywords: Zebrafish; Carica papaya; Aeromonas hydrophila; Immunity; Herbal antibiotic
