Abstract
SummaryParasite burden varies widely between individuals within a population, and can covary with multiple aspects of individual phenotype. Here we investigate the sources of variation in faecal strongyle eggs counts, and its association with body weight and a suite of haematological measures, in a cohort of indigenous zebu calves in Western Kenya, using relatedness matrices reconstructed from single nucleotide polymorphism (SNP) genotypes. Strongyle egg count was heritable (h2 = 23·9%, s.e. = 11·8%) and we also found heritability of white blood cell counts (WBC) (h2 = 27·6%, s.e. = 10·6%). All the traits investigated showed negative phenotypic covariances with strongyle egg count throughout the first year: high worm counts were associated with low values of WBC, red blood cell count, total serum protein and absolute eosinophil count. Furthermore, calf body weight at 1 week old was a significant predictor of strongyle EPG at 16–51 weeks, with smaller calves having a higher strongyle egg count later in life. Our results indicate a genetic basis to strongyle EPG in this population, and also reveal consistently strong negative associations between strongyle infection and other important aspects of the multivariate phenotype.
Highlights
Gastrointestinal parasite infections of livestock are responsible for large economic losses in pastoral systems (Keyyu et al 2003)
We explored the effect of varying the quality control parameters and the number of single nucleotide polymorphism (SNP) included in the identity by state (IBS) matrix on the resulting estimates of heritability; details are given in Supplementary Tables 1 and 2; in general, estimates of heritability for strongyle eggs per gramme of faeces (EPG) increased with increasing marker density
In this study we have used relationship matrices reconstructed from SNP genotypes to demonstrate evidence for heritable variation in strongyle EPG in EASZ
Summary
Gastrointestinal parasite infections of livestock are responsible for large economic losses in pastoral systems (Keyyu et al 2003). They reduce weight gain and fertility, and may even cause direct losses through mortality (Wymann et al 2008). Management of parasite infection requires an understanding of the causes of variation in parasite burdens, variation which can be substantial even between individuals within a population. In indigenous East African Shorthorn Zebu (Bos indicus, EASZ) calves in Western Kenya, most individuals experience an apparently low level of strongyle worm infection, whilst others experience a high worm burden and suffer severe consequences (Thumbi et al 2013a). We explore possible causes of this variation, and quantify its covariation with other variables
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