Combination anthelmintics effectively control ML-resistant parasites; a real-world case history.

Routine investigation into an ill-thrift situation with grazing cattle led to the discovery of the first reported case of macrocyclic lactone (ML) resistance in cattle in the USA. Research revealed that resistant parasites were originating on pastures in southeastern USA and were not an anomalous resident population on Wisconsin pastures. Prior to using anthelmintics in combination, ML-resistant Cooperia and Haemonchus spp. were shown to survive treatment with single-active MLs and were being transported in shipped cattle and seeding summer grazing pastures. Treatment and management strategies implemented in 2011 and 2012 suggested that ML-surviving parasites were introduced into the conditioning facility and surviving treatment with ML. Data also demonstrated the use of combination ML+oral levamisole was highly effective in minimizing the transport of ML-surviving parasites from southeastern USA to Wisconsin pastures. The value of fecal egg count monitoring and PCR evaluation of nematode species under production conditions are confirmed.