**8.3 Genetic and phenotypic correlation between resistant traits**

Estimations of phenotypic and genetic correlation explained the amount to which genes affect two different traits and the phenotypic correlation guides the number of relations between two traits. Correlation evaluations are important in the measurement of the appropriateness of indicator traits as indirect criteria in programs related to breeding. Mandonnet *et al*., [88] under tropical conditions, stated positive

*Anthelmintic Drug Resistance in Livestock: Current Understanding and Future Trends DOI: http://dx.doi.org/10.5772/intechopen.104186*

(0.37–0.58) and negative (0.56–0.79) genetic correlations between FEC and PCV and eosinophil and FEC amount in goats. Costa *et al*., [66] in Brazil also describe a highly negative and significant relationship between changed PCV and FEC or hemoglobin −0.53 and − 0.45 in *H. contortus* infected goats. Very strong negative correlations between IgA activity and FEC have been found in *Teladorsagia circumcincta* infected Scottish Blackface lambs (−0.97, s.e. 0.11 and − 0.78, s.e. 0.18, respectively) and also in resistance-related traits and burdens of worms [90].

#### **8.4 Genetic and phenotypic parameters for production traits**

Host live weight is a production trait that has been considered as an important parameter while assessing the genetic resistances of the host toward GI nematode parasites. The heritability estimates of live weight (LWT) varied widely ranging from 0.13 in Australian Angora goats to 0.50 in Texan Angora goats [91]. Likewise, heritability estimates have been reported in South Africa goats breed as 0.29 and 0.35 [92]. It has been shown that resistance to infection by nematode parasites may not necessarily equate to resistance to the effects of the parasite challenge in grazing animals [86]. The association between FEC and productivity varies in magnitude and direction depending on the breed and the environment in which the evaluation was done. The genetic correlations between packed cell volume (PCV) and packed cell volume decline (PCVD) and production (live weight and wool growth) are either negligible or favorable [93].

Several studies around the globe have been conducted to assess the genetic potential of sheep and goats breeds that are resistant to gastrointestinal nematodes in the last three to four decades [82, 83, 87, 93]. The selection of breeds that are resistant to gastrointestinal nematode parasites is assuming the most promising alternate control method of gastrointestinal nematodes. Improved resistance toward nematodes control leads to reduced cost of anthelmintic treatment and diminished production losses associated with worm burden. Australia and New Zealand initiate programs on breeding for resistance and adopt them successfully by utilizing phenotypic markers [94]. Approximately 96% of the world's goat population is kept by smallholders in developing countries, and genetic improvement programs are rare [95].

#### **8.5 Phenotypic traits as indicators of GI resistance**

Host selection for resistance has based mostly on quantitative measurement of phenotypic traits. These traits have been measured to check the response of the host being evaluated for resistance, which are biochemical, immunological, parasitological, and pathological features [84]. For the development of high-resistant breeds, it is necessary to identify the high-resistant individuals. Criteria for the selection of parasitic resistance are commonly based on two traits, i.e., packed cell volume, which indicates anemia, and fecal egg count, which measures the amount of infection. There is variation in the development of resistance between the animals of different breeds and within the same breeds, which is because of their genetic makeup. The scientists are working to investigate the cause of the development of resistance, and up to some extent they succeeded in finding some reasons while the others are under investigation [84].
