الفهرس 
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Abstract
Poultry are regarded; without any doubt, the most appropriate source of animal protein supply of high nutritive value for humans allover the world. This is due to efficiency cost of production and its short life cycle. In Egypt, a great attention was directed to the poultry industry to meet the increasing demand of animal protein. Infectious diseases are important in broiler industry due to high mortality, retardation of growth, as well as the preventive and therapeutic use of antimicrobials. Moreover, economic losses may result from loss of uniformity of the flock and condemnations in the slaughterhouse.
One of the major problems in poultry farms is the control of bacteria that causing respiratory and enteric diseases. Accordingly, great attention and efforts should be paid to eliminate these microorganisms causing different affections in poultry.
Escherichia coli infection is one of the most important diseases of chickens, resulting in significant economic losses as well as high morbidity and mortality among baby chicks, broilers and layers.
In the current study, the prevalence of avian colibacillosis was studied in 300 broiler chickens; from different farms in Beni-Suef, EL-Minia, El-Fayoum, Assiut and Sohag Governorates, by isolation and identification of E. coli from different pathological lesions by conventional methods (morphology, culturing and biochemical) as well as API20E kits. Also, antimicrobial sensitivity testing of E. coli isolates against the most important antimicrobials with detection of MDR E. coli isolates was applied. Moreover, genotyping of some virulence and antimicrobial resistance-associated genes in the MDR isolates was conducted using PCR technique.
A total of 300 pooling samples were collected aseptically from heart blood as well as the affected organs; air sacs, pericardial sac, and liver of slaughtered diseased and freshly dead broiler chickens.
In the current study, the results revealed that the overall prevalence of E. coli in the diseased broiler chickens in different Governorates was 26.7% where 80 E. coli isolates were recovered from 300 diseased broiler chickens. Regarding Governorates, the highest prevalence was recorded in El-Fayoum as 33.3% followed by El-Minia as 25% and then, both of Beni-Suef and Assiut as 22.5% while the lowest prevalence was recorded in Sohag as 17.5%.
In the present work, all E. coli isolates (n=80) tested for their susceptibility to 11 different antimicrobial drugs to detect the drug of choice for treatment as well as to detect MDR isolates for further analyses of the isolates. The results of in-vitro antimicrobial susceptibility tests revealed that E. coli isolates showed high sensitivity to colistin sulphate only (70%). On the other hand, high resistances were recorded to all other antimicrobials including amoxicillin (97.5%), cefotaxime sodium and florophenicol (95% for each), apramycin, ciprofloxacin and gentamicin (92.5% for each), streptomycin (90%), enrofloxacin (87.5%) and both of trimethoprim-sulphamethoxazol and doxycycline HCl (77.5% for each).
Moreover, in the current study, multidrug resistance (MDR) was detected in all E. coli isolates (100%) which observed a resistance to three or more antimicrobials of different categories.
Moreover, in the current work, Polymerase Chain Reaction (PCR) technique was applied on 10 MDR E. coli isolates to detect the 3 resistance-associated genes including genes encoded the resistance to quinolones (qnrA), resistance to tetracycline (tetA) and resistance to aminoglycosides (aac(6′)-Ib-cr). The results showed that 100% of the tested isolates harbored both qnrA and tetA genes meanwhile aac(6′)-Ib-cr gene was not detected in any isolate.
On the other hand, PCR technique was applied on 10 MDR E. coli isolates to detect 5 virulence-associated genes including iron uptake encoding gene (iutA), avian haemolysin gene (hly), enterotoxin encoding genes including heat stable enterotoxin (sta) and heat labile enterotoxin (lt) and enteroaggregative toxin encoding gene (arginine succinyltransferase A; astA). The results revealed that all the tested isolates (100%) harbored both iutA and astA genes meanwhile no isolates (0%) harbored hly, sta and lt genes.