الفهرس 
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Abstract
Yersiniosis is an infectious disease which produces high mortalities and severe
economic losses in freshwater fish farms. Heavy mortalities were found in
Oreochromis niloticus, Clarias gariepinus.
at dakahlia. A total number of 200 freshly dead fish were sampled and transferred
as soon as possible to the lab for clinical examination and bacteriological
identification. The examined fish showed the septicemic signs of Yersiniosis
which appeared as generalized erythema, petechial hemorrhages on the skin, fin
erosions, protruded hemorrhagic vent with red erythematic hemorrhages on mouth
and lips. Postmortem examination revealed engorged gall bladder, hemorrhagic
gills, liver, gastrointestinal tract (GIT)and darkened congested spleen. After
bacterial isolation from spleen, kidney, liver on specific culture media, diagnosis
was achieved by conventional biochemical tests, serotyping and polymerase chain
reaction (PCR). Antimicrobial was conducted on bacterial isolates in order to find
the most suitable antibiotic for controlling this bacterial infection. from this study,
it was found that the recommended antibiotic for controlling this bacterial
infection is ciprofloxacin or Sulphamethoxazole-Trimethoprim combination.
In this study Yersinia ruckeri is a common fish pathogen, it causes one of the
most significant septicemic diseases responsible for mass mortality in
freshwater fishes and consequently high economic losses. This study was
carried out to investigate prevalence of Y.ruckeri among 0. niloticus and C.
gariepinus at Dakahlia Governorate and to characterize the isolates
phenotypically and genotypically in addition to detection of virulence genes
(yrp,yrIIm,yhlA,yhlB,yrInv) in them by PCR assay and multi-drug resistance
genes(blaTEM,qnrS,tetAgene).
Therefore, (100) samples of O. niloticus and (100) samples of C. gariepinus
collected from different localities at Dakahlia Governorate during the period
fromApril 2019 to April 2020. Fish samples were subjected to clinical and postmortem
examination then bacteriological examination from liver, kidney and
spleen. The suspected isolates were characterized by cultural and morphological
characters, some conventional biochemical tests and API 20E system then by
PCR assay. 24 diseased fish were characterized as infected with Y.ruckeri [10
from O. niloticus (10%) and 14 from C. gariepinus (14%)] with 44 Y.ruckeri
isolates with percentage of 22%. The phenotypic characterization of the isolates
revealed that they were homogenous. Furthermore, 16SrRNA gene (specific
common gene) was demonstrated in all Y.ruckeri isolates by PCR. Results of
this study indicated that polymerase chain reaction is very reliable and rapid
method for identification of Y.ruckeri isolates which may be helpful in
prevention and control of yersiniosis.