Transmission of Escherichia coli O157:H7 to cattle by house flies

Abstract

The main reservoir of Escherichia coli O157:H7 is the digestive tract of cattle; however, the ecology of this food-borne pathogen is poorly understood. House flies ( Musca domestica L.) might play a role in dissemination of this pathogen in the cattle environment. In our study, eight calves were individually exposed to house flies that were orally inoculated with a mixture of four strains of nalidixic acid-resistant E. coli O157:H7 ( Nal REcO157) for 48 h. Another eight calves were individually exposed to uninoculated flies and served as the control. Fresh cattle feces (rectal sampling) and drinking water were periodically sampled and screened for Nal REcO157 up to 19 days after the exposure. At the end of the experiment, all calves were euthanized and the lumen contents of rumen, cecum, colon, and rectum as well as swab samples of gall-bladder mucosa and the recto-anal mucosa were screened for Nal REcO157. On day 1 after the exposure, fecal samples of all eight calves and drinking-water samples of five of eight calves exposed to inoculated flies tested positive for Nal REcO157. The concentration of Nal REcO157 in feces ranged over time from detectable only by enrichment (<10 2) to up to 1.1 × 10 6 CFU/g. Feces of all calves remained positive for Nal REcO157 up to 11 days after the exposure and 62% were positive until the end of experiment. Contamination of drinking water was more variable and all samples were negative on day 19. At necropsy, the highest prevalence of Nal REcO157 was in the recto-anal mucosa region, followed by rectal and colonic contents.