Evaluation of different models to segregate Pelibuey and Katahdin ewes into resistant or susceptible to gastrointestinal nematodes

Abstract

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.

Comparison of two phenotypical methods to segregate resistant and susceptible lambs to parasitic nematodes

“…The classified groups showed significant differences (P<0.05) between RES and SUS lambs in both the FEC and PCV. The RES lambs showed lower FEC and higher PCV than SUS lambs, as indicated in another study with Pelibuey ewes during lactation (Palomo-Couoh et al 2016).…”

“…Where Po = total proportion of concordance observed and Pe = expected random proportion. The concordance value (Youden's J), sensitivity, specificity, predicted positive and negative values, the proportion of false positives and false negatives, and the accuracy were calculated according to Palomo-Couoh et al (2016) relative to an assumed reference method (M3SE). To perform the validity test, the data were grouped in 2 × 2 contingency tables, considering as reference the method of M3SE, because the mean values together with the standard deviation are part of the theory of the selection of animals used to calculate the genetic progress of phenotypic characteristics (Morris et al 2000, McRae et al 2014.…”

“…The other method uses practical rules and categorises animals as resistant when they have a FEC <1000 eggs per gram of faeces (EPG), intermediate when the FEC is between 1000 and 2500 EPG and susceptible if it is >2500 EPG (Fakae et al 2004). In recent years, hair sheep have been segregated using the quartile method (Palomo-Couoh et al 2016, Zaragoza-Vera et al 2019. Others studied indicators include PCV, total plasma protein (TPP), pepsinogen, blood cell count and immunoglobulin, mainly IgA and IgE.…”

Phenotypic expression of parasite susceptibility to Haemonchus contortus in Pelibuey sheep

“…Intermediate animals were also recently identified in groups of adult hair sheep classified according to their FEC during the post-partum period (Palomo-Couoh et al, 2016). In that study, ewes classified as resistant had lower FEC than those classified as intermediate or susceptible (P < 0.001), while ewes classified as susceptible had higher FEC than those classified as intermediate and resistant (P < 0.001) during a 10-week post-partum period.…”

Factors affecting the productivity and the efficiency at weaning of hair ewes in a dry tropical environment

“…However, differences exist among hair sheep. On one hand, Dorper and Katahdin sheep are less resistance to internal parasites than Pelibuey and Blackbelly (Notter 2000;Palomo-Cohuo et al, 2016). On the other hand, adult ewe body size is higher in Katahdin and Dorper than Blackbelly (Notter 2000) and Pelibuey (Chay-Canul et al, 2016).…”