Abstract
The present study was carried out to characterize pathogenic E. coli in apparently healthy and diarrheic neonatal calves with special reference to the hybrid E. coli strains and evaluate their clinical and hematobiochemical consequences. One hundred and seventy calves (age 1-30 days) were divided into two groups: apparently healthy (n = 70) and diarrheic (n=100). Animals were subjected to thorough clinical, hematobiochemical and bacteriological examinations. Clinically, diarrheic calves showed various degree of diarrhea with the presence of cardinal signs of dehydration in moderate and severe cases. There was a significant increase (p<0.05) in the hemogram parameters with uremia and hyperkalemia in calves with severe diarrhea. The O-H serotyping of cultural and biochemically positive isolates identified 31 isolates belonging to 12 serotypes including O44:H18, O55:H7, O146:H21, O113:H4, O121:H7, O26:H11, O91:H21, O111:H2, O8, O127: H6, O86 and O128:H2. Molecular characterization of E. coli isolates on three toxin genes: heat-stable enterotoxin (sta), shiga toxin type 1 and 2 (stx1 and stx2) revealed two well-known pathotypes (EPEC O44:H18, O55:H7, O146:H21, O113:H4, O121:H7 and EHEC O26:H11 O91:H21 O111:H2) with high frequency of enterohemorrhagic E. coli (EHEC). Molecular analysis also showed a number of E. coli isolates that carry sta and stx1 or sta and stx2 gene and belonged to O8, and O127:H6, O86 and O128:H2. These isolates were identified as hybrid E. coli strains (ETEC-STEC) and found in both apparently healthy and diarrheic calves. In conclusion, the present study identified high frequency of pathogenic E. coli in both apparently healthy and diarrheic calves. Serological and molecular analysis of E. coli isolates showed that high frequency of EHEC and presence of a new phenotype, STEC–ETEC hybrid, revealing their importance in the etiopathogenesis of diarrhea in calves and reinforcing the role of these animals as a reservoir of potentially pathogenic E. coli for humans.

Key words: Escherichia coli, hybrid, calf, diarrhea