Fachbereich Veterinärmedizin



    Effect of repeated benzimidazole treatments with increasing dosages on the phenotype of resistance and the beta-tubulin codon 200 genotype distribution in a benzimidazole-resistant cyathostomin population (2004)

    Zeitschriftenartikel / wissenschaftlicher Beitrag
    Drogemuller, Michaela
    Failing, Klaus
    Schnieder, Thomas
    von Samson-Himmelstjerna, Georg (WE 13)
    Veterinary Parasitology; 123(3/4) — S. 201–213
    ISSN: 0304-4017
    Pubmed: 15325046
    Institut für Parasitologie und Tropenveterinärmedizin

    Robert-von-Ostertag-Str. 7-13
    Gebäude 35, 22, 23
    14163 Berlin
    +49 30 838 62310

    Abstract / Zusammenfassung

    This study was designed to investigate the effect of repeated treatments with increasingly high fenbendazole (FBZ) dosages on the phenotype and genotype of a benzimidazole (BZ)-resistant cyathostomin population. An experimentally infected horse was treated repeatedly with FBZ dose rates between 7.5 and 30.0 mg/kg body weight (bw) over approximately 2 years. Faecal egg counts (FECs) and larval cultures were performed weekly. A total of 45 faecal egg count reduction tests (FECRTs) were analysed, revealing a high variability during the course of experiment with a mean value in faecal egg count reduction (FECR) of -17% (S.D. +/- 78). The FECR was always < 90%, providing the evidence of BZ resistance. Nine egg hatch tests were performed during the course of the experiment and revealed LD(50) values between 0.20 and 0.31 microg/ml thiabendazole (TBZ) and LD(96) values of > 0.36 microg/ml TBZ, confirming the phenotype of resistance. The LD(99) varied between 0.40 and 0.63 microg/ml TBZ. Despite consecutive treatments, no noticeable increase of the LD(50), LD(96) and LD(99) values was detected for the duration of the experiment. The molecular analysis of the codon 200 of 106 third stage larvae (L3) was carried out following repeated treatments with 30 mg FBZ/kg bw. Out of these larvae 32% were homozygous TTC/TTC, 60% showed the heterozygous TTC/TAC genotype, and 8% were homozygous TAC/TAC. The resulting allele frequencies were 62% for TTC and 38% for TAC. These findings suggest that repeated BZ treatments with increasing dosages do not alter significantly the FECRT and EHT characteristics of a BZ-resistant cyathostomin population. Furthermore, it may also be concluded that, in contrast to sheep trichostrongyles, such a selection regime does not result in beta-tubulin codon 200 TAC allele autocracy.