This study is conducted to evaluate the protective effectiveness live-attenuated E.Coli vaccine. A 140 broiler chicks were classified into seven groups; the first two groups (1 & 2) were labeled as challenge control for E.Coli O78 and O91 strains, the second two groups (4 & 5) were vaccinated with the E.Coli live- attenuated vaccine at day 1 , the third two groups (3 & 6) treated with Difloxacin "second generation fluoro-quinolone" 1ml/L drinking water for first three days of life then vaccinated at day 7 with the E.Coli live- attenuated vaccine. The last group (7) was kept as control negative group.

The challenge was directed to the 21-day-old chicks with either homologous O78 or heterologous O91 E.Coli strains 1.2x10^9 Colony Forming Unit/mL injected intratracheally using 0.5 ml/ bird of each avian pathogenic E.Coli (APEC) strain. Clinical signs, gross and histo-pathological lesions, intestinal total count of E.Coli (CFU), the chick body-weight and the weight of lymphoid organs were calculated at 28 days of age. The results postulated that Poulvac® E.Coli vaccine gives a significant protective effect against the challenge with homologous O78 and the most common heterologous serotype O91. Based on for-mentioned parameters , vaccinated birds at 7 days of age (Groups 3&6) which was treated with Difloxacin and challenged with both homologous and heterologous E.Coli serotypes (O78 & O91) showed superior protection over those vaccinated at day one of age (Groups 4&5) which challenged with O78 & O91 E.Coli serotypes and did not received any antibiotic.