The Effect of Treating Beef Cattle on Pasture with Ivermectin on the Prevalence and Intensity of Thelazia spp. (Nematoda: Thelazioidea) in the Vector, Musca autumnalis (Diptera: Muscidae)

Abstract

Trihedron sticky traps were used to monitor the prevalence and intensity of Thelazia spp. in face flies associated with 2 groups of cattle. Traps were set for 24 hr once a week beginning 2 wk before cattle were turned out to pasture following winter housing and continued until flies reached diapause in October. The 50 heifers in group 1 received 1 treatment with ivermectin (Ivomec Pour-On for cattle, 500 micrograms/kg body weight) on the day of turnout onto pasture. The 50 cows and their calves in group 2 were not treated. Treating cattle with ivermectin delayed the onset of infection in Musca autumnalis by 20 days and reduced the prevalence to 2.2% compared to 26.9% in flies from the control pasture. Although residues of ivermectin eradicate many fly larvae developing in cattle feces, there was no significant difference in the number of flies counted on the faces of cattle or in the number of flies caught on sticky traps on the 2 pastures (P > 0.05).