Molecular identification and histopathological study of natural Streptococcus agalactiae infection in hybrid tilapia (Oreochromis niloticus)

AIM: The main objective of this study was to emphasize on histopathological examinations and molecular identification of Streptococcus agalactiae isolated from natural infections in hybrid tilapia (Oreochromis niloticus) in Temerloh Pahang, Malaysia, as well as to determine the susceptibility of the pathogen strains to various currently available antimicrobial agents.

MATERIALS AND METHODS: The diseased fishes were observed for variable clinical signs including fin hemorrhages, alterations in behavior associated with erratic swimming, exophthalmia, and mortality. Tissue samples from the eyes, brain, kidney, liver, and spleen were taken for bacterial isolation. Identification of S. agalactiae was screened by biochemical methods and confirmed by VITEK 2 and 16S rRNA gene sequencing. The antibiogram profiling of the isolate was tested against 18 standard antibiotics included nitrofurantoin, flumequine, florfenicol, amoxylin, doxycycline, oleandomycin, tetracycline, ampicillin, lincomycin, colistin sulfate, oxolinic acid, novobiocin, spiramycin, erythromycin, fosfomycin, neomycin, gentamycin, and polymyxin B. The histopathological analysis of eyes, brain, liver, kidney, and spleen was observed for abnormalities related to S. agalactiae infection.

RESULTS: The suspected colonies of S. agalactiae identified by biochemical methods was observed as Gram-positive chained cocci, β-hemolytic, and non-motile. The isolate was confirmed as S. agalactiae by VITEK 2 (99% similarity), reconfirmed by 16S rRNA gene sequencing (99% similarity) and deposited in GenBank with accession no. KT869025. The isolate was observed to be resistance to neomycin and gentamicin. The most consistent gross findings were marked hemorrhages, erosions of caudal fin, and exophthalmos. Microscopic examination confirmed the presence of marked congestion and infiltration of inflammatory cell in the eye, brain, kidney, liver, and spleen. Eye samples showed damage of the lens capsule, hyperemic and hemorrhagic choroid tissue, and retina hyperplasia accompanied with edema. Brain samples showed perivascular and pericellular edema and hemorrhages of the meninges. Kidney samples showed hemorrhage and thrombosis in the glomeruli and tubules along with atrophy in hematopoietic tissue. Liver samples showed congestion of the sinusoids and blood vessel, thrombosis of portal blood vessel, and vacuolar (fatty) degeneration of hepatocytes. Spleen samples showed large thrombus in the splenic blood vessel, multifocal hemosiderin deposition, congestion of blood vessels, and multifocal infiltration of macrophages.

CONCLUSION: Therefore, it can be concluded that pathological changes in tissues and organs of fish occur proportionally to the pathogen invasion, and because of their high resistance, neomycin and gentamicin utilization in the prophylaxis or treatment of S. agalactiae infection should be avoided.