الفهرس 
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Abstract
Neonatal calf diarrhea is one of the most important problems in young calves causing great economic losses. In the current study, a total number of 150 fecal samples from diarrheic and apparently healthy in-contact cattle and buffalo calves under 3 months old were examined bacteriologically for isolation of E. coli. All isolates were confirmed by PCR using 16SrRNA gene. Multiplex PCR was applied for detection of virulence genes and for studying the association of these genes with different E. coli pathotypes. One hundred and six of 150 isolates were positive for at least one virulence gene. The rate of f5, st, lt, eae, stx1, stx2, hylA and astA genes were 17.3%, 27.3%, 6.7%, 10%, 37.3%, 17.7%, 9.3% and 20.7%, respectively. Distribution of E. coli pathotypes revealed presence of ETEC, EPEC, STEC, EAEC, ET.EP. and ET.ST. at a rate of 12.7%, 2.7%, 30.7%, 7.3%, 2.7% and 14.7%, respectively. The rate of pathogenic E. coli infection in calves was higher in winter season and in the first weeks of life. Molecular serogrouping of E. coli isolates was carried using 10 O serogroups and revealed presence of O91 serogroup in the highest rate (62.7%), with higher rate of O26, O91, O121 and O45 in diarrheic calves. Comparing between molecular serogrouping and agglutination test revealed the ability of multiplex PCR to detect all isolates which were positive by agglutination and also 8 negative isolates. Five intestinal samples from four dead calves grossly showing congestion and distention of intestinal loops were bacteriologicaly positive for E. coli with higher rate of eae gene. Histopathological examination revealed heamorrhagic enteritis, shortening and fusion of intestinal villi, desquamation of lining epithelium of intestinal mucosa with congestion of blood capillaries and intense leucocytic infiltration.