RESUMEN
The objective of the study was to evaluate phenotypic resistance against gastrointestinal nematodes in Blackbelly, Pelibuey and Katahdin ewes before pregnancy in the humid tropics of Mexico. Individual faecal and blood samples were taken in 59 Pelibuey, 69 Blackbelly and 73 Katahdin ewes. The egg count per gram of faeces (EPG) of gastrointestinal nematodes (GINs) was determined. The percentage of packed cell volume (PCV) and body condition score (BCS) of each animal were also recorded. The ewes were segregated as susceptible, intermediate or resistant based on the EPG using the quartile method. The data were analysed using the general linear method, and the means between breeds were compared by Tukey's test. The relationships between the EPG, PCV and BCS were evaluated by Spearman correlation. The Katahdin ewes showed the highest EPG counts (3613.6 ± 5649) compared to the Blackbelly and Pelibuey ewes (576.1 ± 1009 and 56.8 ± 187, respectively, P < 0.01). The PCV values between breeds were similar (P > 0.05). The susceptible ewes had the highest EPG counts and the lowest PCV percentage (5069 ± 6404 and 22.8% ± 8.1% respectively) compared to the resistant ewes (P < 0.01). A higher percentage of Katahdin ewes were susceptible compared to the other breeds (P < 0.05). The main GIN species were Haemonchus contortus, Trichostrongylus colubriformis and Cooperia curticei. In conclusion, Katahdin ewes showed susceptibility to GIN compared to Blackbelly and Pelibuey ewes before the pregnancy period in the humid tropics of Mexico.