Transmission of Babesia spp by the cattle tick (Boophilus microplus) to cattle treated with injectable or pour-on formulations of ivermectin and moxidectin

Objective To assess the efficacy of ivermectin and moxidectin to prevent tr ansmission of Babesia bovis and Babesia bigemina by Boophilus microplus to cattle under conditions of relatively intense experimental challenge. Design Naive Bos taurus calves were treated with either pour-on or injectab le formulations of either ivermectin or moxidectin and then exposed to larv ae of B microplus infected with B bovis or larvae or adults of B microplus infected with B bigemina. One calf was used for each combination of haemopa rasite, B microplus life stage, drug and application route. Procedure Groups of calves were treated with the test drugs in either pour- on or injectable formulation and then infested with B microplus larvae infe cted with B bovis or B bigemina. B bigemina infected adult male ticks grown on an untreated calf were later transferred to a fourth group of animals. Infections were monitored via peripheral blood smears to determine haemopar asite transmission. Results Cattle treated with either pour-on or injectable formulations of iv ermectin and moxidectin became infected with B bovis after infestation with infected larvae. Similarly, larvae infected with B bigemina survived to th e nymphal stage to transmit the haemoparasite to animals treated with each drug preparation. Cattle treated with pour-on formulations of ivermectin an d moxidectin then infested with adult male ticks infected with B bigemina d id not become infected with B bigemina whereas those treated with the injec table formulations of ivermectin and moxidectin did show a parasitaemia. Conclusions Injectable or pour-on formulations of ivermectin and moxidectin do not prevent transmission of Babesia to cattle by B microplus. Use of th ese drugs can therefore not be recommended as a primary means of protecting susceptible cattle from the risk of Babesia infection.