Persistence of ivermectin in plasma and faeces following administration ofa sustained-release bolus to cattle

Six calves (weight 210 to 230 kg) were dosed with an intra-ruminal slow-rel ease bolus prepared to deliver ivermectin at a low daily dosage for 135 day s. Ivermectin concentrations in jugular blood 160 days post-treatment were determined by high performance liquid chromatography (HPLC) using fluoresce nce detection. Ivermectin plasma concentrations increased gradually to achi eve the steady-state concentration (20 ng ml(-1)) at approximately four day s post-treatment, which was maintained for 120 days. The ivermectin peak pl asma concentration (28.5 ng ml(-1)) was attained at 15 days post-administra tion of the bolus. The faecal ivermectin concentration rose to a maximal co ncentration of 4.1 mu g g(-1) at four days post-treatment, dropping to a st eady-state concentration of around 1.18 mu g g(-1) which was maintained up to 120 days post-treatment. Ivermectin was detected in both plasma (0.05 ng ml(-1)) and faeces (2.67 ng g(-1)) up to 160 days. The high levels of iver mectin recovered in faeces indicate that a large proportion of the dose rel eased by the bolus (80 to 90 per cent) is excreted in faeces.