19. Robust Immunity is Linked to Fewer Gastrointestinal Parasites.

Abstract Host genetic background shapes immune function and plays an important role in the host’s ability to limit or eliminate pathogens. Recent evidence from our group demonstrated that Florida Native (FN) sheep exhibit superior resistance to multiple gastrointestinal parasite taxa. However, the mechanisms underlying this resistance require further investigation. The objective of this study was to evaluate the influence of breed on immune responsiveness and its association with gastrointestinal parasite resistance. A cohort of 44 FN and 16 Katahdin (KT) lambs, aged 4-5 months, raised under grazing conditions and naturally exposed to parasites, was used. Fecal samples were collected weekly from day 0 to 28 to determine Trichostrongylidae family fecal egg count (FEC) using the modified McMaster’s technique as a proxy for parasite resistance. Lambs were challenged with keyhole limpet hemocyanin (KLH) to evaluate humoral and cellular immune responses. Intramuscular KLH injections were administered on days 0 and 14. Blood sample collected on day 28 was used to quantify plasma anti-KLH IgG concentrations via indirect ELISA as an indicator of humoral immune response (HIR). On day 28, a delayed-type hypersensitivity test (DTH) was performed using intradermal KLH and PBS injections, and changes in skinfold thickness after 24 hours were used to estimate cellular immune response (CIR). An immune competence index (ICI) was created adding the z-scores of HIR on day 28 (HIR-28) and CIR. The fixed effect of breed on HIR-28, CIR and ICI data were analyzed using linear models, with influential points discarded using Cook’s D criteria and HIR-28 natural-log transformed to achieve normally distributed residuals. Associations of breed and immune parameters with FEC were assessed using generalized linear models assuming a negative binomial distribution with a log link to account for overdispersion in the count data. FN lambs exhibited a higher (P < 0.05) HIR-28 than KT lambs (2.12±0.042 vs. 1.83±0.068), suggesting a more robust humoral immune response. No significant differences were found between breeds for CIR (P = 0.90) and ICI (P = 0.40). Consistent with previous observations, FN lambs had significantly lower FEC than KT lambs. Additionally, a tendency for association was observed between FEC and HIR-28 (P = 0.07), whereas no associations were found between FEC and CIR (P = 0.30) or ICI (P = 0.60). These findings indicate that stronger humoral immune responsiveness may contribute to the superior gastrointestinal parasite resistance observed in Florida Native sheep. Animals with more robust humoral responses tend to have lower parasite burdens regardless of breed, highlighting the role of host genetics in shaping immune-mediated parasite control.