Prevalence of non-O157 Shiga-toxigenic Escherichia coli (STEC) in muscoid flies from confined cattle environment

Muscoid flies, especially house flies (Musca domestica L.) (HF) and stable flies (Stomoxys calcitrans L.) (SF), build very large populations in the confined cattle environment. The larval development of these flies takes place primarily in the cattle manure, which serves as a reservoir of Shiga toxigenic Escherichia coli (STEC). This study aimed to assess the prevalence of seven serotypes of non-O157 STEC (O104, O26, O45, O145, O103, O121, O111) in adult HF and SF from cattle feedlots and dairies. House flies (n=463) from nine feedlots and three dairies and stable flies (n=180) from a feedlot were collected. Flies were surface sterilized, and individually tested for STEC by: 1) direct plating and 2) enrichment followed by immunomagnetic separation. Individual serotypes and virulence traits were detected by multiplex PCR. Out of 463 HF, 159 (34.3%) carried E. coli serotypes of interest (O104, O103, O45, O121, O26, and O145) but STEC was found in 1.5% of HF. Escherichia coli O103 and O104 harbored stx1and ehxA and E. coli O45 carried stx1, eae, and ehxA. In contrast, SF appear to have a very limited vector competence for STEC. Only two serotypes (O45 and O26) were detected in two SF (prevalence 1.1%) and neither of them harbored virulence genes tested. The results from our study indicate that HF but not SF play a role as a vector and/or reservoir of STEC in confined beef cattle.