Stability of ivermectin in rumen fluids

To determine whether ivermectin is metabolized in the rumen, in vitro studies were conducted with the tritium-labelled H2B1a component of ivermectin in rumen fluid from sheep and cattle. No detectable metabolism occurred over 24 h in in vitro incubations at 38 degrees C. The viability of the microbes in the rumen fluids was demonstrated by the conversion of 17% and 11% of [14C]cellulose to 14CO2 in 24 h in the incubations with sheep and steer rumen fluids respectively. The results indicate that ivermectin is not metabolized in the rumen. Based on the lack of in vitro metabolism of ivermectin in rumen fluid, the similarity of in vitro liver microsomal metabolism with in vivo metabolism of the avermectins and the physicochemical properties of the avermectins, any disappearance of ivermectin in vitro from rumen fluid is probably a result of binding to solids or surfaces. Apparent discrimination by dung beetles, where observed, between control faeces and faeces from cattle or sheep treated with ivermectin or abamectin therefore must be attributable to chance, to factors unrelated to treatment or to factors such as changes in amino acid composition rather than the production of volatile metabolites of ivermectin.