In this work we demonstrated that the acquisition of benzimidazole (BZ) resistance in the small-ruminant parasite Teladorsagia circumcincta is linked to the selection of individuals that are characterized by a tyrosine (Tyr) at amino acid 200 of their isotype 1 beta-tubulin gene. This mutation appears to be recessive, since only homozygous mutant (Tyr/Tyr) individuals survived after BZ treatment of two resistant populations in which the three genotypes (rr, rs, ss) were initially present. In comparison with natural BZ-susceptible populations, a decrease in the restriction polymorphism (RFLP) of the isotype 1 beta-tubulin gene was observed in natural resistant populations. It seems that this decrease in beta-tubulin polymorphism results from the selection of homozygous mutant individuals.

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We compared, some fitness-related traits of benzimidazole resistant (rr) and susceptible (rS, SS) worms of Teladorsagia circumcincta, a gastrointestinal parasite of the small ruminants, under laboratory conditions. PCR was used to determine the genotypes (rr, SS, rS) and the fitness of each was compared within the same strain. There was no significant difference in egg production, development rate from egg to infective larvae stage, establishment of these larvae in the host or the survival of adult worms and infective larvae for the 3 genotypes. The same results were obtained for the establishment rate of larvae in the host and the production of infective larvae under conditions of strong competition between resistant and susceptible worms. The fact that there were no differences in fitness suggests that the installation of benzimidazole resistance in a worm population is irreversible. This agrees with field observations.

Four species of lungworms of the Metastrongylus genus are found sympatrically in the lungs of the wild boars (Sus scrofa L.) on the Chambord game reserve (France): Metastrongylus asymmetricus, M. confusus, M. pudendotectus, and M. salmi. These species are difficult to identify using morphological characters alone, and epidemiological evidence raises doubt about the existence of 4 distinct species. Two molecular approaches (sequencing of ribosomal DNA Internal Transcribed Spacer 2 [ITS 2] and Random Amplified Polymorphic DNA assay [RAPD]) were used to evaluate the genetic similarities between the 4 taxa. The ITS 2 sequences of M. salmi and M. confusus were identical, whereas the sequences of the other species were distinct. On the other hand, RAPD analysis indicated unambiguously that the 4 species are genetically different. These results reveal the limitation of using only 1 molecular approach for taxonomic studies and indicate the need to examine different regions (ribosomal, mitochondrial DNA) in the genome of a species in complement with morphological and epidemiological data.