Understanding the transmission dynamics of O157:H7 super-shedding infections in feedlot cattle.

Background: The presence of O157:H7 ( O157:H7) super-shedding cattle in feedlots has the potential to increase the overall number (bio-burden) of O157:H7 in the environment. It is important to identify factors to reduce the bio-burden of O157 in feedlots by clarifying practices associated with the occurrence of super-shedders in feedlot cattle.

Methods: The objective of this study is to (1) identify host, pathogen, and management risk factors associated with naturally infected feedlot cattle excreting high concentrations of O157:H7 in their feces and (2) to determine whether the ingested dose or the specific strain of O157:H7 influences a super-shedder infection within experimentally inoculated feedlot cattle. To address this, (1) pen floor fecal samples and herd parameters were collected from four feedlots over a 9-month period, then (2) 6 strains of O157:H7, 3 strains isolated from normal shedder steers and 3 strains isolated from super-shedder steers, were inoculated into 30 one-year-old feedlot steers. Five steers were assigned to each O157:H7 strain group and inoculated with targeted numbers of 10, 10, 10, 10, and 10 CFU of bacteria respectively.

Results: In the feedlots, prevalence of infection with O157:H7 for the 890 fecal samples collected was 22.4%, with individual pen prevalence ranging from 0% to 90% and individual feedlot prevalence ranging from 8.4% to 30.2%. Three samples had O157:H7 levels greater than 10 MPN/g feces, thereby meeting the definition of super-shedder. Lower body weight at entry to the feedlot and higher daily maximum ambient temperature were associated with increased odds of a sample testing positive for O157:H7. In the experimental inoculation trial, the duration and total environmental shedding load of O157:H7 suggests that the time post-inoculation and the dose of inoculated O157:H7 are important while the O157:H7 strain and shedding characteristic (normal or super-shedder) are not.

Discussion: Under the conditions of this experiment, super-shedding appears to be the result of cattle ingesting a high dose of any strain of O157:H7. Therefore strategies that minimize exposure to large numbers of O157:H7 should be beneficial against the super-shedding of O157:H7 in feedlots.