Detection and virulent profiles of multidrug-resistant Shiga toxin-producing Escherichia coli (STEC) from horses in Nigeria

Shiga toxin-producing Escherichia coli (STEC) are significant zoonotic pathogens worldwide, causing diseases ranging from diarrhea to hemolytic uremic syndrome (HUS). Ruminants, mainly cattle, are considered the primary reservoirs; however, the role of horses in the epidemiology of STEC remains poorly understood in Nigeria. A total of 231 rectal swabs were collected from horses across selected equine facilities in five Nigerian states. Samples were processed following standard procedures, and isolates were confirmed using the Analytical Profile Index (API 20E) kit. Antibiotic susceptibility testing (AST) was performed using the Kirby-Bauer disk diffusion method with a panel of nine antibiotics, and the isolated Escherichia (E.) coli were classified as sensitive or resistant according to CLSI guidelines. The Multiple Antibiotic Resistance (MAR) index was calculated to assess the risk of contamination. The E. coli isolates were screened for selected Shiga toxin genes (stx1, stx2 and eae) using polymerase chain reaction (PCR). Fourteen isolates (6%) were identified as Shiga toxin-producing E. coli; of these, 78.6% (11/14) expressed at least one virulence gene: 42.9% (6/14) carried stx2, 35.7% (5/14) carried eae, and no isolate carried stx1. All confirmed STEC isolates exhibited multidrug resistance (MDR). The isolates showed complete resistance (100%) to cefuroxime and ciprofloxacin, while all were susceptible to ofloxacin. MAR indices ranged from 0.2 to 0.8, with 77.8% of isolates exhibiting MAR indices ≥0.5. This study reports the detection of MDR STEC in horses in Nigeria, revealing high-level resistance to clinically important antibiotics and significant carriage of virulence genes. The high MAR indices indicate high-risk contamination sources. The detection of MDR STEC in this study underscores a significant health risk to horses, with potential for zoonotic transmission to humans and the environment.