This study evaluated four models based on the number of eggs per gram of faeces (EPG) to segregate Pelibuey or Katahdin ewes during the lactation period into resistant or susceptible to gastrointestinal nematodes (GIN) in tropical Mexico. Nine hundred and thirty EPG counts of Pelibuey ewes and 710 of Katahdin ewes were obtained during 10 weeks of lactation. Ewes were segregated into resistant, intermediate and susceptible, using their individual EPG every week. Then, data of every ewe was used to provide a reference classification, which included all the EPG values of each animal. Then, four models were evaluated against such reference. Model 1 was based on the 10-week mean EPG count ± 2 SE. Models 2, 3 and 4 were based on the mean EPG count of 10, 5 and 2 weeks of lactation. The cutoff points for the segregation of ewe in those three models were the quartiles ≤Q1 (low elimination) and ≥Q3 (high elimination). In all the models evaluated, the ewes classified as resistant had lower EPG than intermediates and susceptible (P < 0.001) while ewes classified as susceptible had higher EPG than intermediate and resistant (P < 0.001). According to J Youden test, models presented concordance with the reference group (>70 %). Model 3 tended to show higher sensitivity and specificity with the reference data, but no difference was found with other models. The present study showed that the phenotypic marker EPG might serve to identify and segregate populations of adult ewes during the lactation period. All models used served to segregate Pelibuey and Katahdin ewes into resistant, intermediate and susceptible. The model 3 (mean of 5 weeks) could be used because it required less sampling effort without losing sensitivity or specificity in the segregation of animals. However, model 2 (mean of 2 weeks) was less labour-intensive.

This study compared the phenotypic susceptibility of Pelibuey and Katahdin female lambs against gastrointestinal nematodes (GINs) under grazing conditions in the hot humid tropics of Mexico. The study was performed during the rainy season (August to October, 2014). It included 27 Pelibuey and 12 Katahdin female lambs from 6 months of age and live weight of 21.0 ± 3.7 and 23.3 ± 3.6 kg, respectively. Lambs were reared free of GIN infection before the study. The study lasted 91 days. Animals were weighed and sampled (blood and feces) on days 0 and 28 and every 7 days onwards. Fecal samples were obtained to determine fecal eggs of GIN per gram (EPG), and blood samples were used to determine the packed cell volume (PCV), the peripheral eosinophil counts (PECs), and optical densities (ODs) for IgA. The EPG counts were significantly lower for Pelibuey lambs compared to Katahdin throughout the study (P < 0.001). Similarly, Pelibuey lambs had higher mean PCV (P < 0.01) and PEC (P < 0.05) than Katahdin lambs during the study. The total weight gain and OD for IgA were similar between breeds (P > 0.05). Negative associations (P < 0.05) between EPG and PCV or PEC were moderate to strong for the lambs of both breeds. No association was found between EPG and IgA. In conclusion, Pelibuey lambs showed phenotypic evidence of higher resistance to natural GIN infections compared to Katahdin lambs sharing the same grazing conditions in the hot humid tropics. The most accurate phenotypic markers to identify a difference in susceptibility were EPG and PEC.