Short communicationTransmission of Escherichia coli O157:H7 to cattle by house flies
Introduction
Enterohemorrhagic Escherichia coli O157:H7 is an important causative agent of hemorrhagic colitis and hemorrhagic uremic syndrome in humans (Rangel et al., 2005). Although the number of human infections in the United States has declined over the past several years (402 human infections in the United States in 2004) (FoodNet Surveillance Annual Reports, 1997–2004); the outbreaks in 2006 (CDC, Health and Safety Topics, 2006) have underscored the importance of this human food-borne pathogen. The main reservoir for E. coli O157:H7 is the intestinal tract of healthy cattle; individual cattle are transiently colonized and shed E. coli O157:H7 in their feces (Bach et al., 2002). The sources of E. coli O157:H7 that colonize cattle are not well understood and little is known about the ecology of E. coli O157:H7 in the environment (Bach et al., 2002, Sargeant et al., 2003). Additionally, high variability of prevalence of E. coli O157:H7 among cattle suggests the possibility of a reservoir of E. coli O157:H7 external to the cattle. E. coli O157:H7 has been detected in non-bovine animals including sheep, horses, dogs, and wild birds (Bach et al., 2002); however, the mode of dissemination of this pathogen in the environment is not well understood.
One of the potential means of spread of this pathogen in the environment is by insects that develop in animal feces/manure (primarily house flies, Musca domestica L.). House flies (HF) commonly build up very large populations on cattle farms and other animal facilities. Previously, a laboratory-based study demonstrated that E. coli O157 ingested by HF remain viable in the fly excreta and the flies were able to carry and disseminate E. coli for several days (Kobayashi et al., 1999). In Japan, HF were implicated in transmission of E. coli O157:H7 from reservoir animals to other animals and humans (Moriya et al., 1999). Alam and Zurek (2004) reported the E. coli O157:H7 prevalence of 2.9 and 1.4% in HF collected in a cattle feedlot from feed bunks and cattle-feed storage, respectively. E. coli O157:H7 counts ranged from 3.0 × 10 to 1.5 × 105 CFU/fly. PCR analysis revealed that 90.4, 99.2, and 99.2% of the E. coli O157:H7 isolates (n = 125) possessed the virulence genes stx1, stx2, and eaeA, respectively (Alam and Zurek, 2004). They suggested that HF in cattle farms play a role in the dissemination of E. coli O157:H7 among animals and to the surrounding environment.
Our objectives were: (a) to assess whether HF can transmit E. coli O157:H7 to cattle; (b) to evaluate duration and concentration of E. coli O157:H7 in cattle feces and drinking water after the fly exposure; (c) to assess the spatial prevalence and concentration of E. coli O157:H7 in cattle's digestive tract at the end of the experiment.
Section snippets
Bacteria
We used four strains of E. coli O157:H7 resistant to nalidixic acid (50 μg/ml; NalR). The strains were originally isolated from feces collected from feedlots and all were positive for the virulence genes stx1, stx2, and eaeA (Sargeant et al., 2003). Bacterial suspensions from the overnight culture of each NalREcO157 strain on blood agar (Remel Laboratories, Lenexa, KS, USA) were prepared and adjusted to 5.0 McFarland turbidity in sterile phosphate-buffered saline (PBS) (pH 7.2; MP Biomedicals,
Results and discussion
None of the fecal samples of the 16 calves or the drinking water sampled from each pen before the exposure to HF was positive for NalREcO157. All samples (feces, water, and necropsy) from the eight control calves exposed to uninoculated HF were negative for the pathogen throughout the experiment.
On day 1 after the exposure, fecal samples of all calves exposed to inoculated flies and 62% of their drinking-water samples tested positive for NalREcO157 (Table 1). Feces of all exposed calves were
Acknowledgments
We thank Jan Metlevski for technical assistance and Dr. A. Broce for reviewing the manuscript. This study was supported by a U.S. Department of Agriculture Grant (2005-34359-15684). This is contribution 06-349-J of the Kansas Agricultural Experiment Station.
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