Avian pathogenic E. coli, the causative agent of colibacillosis harbors several putative virulence genes. In this study we examined by polymerase chain reaction (PCR) the presence of 4 of those genes in 52 strains of Esherichia coli isolated from birds with colibacillosis and 11 strains from feces of normal birds from Razavi Khorasan province. Isolates from normal birds were negative for iss gene sequence, but it was detected in 38.4% of disease isolates. The tsh gene sequence was detected in 67.3% of disease isolates, but only 27.3% of healthy isolates were positive for tsh gene sequence. The cvi gene sequence was detected in 53.84% of disease isolates but it was detected in 90.9% of healthy birds. Fourthy three (82.6%) isolates from colibacillosis cases and 90.9% of isolates from healthy birds possessed iuc sequences. The presence of iss, cvi, and tsh in disease isolates were significantly higher than normal ones. 12 patterns were found among disease isolates which two of them (iss+, cvi+, iuc+, tsh+ and iss-, cvi+, iuc+, tsh+) were the most ones. 22 of 52 strains from colibacillosis cases and 11 strains from normal birds were studied for the susceptibilities to nalidixic acid, norfloxacin and enrofloxacin. All of 22 disease isolates were resistant to quinolones (nalidixic acid) while 59%, 72.7% and 36.3% of 22 disease isolates were resistant to ciprofloxacin, enrofloxacin, and norfloxacin, respectively. incidences of virulence factors in quinolone and fluoroquinolone resistant isolates from diseased birds were not significantly different from quinolone and fluoroquinolone susceptible isolates.

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