Phenotypic assessment of resistance in a flock of Pelibuey lambs experimentally infected with Haemonchus contortus.

The aim of this study was to determine the phenotypic variables commonly evaluated to segregate Pelibuey lambs as resistant or susceptible to Haemonchus contortus infection. Twenty-one lambs were infected over 3 consecutive weeks with 200 larvae per kg of body weight. The lambs were dewormed 42 days after the first infection. A second infection was performed during the growth stage (32 weeks of age), and a third infection at 1 year of age. After the first infection, the lambs were segregated as resistant (<761 eggs per gram of feces, EPG) or susceptible (>761 EPG). Packed cell volume (PCV) and total plasma protein (TPPr) during the first stage were lower (25.14% and 6.56 g/dL) than during the second stage (30.06% and 7.1 g/dL, respectively). A notable decrease in EPG values in both groups occurred from the first to the second experimental infection (resistant: 510 ± 663 to 26 ± 83 EPG; susceptible: 3621 ± 6246 to 76 ± 361 EPG). Pelibuey lambs can be segregated as susceptible or resistant to H. contortus infection from 5 months of age following the first infection. The most informative phenotypic markers for segregation identified in the present study were EPG, PCV, and TPPr.