Temporal changes in circulatory blood cell parameters of sheep genetically different for faecal worm egg count and diarrhoea from late summer to spring in a Mediterranean environment

Abstract

Differences in haematology traits were investigated in worm-resistant and worm-susceptible Merino sheep that were genetically prone or less prone to developing diarrhoea in a winter rainfall region. The experiment was a 2 × 2 × 2 × 5 design. Male and female sheep aged 8–9 months old identified as having high (H) or low (L) breeding values for dags (breech soiling) and for worm egg counts (WEC) were allocated to one of four genetic groups: HH, HL, LH and LL (the first letter refers to dags and the second to WEC). The animals were sampled five times from autumn through winter to spring, in March, May, June (ewes only), July (rams only), August and in September after exposure to a natural paddock infection dominated by Trichostrongylus, Teladorsagia, Chabertia and Oesophagostomum. Measurements were packed cell volume, haemoglobin, red blood cell count, differential white cell counts, WEC and faecal consistency score. At all sampling times, there were large (P < 0.01) differences between groups within sex for both the faecal consistency score and WEC. The genetic effect for WEC was large (P < 0.01), as worm-susceptible sheep shed 10- (ewes) and 6-fold (rams) as many worm eggs at their WEC peak as the worm-resistant sheep at the end of the experiment. In the high-dag groups, the faecal consistency score was ~0.5 units higher for rams and 0.4 units higher for ewes, throughout the experiment. The optimum time to measure dags and WEC in this environment appears to be 6–8 weeks after the start of the winter rain. No differences were found among any of the treatment groups for any haematology trait. The haematogram changed significantly throughout the experiment, but genetic selection for low-dag score (reduced diarrhoea) or low WEC did not provide an acceptable discriminator in circulatory haematology traits between these genetically distinct groups of Merino sheep. It was concluded that circulatory haematology traits do not offer opportunities to select against diarrhoea or increased worm resistance in sheep.