Reduced efficacy of ivermectin against Ostertagia in a Dutch cattle herd

Abstract

A pilot study on reduced ivermectin efficacy against Ostertagia ostertagi following the detection of a high number of strongyle-type eggs in a 1.5-year-old bull during the first part of the pasturing period in 2015 was conducted. This finding was remarkable because of the pasturing history, treatment history and time after turnout (June). The study involved one beef cattle herd and followed as far as possible the World Association for the Advancement of Veterinary Parasitology guidelines regarding the faecal egg count reduction test. We observed poor ivermectin efficacy (65%) compared with the expected efficacy of >95%. This also has consequences for practitioners, who should ensure the correct application of anthelmintics and perform repeated faecal examinations following the use of parasitical agents for persistent problems. Our finding also should have consequences for the pharmaceutical industry: the administration of registered anthelmintics via the most efficacious route is preferable.