Phenotype characterization of Pelibuey native lambs resistant to Haemonchus contortus.

Specific phenotypic traits of the blood-feeder nematode Haemonchus contortus were characterized on Pelibuey native, cross-breed sheep in a tropical region of Mexico with background of anthelmintic problems. Data were collected on 4-month-old lambs (n= 63) and analyzed weekly in three stages: stage 1 (S1, not infected (0-3 weeks)); S2, first infection with 350 H. contortus infective larvae (L(3)) per kg of body weight by oral route (3-10 weeks); and S3, second infection with 175 H. contortus L(3) per kg of body weight by oral route (10-18 weeks). Once infection took place, sheep grazed in a free-nematode experimental paddock, and hemoncosis-related traits, such as the number of eggs per gram of feces (epg), packed cell volume (%pvc), and eosinophils (%eo), were measured each week. Results were analyzed on specific traits and stages using SAS statistics. Statistical differences of the main phenotypic traits (P < 0.05) were observed throughout S2 on Pelibuey high (hr) and low (lr) responders, respectively: epg mean, 475 +/- 340.5 and 1439 +/- 243.6; %pvc mean, 31 +/- 0.3 and 27 +/- 0.2; and %eo mean, 2.5 +/- 0.3 and 1.99 +/- 0.2. In addition, lr and hr sheep grazed for 3 months on an area infected with Haemonchus, Cooperia, and Strongyloides, and the hr sheep showed a low number of epg after being exposed to multiple infections, so that no further treatment was required. These results showed that the selection of animals resistant to nematodes can be an alternative method of control against anthelmintic resistance problems in order to improve animal health and avoid a negative environmental impact.