EVALUATION OF NATURAL RESISTANCE STATUS OF SHEEP AGAINST HAEMONCHUS CONTORTUS INFECTION

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Date
2023-02
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SRI VENKATESWARA VETERINARY UNIVERSITY TIRUPATI - 517 502. (A.P.) INDIA
Abstract
Anthelmintic resistance is a major concern throughout the world, especially for the control of Haemonchus contortus. Selection of genetically resistant animals is one alternative to control infection that minimize herd use of anthelmintics. The aim of the current study was to explore associations between phenotypic traits for H. contortus and polymorphisms in the DRB1 region of major histocompatibility complex and GALNT6 gene involved in gastrointestinal mucus production in a Nellore (n=95), Deccani (n=28) and Kenguri (n=34) sheep naturally infected with H. contortus. Blood and faecal samples were collected to evaluate faecal worm egg counts (FEC), packed cell volume (PCV), haemoglobin (Hb), eosinophilia and for DNA isolation. Animals were grouped into high FEC (susceptible) and low FEC (resistant) group based on EPG counts. FEC and circulating eosinophilia were higher in susceptible group. Log FEC was negatively correlated (P < 0.01) with PCV, and Hb estimates. The second exon of DRB1 and intron variant of GALNT6 genes were amplified by polymerase chain reaction from DNA samples of resistant and susceptible sheep. PCR products were characterized by the restriction fragment length polymorphism technique using restriction enzymes. Two genotypes (‘bb’ and ‘ab’) were observed following digestion of Ovar-DRB1 with PstI. Genotype frequencies were 0.78 for ‘bb’ and 0.22 for ‘ab’. The frequency of ‘a’ and ‘b’ alleles were 0.11 and 0.89, respectively. The genotype frequencies differed significantly between both the groups (P<0.05). A generalized linear model was used to test effects of genotype on FEC, Hb and PCV. The ‘bb’ genotypes had higher (P<0.05) log FEC value than ‘ab’ genotypes and ‘b’ allele was linked with susceptibility to haemonchosis in sheep. The mean FEC of Nellore sheep was high (P<0.01) indicating susceptibility of the breed and also in which the frequency of ‘b’ allele was more compared to other two breeds. Ovar-DRB1 genotypes associated to FEC did not affect PCV and Hb. Digestion of GALNT6 amplicons with AluI revealed undigested single fragment referred to ‘aa’ genotypes. Since monomorphic no further association studies could be conducted for this gene. In conclusion, Ovar-DRB1 could be a reliable marker for selecting sheep resistant against H. contortus.
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