Occurrence of Listeria species in raw milk in farms on the outskirts of Mexico city

Listerosis may be transmitted by direct contact with infected animals or by consumption of contaminated vegetables or meat and milk products. In Mexico, raw milk is widely consumed and the incidence of milkborne disease is unknown. A total of 1300 raw milk samples were obtained from 20/bulk tanks at four different dairy farms in southeast of Mexico City from June 1998 to June 1999. The samples were enriched for 48 h at 30°C and plated onto McBride's Modified Agar (MMA). Suspect colonies were biochemically tested to confirm identity. Overall, 23% of all raw milk samples examined tested positive for Listeria species; 13% were positive for L. monocytogenes (45.6% were serotype-4b and 54.4% were serotype 1); 6% for L. ivanovii; 4% for L. seeligeri and 1% for L. innocua. L. monocytogenes contamination was more frequent during the spring and summer months as isolation rates were 12.2%from June to October 1998 and 17% from March to June 1999. Serotype-4b isolates were not pathogenic for the mouse, while for serotype-1, strains DL₅₀ ranged from 1.8×10⁶ to 4×10⁷ CFU ml^-1. Additional studies are needed to assess the public health impact of contaminated milk in Mexico.