Study area
This study was carried out in the Municipality of Lerma de Villada, State of Mexico (19°13’–19°26’N, 99°22’–99°34’W). The Lerma marshes provide a suitable habitat for thousands of migratory waterfowl than arrived annually from Canada and the United States in the Mexican Central Plateau on their migratory route to the south (Barragán Severo, López-López, & Babb Stanley, 2002). Animals were sampled from ten different study units: wild ducks from the Lerma marshes, as well as pigs and poultry from nine backyard farms located within a 6 Km radius of the Lerma marshes boundaries (Figure 1).