Prevalence of non-O157 shiga toxin-producing Escherichia coli (STEC) in house flies (Musca domestica L.) from cattle feedlots and dairies

Shiga toxin-producing Escherichia coli (STEC) are major foodborne pathogens affecting people worldwide. Cattle are asymptomatic reservoirs of STEC and bacteria are released to the environment in animal feces. The present study aimed to assess the prevalence of seven non-O157 STEC serogroups (O104, O26, O45, O145, O103, O121, and O111), in house flies from confined cattle environments. House flies were collected in summer and fall from a total of nine cattle feedlots located in Nebraska, Oklahoma and Texas and three dairy farms in Minnesota, Florida and California. House flies were shipped to the K-State laboratory overnight on ice surface sterilized, and individually tested for STEC by: 1) direct plating and 2) enrichment followed by immunomagnetic separation. Individual serogroups and virulence traits were confirmed using multiplex PCR. Concentration of enterics ranged from 4 log–7 log CFU/fly. Out of 463 house flies, 159 (34.3%) carried E. coli serogroups of interest (O104, O103, O45, O121, O26, and O145) and  34 (7.3%) house flies were positive for more than one serogroup. STEC was found in 7 (1.5%) flies. Escherichia coli O103 and O104 harbored stx1 and ehxA while E. coli O45 carried stx1, eae, and ehxA. STEC prevalence in house flies in cattle feedlots was relatively low; however, due to very large fly populations, this represents many house flies carrying STEC. House flies may play a role as a vector of non-O157 STEC because of their synanthropic nature and unrestricted movement.