Objectives: Avian colibacillosis is caused by an avian pathogenic Escherichia coli (APEC). Despite being known for over a century, avian colibacillosis remains one of the major endemic diseases afflicting the poultry industry worldwide. Genetic determinants for serum complement resistance in avian E.coli are found on colicin V encoding plasmids. On such gene, iss, first describe for its role in the serum resistance associated with col V plasmid from a human E.coli isolate, occurs much more frequently in isolates from bird with colibacillosis. The aim of this study was to survey the frequency of iss-harboring avian pathogenic Escherichia coli isolates from dead in shell embryos in hatcheries of Mazandaran province. Materials & Methods: 175 eggs from A, B and C hatcheries including 30 eggs from A, 76 eggs from B and 69 eggs from C hatchery with embryos were collected. After transferring to laboratory, the heart, liver and lung of each dead embryo was sampled for bacteriological culture. After isolating E.coli, Virulence of each E.coli isolate was evaluated based on PCR test in order to identification iss gene. Antibiogram test according to Kirby and Bauer method were carried out on 40 iss positive isolated. Twelve antibiotics discs were used including oxytetracycline (T),doxycycline(D),ceftriaxone(CRO), furazolidone(FR), Lincospectin(LS),florfenicol(FF),enrofloxacine(NFX),neomycine(N),gentamycine(GM),trimethopri m(TMP),trimethoprimsulfamethoxazol(SXT),amoxicillin-clavulanic acid(AMC). Results & Conclusion: After bacteriological culture, a total of 98 E.coli isolates were confirmed. The result of PCR test indicated that 78 isolates possessed iss(79.5%). The result of antibiogram test indicated 100% isolates were sensitive to CRO while 85.5% isolates were resistance to AMC. Resistance for T,LS,FF,NFX,D,N,AMC,TMP,SXT were reported 17.5%,45%,2.5%,32.5%, 22.5%, 25, 85.5%, 12.5%, 12.5% respectively. None of isolates were resistant to GM,CRO,FR. Overall this study indicates that the high pathogenicity iss-positive APEC strains are mostly attributed to embryonic mortalities in chickens.

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