LONGITUDINAL-STUDY OF ESCHERICHIA-COLI O157-H7 DISSEMINATION ON 4 DAIRY FARMS IN WISCONSIN

A 14-month longitudinal study was conducted on four dairy farms (C, H, R, and X) in Wisconsin to ascertain the source(s) and dissemination o f Escherichia coli O157:H7. A cohort of 15 heifer calves from each far m were sampled weekly by digital rectal retrieval from birth to a mini mum of 7 months of age (range, 7 to 13 months), Over the 14 months of the study, the cohort heifers and other randomly selected cattle from farms C and 14 tested negative, Farm R had two separate periods of E. coli O157:H7 shedding lasting 4 months (November 1995 to February 1996 ) and 1 month (July to August 1996), while farm X had at least one pos itive cohort animal For a 5-month period (May to October 1996), Heifer s shed O157:H7 strains in feces far 1 to 16 weeks at levels ranging fr om 2.0 x 10(2) to 8.7 x 10(4) CFU per g, E coli O157:H7 was also isola ted from other noncohort cattle, feed, flies, a pigeon, and water asso ciated with the cohort heifers on farms R and/or X, When present in an imal drinking water, E coli O157:H7 disseminated through the cohort ca ttle and other cattle that used the plater source, E, coli O157:H7 was found in water at <1 to 23 CFU/ml, Genomic subtyping by pulsed-field gel electrophoresis demonstrated that a single O157:H7 strain comprise d a majority of the isolates from cohort and noncohort cattle, water, and other positive samples (i,e,, from feed, flies, and a pigeon, etc. ) on a farm, The isolates from farm R, displayed two predominant XbaI restriction endonuclease digestion profiles (REDP), REDP 3 and REDP 7, during the first and second periods of shedding, respectively. Six ad ditional REDP that were greater than or equal to 89% similar to REDP 3 or REDP 7 were identified among the farm R isolates, Additionally, th e REDP of an O157:H7 isolate from a heifer on farm R in 1994 was indis tinguishable from REDP 3, Farm X had one O157:H7 strain that predomina ted (96% of positive samples had strains with REDP 9), and the REDP of an isolate from a heifer in 1994 was indistinguishable from REDP 9, T hese results suggest that E. coli O157:H7 is disseminated from a commo n source on farms and that strains can persist in a herd for a 2-year period.