Stable Flies (Stomoxys calcitrans L.) from Confined Beef Cattle Do Not Carry Shiga-Toxigenic Escherichia coli (STEC) in the Digestive Tract.

Background and Objective: Stable flies (Stomoxys calcitrans L.) are very common around confined and pastured cattle, and due to their painful bites they are very important animal pests. Cattle are asymptomatic reservoirs of foodborne pathogens, Escherichia coli O157:H7 and other Shiga-toxigenic E. coli serotypes (STEC). In the present study, the potential of stable flies to carry STEC in a beef cattle feedlot was assessed.
Methods: Stable flies (n = 180) were collected over 3 summer months and processed individually for STEC-8 that included the serotype O157 and seven non-O157 serotypes (O26, O45, O103, O104, O111, O121, and O145). Isolation and detection of STEC was based on direct plating as well as the enrichment/immunomagnetic separation approach. Modified Posse agar (mP) was used for culturing non-O157 serotypes and sorbitol MacConkey agar with cefixime and potassium tellurite (CT-SMAC) for E. coli O157. Multiplex polymerase chain reactions were used for differentiation of individual serotypes and detection of virulence genes (stx1, stx2, eae, and ehxA).
Results and Conclusions: Of 180 stable flies, 67 (37.2%) carried enterics on mP (mean: 3.6 ± 1.05 × 10(6) colony-forming units [CFU]/fly) and 55/180 (30.5%) were positive for bacteria on CT-SMAC (mean: 1.2 ± 1.08 × 10(4) CFU/fly). However, stable flies positive for E. coli serotypes of interest were very rare (prevalence: 1.1%). The three serotype-positive isolates, two E. coli O26 and one E. coli O45, were recovered from two flies and neither of them harbored the virulence genes. We conclude that stable flies likely do not play a role as a biological vector and/or reservoir of STEC-8 in cattle feedlots.