Assessment of monthly tick count variation and mapping of genomic regions associated with cattle resistance to African ticks

Ticks are among the most important livestock parasites and cause annual economic losses amounting to US$20–30 billion worldwide. Currently there is no sustainable tick control method. Acaricides and tick vaccines are expensive and difficult to manage. Genomic selection for host resistance presents an alternative tick control strategy that is less costly and more sustainable. The primary objective of this study was to assess variation in monthly tick count, as well as identify genomic regions associated with host resistance to ticks in South African
Nguni cattle. Tick counts on the animal’s perineum were recorded in the month of November for 2 years (2012 and 2014) from 586 Nguni cattle reared in 4 herds under natural grazing conditions. Tick count was transformed using log10(x + 1) and the resulting values were examined for normality. DNA of the animals was genotyped using the Illumina BovineSNP50 assay. After quality control (call rate >90%, minor allele frequency >0.02), 40,436 SNPs were retained for analysis. Heritability estimates were computed using mixed linear models, fitting univariate sire models. Fixed effects considered were location, sex, year and age as a covariate. An association analysis for tick resistance was carried out using a genome-wide association (GWA) analysis of the GenABEL
package. The Bonferroni genome-wide (P < 0.05) corrected significance threshold was 1.24 × 10⁻⁶, with 2.47 × 10⁻⁵ as the suggestive significance threshold (P < 0.10) (i.e., one false positive per genome scan) in the GWA analysis. A heritability estimate of 0.33 ± 0.17 was obtained for perineum tick count trait. Genome-wide significant regions were identified on chromosome 1, while other regions significant at the suggestive level were found on chromosomes 1, 11 and 15. The observed genetic variation of tick count in Nguni cattle forms the basis to explore genetic architecture of cattle resistance to ticks and may provide the potential for marker-assisted selection as an alternative tick control strategy.