Cooperia is the most prevalent helminth parasitizing calves in Brazil. Three species of this genus occur most often: C. punctata, C. pectinata and C. oncophora. Six calves from dairy farms in the south of the State of Minas Gerais aged six to 15 months were killed and necropsied each month over a period of two years. The Cooperia species were identified, counted, and the numbers related to the calves' age. The worm burdens due to the three species of Cooperia were statistically different. C. punctata was the most prevalent species and had a positive correlation with the age of the calves; C. pectinata appeared with lower intensity but was always present, and C. oncophora was not found in calves older than 11 months. These results show the existence of different degrees of resistance to Cooperia species among calves as the three species did not behave similarly. It seems to be an acquired resistance. C. punctata appears to be less immunogenic than C. pectinata and C. oncophora. As C. punctata and C. pectinata are more pathogenic than C. oncophora, it seems that this pathogenicity can be related to immunogenic aspects associated with the species.

A polymorphic set of 14 kDa excretory–secretory (E–S) antigen-encoding cDNAs, with similarity to a previously characterized 15 kDa E–S antigen of Haemonchus contortus, was cloned from Cooperia punctata. Five cDNAs encoding predicted proteins of 70–80% identity were sequenced. Nucleotide sequence data reported in this manuscript are available in the GenBankTM databases under the accession numbers AF352718, AF352719, AF352720, AF352721, AF352722, AF352723, AF352724, AF352724, AF352726 and AF352727. Genomic analyses of individuals proved the existence of three 14 kDa E–S antigen-encoding genes, excluding that the differences reflected polymorphisms between individuals in a population. Southern blots indicated the presence of additional members of this gene family. Thus, despite the fact that heterologously expressed C. punctata 14 kDa E–S products are shown to be recognized by immune sera, potential pitfalls in the development of a recombinant vaccine are presented by this genetic diversity. Vaccine design could be further rationalized by knowledge of the function, and possible redundancy in function, of the E–S products which is presently lacking. The limitations encountered in assigning a function to the 14/15 kDa family of E–S proteins that is thus far unique to the trichostrongyloid nematodes are discussed.