Avian pathogenic Escherichia coli (APEC) causes a wide range of diseases in chickens called colibacillosis, resulting a substantial economic loss to the poultry industry. This dissertation aims at addressing this disease by exploring key virulence-associated genes (VAGs), swarming motility (SM), and multi-locus sequence types (MLST) of E. coli isolates obtained from colibacillosis-infected or asymptomatic commercial hens. Secondly, by classifying the E. coli isolates into different virulence categories based on the presence of five key VAGs [iroN, ompT, hlyF, iutA, and iss]. Finally, by performing embryo lethality assays and experimental infection studies to establish the effect of these VAGs. Our findings showed a significantly higher proportion of E. coli isolates obtained from colibacillosis lesions possessed the ompT gene compared to the isolates of asymptomatic commercial hens. A trend of a higher occurrence of the iutA gene was also observed in the isolates of colibacillosis cases. Based on the presence of all five VAGs, we categorized 87.5% of the isolates obtained from colibacillosis lesions into the virulent category and 64.71% of the isolates obtained from asymptomatic commercial hens into the avirulent category. During the embryo lethality assay, we found an interaction effect of virulence categories and SM on embryo mortality. Motile and hyper-motile isolates of virulent and moderately virulent categories caused significantly higher embryo mortality than the non-motile isolates of the same categories. Isolates of the avirulent category significantly reduced the relative embryo weight of the remaining live embryos. The MLST of the isolates did not have any influence on embryo lethality, or relative embryo weight. Yolk sac retention of the remaining live embryo was unaffected by virulence category, MLST, and SM of the isolates. During the experimental infection studies, we observed higher mortality and lesion scores in layer chicks inoculated intratracheally by virulent isolates compared to avirulent isolates. These two classes of isolates also had a different colonization pattern in the extra-intestinal tissues. The avirulent isolates preferably colonized deeper respiratory tracts such as airsacs whereas, the virulent isolates colonized systemic organs such as the liver. Overall, we expect this dissertation will establish the contribution of five key VAGs on embryo and chick mortality, lesion development, and colonization pattern of E. coli isolates. These findings will facilitate the selection of VAGs for field diagnosis of APEC.
| Identifer | oai:union.ndltd.org:MSSTATE/oai:scholarsjunction.msstate.edu:td-7160 |
| Date | 10 May 2024 |
| Creators | Ovi, Fozol Korim |
| Publisher | Scholars Junction |
| Source Sets | Mississippi State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
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