Antimicrobial susceptibility and factors affecting the shedding of E. coli O157:H7 and Salmonella in dairy cattle.
OBJECTIVE
To examine factors affecting faecal shedding of the foodborne pathogens Escherichia coli O157:H7 and Salmonella in dairy cattle and evaluate antimicrobial susceptibility of these isolates.


METHODS
Faecal samples were obtained in replicate from lactating (LAC; n = 60) and non-lactating (NLAC; n = 60) Holstein cattle to determine influence of heat stress, parity, lactation status (LAC vs NLAC) and stage of lactation [</=60 or >60 days in milk (DIM)] and cultured for E. coli O157:H7 and Salmonella. A portion of the recovered isolates were examined for antimicrobial susceptibility using the broth microdilution technique.


RESULTS
No effects of heat stress were observed. Lactating cows shed more (P < 0.01) E. coli O157:H7 than NLAC cows (43% vs 32%, respectively). Multiparous LAC cows tended to shed more (P = 0.06) Salmonella than primiparous LAC cows (39% vs 27%, respectively). Parity did not influence (P > 0.10) bacterial shedding in NLAC cows. Cows </=60 DIM shed more (P < 0.05) Salmonella than cows >60 DIM. Seventeen Salmonella serotypes were identified with the most prevalent being Senftenberg (18%), Newport (17%) and Anatum (15%). Seventy-nine of the Salmonella isolates were resistant to at least one of the seven antibiotics. Escherichia coli O157:H7 isolates were resistant to 11 different antibiotics with multiple resistance to nine or more antibiotics observed in five isolates.


CONCLUSIONS
This study demonstrated differences in the shedding patterns of foodborne pathogens due to the stage of the milk production cycle and may help identify times when on-farm pathogen control would be the most effective.
Fitzgerald
A C
AC
Department of Animal and Range Sciences, New Mexico State University, Las Cruces, NM, USA.
Edrington
T S
TS
Looper
M L
ML
Callaway
T R
TR
Genovese
K J
KJ
Bischoff
K M
KM
McReynolds
J L
JL
Thomas
J D
JD
Anderson
R C
RC
Nisbet
D J
DJ
D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology; D004352:Drug Resistance, Microbial; D018432:Drug Resistance, Multiple; D019453:Escherichia coli O157 / Q000187:drug effects* / Q000302:isolation & purification; D005243:Feces / Q000382:microbiology*; D008826:Microbial Sensitivity Tests; D012475:Salmonella / Q000187:drug effects* / Q000302:isolation & purification
D000900:Anti-Bacterial Agents