Transmission and infectious dose of Escherichia coli O157:H7 in swine

Abstract

Escherichia coli O157:H7 is only occasionally isolated from healthy swine, but some experimentally infected animals will shed the organism in their feces for at least 2 months. Potential explanations for the paucity of naturally occurring infections in swine, as compared to cattle, include a lack of animal-to-animal transmission so that the organism cannot be maintained within a herd, a high infectious dose, or herd management practices that prevent the maintenance of the organism in the gastrointestinal tract. We hypothesized that donor pigs infected with E. coli O157:H7 would transmit the organism to naïve pigs. We also determined the infectious dose and whether housing pigs individually on grated floors would decrease the magnitude or duration of fecal shedding. Infected donor pigs shedding <10(4) CFU of E. coli O157:H7 per g transmitted the organism to 6 of 12 naïve pigs exposed to them. The infectious dose of E. coli O157:H7 for 3-month-old pigs was approximately 6 x 10(3) CFU. There was no difference in the magnitude and duration of fecal shedding by pigs housed individually on grates compared to those housed two per pen on cement floors. These results suggest that swine do not have an innate resistance to colonization by E. coli O157:H7 and that they could serve as a reservoir host under suitable conditions.

FIG. 1.

E. coli O157:H7 shed by inoculated donor pigs and naïve pigs exposed to those donors. Day 0 is the day that infected donors were moved into a different room and penned with a naïve pig. •, donor pigs shedding ≥10⁴ CFU/g; ○, donor pigs shedding <10⁴ CFU/g; ♦, naïve pigs exposed to donors shedding ≥10⁴ CFU/g; ⋄, naïve pigs exposed to donors shedding <10⁴ CFU/g. Values below the dotted line were positive by enrichment culture only (<50 CFU/g).

FIG. 2.

Average magnitude of fecal shedding of E. coli O157:H7 by pigs housed one per pen on raised decks and two per pen on cement floors. There was no significant difference between floor types.