Evaluation of pyriproxyfen in cattle by oral treatment: An alternative to control Haematobia irritans

The horn fly, Haematobia irritans, is a bovine ectoparasite that causes large losses to cattle breeders, through lower meat and milk production and hide damage. Currently, the control of this parasite is mainly through chemicals. However, the indiscriminate use of these substances generates resistance. Pyriproxyfen belongs to an insect growth disruptors class with mechanisms of action for the control of immature forms of the insect. The aim of this study was to investigate the potential of pyriproxyfen administered orally to cattle for the control of the horn fly. In vitro bioassays were carried out by evaluating the number of adults emerged from 30 eggs per replicate, in triplicate, added in a substrate (fresh cattle feces) spiked with pyriproxyfen solutions in the concentration range of 2-130 ppb (ng.g^-1). Probit analysis estimated LC₅₀ value of 7.89 ppb and LC₉₀ value of 70.08 ppb. The doses used in the in vivo evaluation were established based on the LC₉₀ values and the anatomical and physiological aspects of the bovine digestive tract. Capsules containing pyriproxyfen at doses of 2.5 mg (G2.5) and 40 mg (G40) were produced and administered orally by bolus applicator for 12 days. The efficacy of pyriproxyfen against the immature forms of H. irritans was determined by incubating eggs in vitro in the feces collected on days 0, +3, +6, +10 and +13. Quantification of pyriproxyfen in feces was performed by UPLC-MS/MS, finding concentrations ranging from 13.4 to 22.6 ppb for G2.5 and between 268.5 to 509.0 ppb for G40. Pyriproxyfen administered orally is eliminated in the active form in the fecal mass and at a dose of 40 mg.day^-1 (0.1 mg/kg/day) generates fecal concentrations able to produce 100 % prevention of adults emergence of H. irritans.