Characterising the innate immune response in breech strike resistant and non-selected sheep to the sheep blowfly (Lucilia cuprina)

Breech strike is a major parasitic problem affecting wool-producing sheep. While skin wrinkles and fecal soiling are recognized risk factors, much of the difference in susceptibility between individual sheep remains unexplained. This study compared the early immune response of 3-year-old Merino ewes genetically selected to be resistant to breech strike to that of the non-selected ewes following a brief Lucilia cuprina larval challenge. Fourteen sheep (seven breech strike resistant and seven non-selected sheep) were challenged with L. cuprina eggs at four random sites on their back. Skin biopsies were collected 31 h post-implantation from the four infested and four mock sites (dental plugs without blow fly eggs) from each sheep and analysed using immunohistochemical staining for different cell biomarkers. A Milliplex ovine cytokine/chemokine assay was used to analyse the local cytokine response at these sites. An infiltration of leukocytes was observed at the larval feeding sites that predominantly comprised neutrophils. Significant increases in lymphocytes expressing T cell markers for CD4, CD1, CD8, T19, γδ-T cell, as well as the B cell marker CD45R, were observed compared to the mock sites. The pro-inflammatory cytokines IL-1α, IL-6, IL-17a, and chemoattractants including IL-8 and MIP-1α were significantly elevated in challenged sites. These results demonstrated a selective innate immune response in sheep following a brief larval challenge, which was similar in breech strike resistant and non-selected sheep, suggesting that the observed resistance to flystrike in the breech strike resistant flock is unlikely to be primarily mediated by local innate immune mechanisms at the tissue level.