THE RELATIONSHIP BETWEEN FECAL EGG COUNT REDUCTION AND THE LETHAL DOSE 50-PERCENT IN THE EGG HATCH ASSAY AND LARVAL DEVELOPMENT ASSAY

The relationship between resistance detected in the faecal egg count r eduction test (FECRT) and the lethal dose 50% (LD50) in the egg hatch assay (EIIA) for benzimidazoles (BZs) and a larval development assay ( LDA) for BZs, levamisole (LEV) and ivermectin (IVM) was examined on 13 sheep farms and 12 goat farms in Denmark. Out of 10 farms where resis tance to BZs was detected according to the FECRT, nine (90%) had LD50 values above 0-5 mu M thiabendazole (TBZ) (0.1 mu g TBZ/ml) in the EHA , indicating resistance to BZs. However, four out of the 12 isolates s usceptible to BZs in the FECRT had LD50 values higher than 0.5 mu M TB Z in the EHA. For all isolates examined, LD50 values for TBZ in the LD A were lower than in the EHA. Four out of 11 and five out of 12 farms with worm populations resistant to BZs according to the FECRT and EHA respectively, had LD50 values lower than 0.5 mu M TBZ in the LDA. Usin g the same cut-off point for resistant isolates in the LDA as in the E HA (0.5 mu M TBZ), these isolates would be considered susceptible to B Zs. All 10 isolates susceptible to BZs according to the FECRT and EHA and two isolates with suspect BZ resistance had LD50 values lower than 0.5 mu M TBZ in the LDA. The above results indicated fairly good agre ement in the detection of BZ resistance between the FECRT, EHA and the LDA. Groups of farms where resistance to LEV was detected according t o the FECRT had higher mean LD50 values compared to those with LEV-sus ceptible or suspected resistant isolates. However, only four out of 12 farms having isolates resistant to LEV had LD50 values higher than 1. 2 mu M LEV (0.28 mu g LEV/ml) recorded previously fore LEV-susceptible strain of Ostertagia circumcincta. This indicated discrepancies in de claring resistance to LEV between the FECRT and the LDA. Isolates fi o m four farms where resistance to IVM was detected in the FECRT had LD5 0 values higher than the susceptible isolates. These were 2.5 to 7.5 t imes higher than those recorded previously for IVM-susceptible strains . (C) 1998 Elsevier Science B.V. All rights reserved.