Abstract
Background:
Gastrointestinal disorders in dogs are important clinical conditions in veterinary medicine, which have multifactorial non-infectious and infectious etiologies. Colibacillosis, caused by Escherichia coli, is an infectious bacterial disease that contributes to host health, and pathogenic variants that are responsible for a wide spectrum of diseases.

Aim:
This study aims to investigate the prevalence rate of gastrointestinal signs in diseased pet dogs, phylogenetic analysis of colibacillosis, measurement of antibiotic sensitivity, and levels of immune and antioxidant markers in dog populations.

Methods:
A total of 127 pet dogs admitted with various gastrointestinal signs were examined clinically. Fresh fecal samples were traditionally and molecularly analyzed to identify E. coli isolates for phylogenetic analysis and antibiotic susceptibility. The cephalic venous blood samples were examined quantitatively by enzyme-linked immunosorbent assay (ELISA) to measure immune markers such as interleukins (IL-1, IL-4, IL-6, and IL-8) and interferon-alpha (INF-α), as well as antioxidants such as catalase (CAT), glutathione (GSH), superoxide dismutase (SOD), and malondialdehyde (MDA).

Results:
Clinically, diarrhea was the most identified gastrointestinal sign in the study dogs. Traditional findings revealed that 14.96% of fecal samples were positive to E. coli isolates that showed a high sensitivity to antibiotics like meropenem, levofloxacin, and ciprofloxacin with a significant resistance to amoxicillin and ampicillin. Molecularly, 78.95% of study isolates were positive to E. coli strains, which were found to be phylogenetically (NCBI-BLAST) identical to the Iraqi strain (LC844819.1). Serologically, elevated levels of IL-1, IL-6, IL-8, and MDA with reduced levels of IL-2, CAT, GSH, and SOD were observed in E. coli-positive dogs. However, no significant alteration was seen in values of INF-α among the dogs with and without colibacillosis.

Conclusion:
The present study represents a multifaceted analysis of the incidence rate of gastrointestinal disorders in pet dogs, prevalence rate of E. coli using traditional and molecular phylogenetic diagnostics, and the association of colibacillosis to immune and antioxidant markers in Iraq

Key words: Canine gastrointestinal infections, Escherichia coli, Interferon, Interleukin, Oxidative stress, Iraq