الفهرس 
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Abstract
>This study deals with the role of pathogenic E. coli as the most important cause of bacterial diarrhea in calves. The purpose of this study was to investigate the pathogenicity and virulence of E. coliaffecting young calves and its role in calf diarrhea as well as investigation of the epidemiological factors associated with E coli infection among calves.
A total of 46 new born calves were followed up for a period of 75 days (0 day till 75 days of age) for evidences of enteritis. These calves were clinically examined daily with special attention for enteritis. Clinical abnormalities suggesting enteritis were recorded in 14 (30.43%) calves of different agesthat showed fever, diarrhea, dehydration, pneumonia and recumbence.
Different parameters were studied to investigate the relationship between calf scour and some associated epidemiological factors. Investigated calves were followed up for a period of 75 days (0 day till 75 days of age) for evidences of enteritis. Different parameters included sex, age of onset of diarrhea, method of feeding, dam parity, calving ease score and housing system. The present study recorded a higher prevalence of calf scour in female calves (48.00%) than male calves (9.52%). Prevalence of calf scour among calves was evaluated in calves of different ages. The results showed that calves were more susceptible during the 1st week, 17.39% followed by the 2nd, 3rd and 4th week, 4.35each.
The obtained results revealed that the prevalence of calf scour was (31.71%) in artificial feeding (milk) and (20%) in natural suckling. The higher prevalence in artificial feeding may be attributed to the bad hygienic measures and bad handling from workers and the high chance of contamination in comparison with the natural suckling.
Concerning housing system of calves, the results revealed that calves reared in separate boxes showed the highest prevalence, 35.71% followed by those reared in yards, 29.63%, then those reared mixed with other animals of different ages and other animal species, 20%. Although such results are unexpected especially for calves kept in separate boxes, but it can be explained on the basis of the very bad hygienic measures in the farm.
Correlation between calf scour and the parity of the dam revealed 31.03% prevalence in dams of 1st parity, 33.33% in dams of 2nd parity and 18% in dams of 3rd parity and more.Correlation between calf scour and the calving ease score revealed 28.57% prevalence in calves of mothers that do not need or need little assistance while it was 50% in calves from dams with caesarian sectionsindicating the danger and effect of surgical interference on the calves especially following dystocia.The prevalence of calf scour in calves obtained from native cross breed was 20%, while it was 31.71% inpure Holstein breed.
Among the important challenge factors observed in this study, that some cows especially first calving heifers do not lick or stimulate their newborn calves and remove secretions from their nostrils and skin to get up and nurse. It was observed that these calves were unable to suckle their dams delaying taking of colostrum at the proper time and predisposing them to infection. Other pure Holstein cows showed severe physiological edema of the udder both periparturient and post parturient. It was observed that these calves were unable to suckle their dams delaying taking of colostrum and predisposing them to infection.Dystocia was another challenge as it was observed that one among two calves obtained after caesarian sections was suffering from severe scouring indicating the danger and drastic effect of surgical interference on the calves especially following dystocia.
In this study,a total number of 14 rectal swaps were collected from the 14 diarrheic calvesobserved in this study and subjected to bacteriological examination for isolation and identification of the pathogenic E. coli. Rectal swaps were inoculated onto TSB and incubated aerobically for 24 hours at 37°C. for propagation of E. coli. Positive TSB isolates were cultured onto MacConkey agar and EMB agar media and incubated aerobically for 24 hours at 37°C. Isolation and identification of 13 (92.85 %) isolates of E. coliamong the 14 collected samples of the 14 diarrheic calveswere confirmed on the basis of their morphology, culturaland biochemical tests using standard bacteriological procedures.The 13 isolates were visualized as blue-black colonies (2-3mm diameter) with green metallic sheen on Eosin Methylene Blue Agar and brightpink colonies on MacConkey agar. Such findings indicated a presumptive identification of E. coli for all 13 isolates that was confirmed using standard bacteriological procedures.The overall prevalence rate of enteric colibacillosis in the present study among 46 calves was (28.26%).
Molecular confirmation of isolated E. coli strains using Polymerase chain reaction was carried out. The experiment was designed to confirm on molecular basis (PCR) the bacteriological findings of E. coli isolation. A monoplex PCR detecting alkaline phosphatase gene (phoA gene) specific for E. coli was performed on E. coli isolates (N=10). In this experiment, E. coli isolates (N=10) were subjected to the PCR using primer pairs targeting the phoA gene specific for E. coli. The results of the amplification of phoAgene using PCRrevealed that, all the 10 E. coli isolatesshowed positive result for the presence of phoA gene, thus confirming their identity as E. coliand proving thatphoAgene is a housekeeping gene present in all E. coli strains.
Polymerase chain reaction was used for detection of virulence genes to distinguish pathogenic E. coli strains from normal gut flora.The present study focused on molecular characterization of pathogenic E. coli causing diarrhea in neonatal calves through applying different conditions of multiplex PCR on isolated E. coli strains for detection of genes encoding virulence factors. These genes encoding virulence factors wereshiga toxin1 (stx1), shiga toxin 2 (stx2), intimin (eae), hemolysin (hylA), and E. coli enterotoxin genes such as heat stable enterotoxin (st), heat labile enterotoxin (lt).
All tested E. coli isolates were positive for intimin (eae)A, attachingand effacing gene (gene species specific) for E. coli(100%). No isolate had shiga toxin1 (stx1), shiga toxin 2 (stx2), hemolysin (hylA), and E. coli enterotoxin genes heat stable enterotoxin (st) and heat labile enterotoxin (lt). Intimin is a virulence factor (adhesin) of EPEC (e.g. E. coli O127:H6) and EHEC (e.g. E. coli O157:H7) E. coli strains. It is an attaching and effacing (A/E) protein, which is responsiblefor enteropathogenic and enterohemorrhagic diarrhea.
In the present study, EPEC represents 100% of the 10 tested E. coli strains. Allisolates were obtained from diarrheic calves which indicated that EPEC is a calf pathogen.