Which species poses the greatest zoonotic risk for E. coli O157:H7?

• A. Pigs
• B. Sheep
• C. Horses
• D. Cattle

Answer: (D)

Cattle pose the greatest zoonotic risk for E. coli O157:H7. They are the main reservoir of this organism, often carrying it in their intestines without signs of illness and shedding it in feces, especially at the recto-anal junction. This prolific shedding, combined with practices in farming, transport, and slaughter, makes carcasses and the environment easy to contaminate. Ground beef and unpasteurized dairy products are well-known vehicles for transmission to humans, so outbreaks are frequently linked to beef products or dairy from cattle operations. While other species like sheep can harbor the bacteria and horses or pigs can carry it in some contexts, the overall likelihood and scale of human exposure are greatest with cattle. The low infectious dose of this organism means even small amounts of contamination can cause illness, underscoring why cattle represent the top zoonotic risk.