A woman recently returned from vacation in Mexico presents with fever and flu-like symptoms after consuming cheese made from unpasteurized milk. The most likely etiologic agent is which organism?

• A. Bordetella pertussis
• B. Listeria monocytogenes
• C. Staphylococcus aureus
• D. Yersinia enterocolitica

Answer: (B)

Unpasteurized dairy products, especially soft cheeses, are classic vehicles for Listeria monocytogenes. This organism can grow at refrigeration temperatures, allowing it to multiply in cheeses stored in the fridge. In healthy adults, ingestion often produces fever and flu-like symptoms or mild gastroenteritis. Listeria is a gram-positive, facultative intracellular rod that invades intestinal cells, disseminates via macrophages, and can cause systemic illness; it’s especially concerning for pregnancy, where it can cross the placenta and affect the fetus. The other organisms fit different clinical pictures: Bordetella pertussis causes whooping cough; Staphylococcus aureus food poisoning typically presents with rapid-onset vomiting from preformed toxins; Yersinia enterocolitica commonly presents with gastroenteritis or mesenteric adenitis and is not the classic association with unpasteurized dairy.