The efficacy and plasma profiles of abamectin plus levamisole combination anthelmintics administered as oral and pour-on formulations to cattle.

In phase I, faecal egg count reduction tests (FECRT) were conducted on six commercial cattle farms to compare the performance of two pour-on and one oral combination anthelmintic. Groups of 12-15 calves were sampled for faecal nematode egg count (FEC) before treatment with either abamectin oral, levamisole oral, an abamectin+levamisole oral combination or one of two abamectin+levamisole combination pour-ons. Samples were collected again 14days after treatment to calculate the percentage reduction in FEC. The proportions of infective stage larvae (L3) in faecal cultures were used to apportion egg counts to, and calculate efficacy against, the main parasite genera. Abamectin oral was effective against Ostertagia except on one farm where resistance was indicated, but had reduced efficacy against Cooperia on four farms. Levamisole oral was effective against Cooperia on all farms, but had variable efficacy against Ostertagia. The abamectin+levamisole oral was effective against both species on all farms. The abamectin+levamisole pour-ons were effective on some farms but not on others. In particular, pour-on 2 failed to achieve 95% efficacy in 45% of evaluations, 4/6 against Cooperia and 1/5 against Ostertagia. On some farms the combination pour-ons were less effective than their constituent actives administered alone as orals. In phase II, 8 groups of 6 calves, grazing parasite-free pasture, were infected with putatively ML-resistant isolates of Cooperia oncophora and Ostertagia ostertagi. Once infections were patent groups were treated with oral or pour-on formulations of abamectin alone, levamisole alone, abamectin+levamisole (two pour-ons) or remained untreated. Blood samples were collected for analysis and after 8days all calves were euthanized and abomasa and intestines recovered for worm counts. All treatments were effective against O. ostertagi and all treatments containing levamisole were effective against C. oncophora. Animals treated with the oral combination had higher Cmax and AUC values for abamectin in plasma than animals treated orally with abamectin alone. In contrast, animals treated with the combination pour-ons tended to have lower plasma levels for abamectin than those treated with abamectin alone as a pour-on, with differences in the Cmax and AUC values approaching statistical significance (p-values ≤0.07). There were no differences detected in plasma concentrations of levamisole. The inconsistent and sometimes poor efficacy of the combination pour-ons on-farm is likely due to reduced levels of abamectin in the plasma and hence less active reaching the target worms in the gut.