PERSISTENT EFFICACY OF DORAMECTIN AGAINST EXPERIMENTAL CHALLENGE WITHOSTERTAGIA-OSTERTAGI IN CATTLE

Two studies were conducted in North America to evaluate the persistent efficacy of doramectin injectable solution against experimental chall enge with infective larvae of Ostertagia ostertagi. In both studies, f our groups of 10 randomly-assigned calves, negative for trichostrongyl e-type eggs on fecal examination, were treated subcutaneously in the m idline of the neck with saline (1 mi 50 kg(-1)) on Day 0 or doramectin (200 mu g kg(-1) = 1 mi 50 kg(-1)) on Day 0, 7, or 14. Two additional calves from the same pool of animals were randomly assigned as larval -viability monitors and received no treatment. Beginning on Day 14 and continuing through Day 28, the 40 treated calves each were given simi lar to 1000 infective larvae of O. ostertagi by gavage daily; the two larval-viability monitors were inoculated in a similar manner with sim ilar to 30 000 larvae as a single dose on Day 28. Animals were slaught ered on Day 42 in one study and on Days 42, 43, or 46 in the second. T he abomasum from each calf was harvested and processed for worm recove ry. A 2% aliquot of abomasal contents plus wash was examined for worm quantification and identification. Geometric mean O. ostertagi burdens were calculated from the log (O. ostertagi count + I) and were used t o estimate percentage reduction. In both studies, doramectin injectabl e solution was greater than or equal to 99.6% efficacious in reducing infection resulting from challenge with infective larvae of O. osterag i for at least 21 days posttreatment; by 28 days posttreatment, effica cy was 87.3% in one study and 99.7% in the other. (C) 1997 Elsevier Sc ience B.V.