Sheep & Goats

Exploring the extent of resistance to gastrointestinal parasites in sheep

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Abstract

Gastrointestinal endoparasites are one of the main health challenges in sheep operations, increasing production costs and impairing animal performance. Selection for gastrointestinal parasite resistance in sheep primarily targets nematodes of the Trichostrongylidae family (TRI), which includes Haemonchus contortus, the most prevalent and pathogenic species in many regions. Fecal egg count (FEC) is widely used as a proxy for resistance to trichostrongylid nematodes; however, the ability of resistant breeds to deal with other endoparasite taxonomical groups requires further exploration. The objective of this study was to evaluate the capacity of TRI-parasite-resistant breeds to cope with endoparasites from different taxa. A cohort of 44 Florida Native (FN) and 16 Katahdin (KT) lambs, aged 4-5 months, raised under co-grazing conditions since birth, was used. Lambs were dewormed at weaning, at 2-3 months of age, and naturally re-infested with parasites before and during the trial. Fecal samples were collected weekly from day 0 to 28 to determine FEC for TRI (nematodes), Strongyloididae (STR; nematodes) and Moniezia spp. (MON; cestodes), as well as fecal oocyst count (FOC) for Eimeria spp. (EIM; protozoan), both measured as eggs/oocyst per gram (EPG, OPG), using the modified McMaster's technique. Blood samples were collected on day 0 and 28 to measure packed cell volume (PCV, %) and total serum protein (TSP, g/dL), as indicators of anemia and nutritional status, respectively, as proxies for health status. FEC and FOC data were analyzed using generalized linear mixed models under a repeated measures framework, assuming a negative binomial distribution with a natural-log link function to account for overdispersion in the count data. The models included the fixed effects of breed and collection point and the random effect of animal. Furthermore, linear mixed models with repeated measures were fitted to analyze Box-Cox-normalized PCV and TSP, including the same fixed and random effects described above. FN showed significantly lower (P < 0.05) overall FEC and FOC compared to KT across all parasite groups: TRI (178.0±34.1 vs. 467.0±140.0), STR (84.7±52.2 vs. 296.0±110.0), MON (54.5±23.2 vs. 112.3±48.9) and EIM (85.9±17.7 vs. 156.4±39.4), demonstrating a stronger ability to suppress parasite establishment and reproduction, potentially reflecting more effective immune responses. Additionally, FN also exhibited significantly higher percentages of PCV (32.0±0.47 vs. 30.1±0.76) and TSP concentrations (9.32±0.052 vs 8.78±0.085) than KT, suggesting a lower parasite-associated impact on the animals' health status. In general, FN displayed greater parasite resistance across multiple endoparasite taxonomic groups, indicating that the ability to cope with gastrointestinal parasites extends beyond nematodes of the Trichostrongylidae family. These findings suggest that parasite resistance may be linked to enhanced immune competence, and the higher PCV and TSP values observed in FN compared to KT further support a connection between parasite resistance and improved overall health status.

Keywords: 2026

How to Cite:

Alvarado-Vargas, A., Boscollo, P., Ladeira, G., Alvares, L., Cavender, C., Diehl, B. & Rezende, F., (2026) “Exploring the extent of resistance to gastrointestinal parasites in sheep”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2287322. doi: https://doi.org/10.31274/wcgalp.24265

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