Evaluation of insecticide ear tags containing ethion for control of pyrethroid resistant Haematobia irritans (L.) on dairy cattle

The characterization of Lucilia cuprina acetylcholinesterase as a drug target, and the identification of novel inhibitors by high throughput screening

Acetylcholinesterase (AChE, EC3.1.1.7.) is the molecular target for the carbamate and organophosphate pesticides that are used to combat parasitic arthropods. In this paper we report the functional heterologous expression of AChE from Lucilia cuprina (the sheep blowfly) in HEK293 cells. We show that the expressed enzyme is cell-surface-exposed and possesses a glycosyl-phosphatidylinositol membrane anchor. The substrates acetyl-, propionyl- and butyrylthiocholine (AcTC, PropTC, ButTC), and also 11 further thiocholine and homo-thiocholine derivatives were chemically synthesized to evaluate and compare their substrate properties in L. cuprina AChE and recombinant human AChE. The Michaelis-Menten constants K(M) for AcTC, PropTC and ButTC were found to be 3-7-fold lower for the L. cuprina AChE than for the human AChE. Additionally, 2-methoxyacetyl-thiocholine and isobutyryl-thiocholine were better substrates for the insect enzyme than for the human AChE. The AcTC, PropTC and ButTC specificities and the Michaelis-Menten constants for recombinant L. cuprina AChE were similar to those determined for AChE extracted from L. cuprina heads, which are a particularly rich source of this enzyme. The median inhibition concentrations (IC(50) values) were determined for 21 organophosphates, 23 carbamates and also 9 known non-covalent AChE inhibitors. Interestingly, 11 compounds were 100- to >4000-fold more active on the insect enzyme than on the human enzyme. The substrate and inhibitor selectivity data collectively indicate that there are structural differences between L. cuprina and human AChE in or near the active sites, suggesting that it may be possible to identify novel, specific L. cuprina AChE inhibitors. To this end, a high throughput screen with 107,893 compounds was performed on the L. cuprina head AChE. This led to the identification of 195 non-carbamate, non-organophosphate inhibitors with IC(50) values below 10μM. Analysis of the most potent hit compounds identified 19 previously unknown inhibitors with IC(50) values below 200nM, which were up to 335-fold more potent on the L. cuprina enzyme than on the human AChE. Some of these compounds may serve as leads for lead optimization programs to generate fly-specific pesticides.

New spot-on formulation containing chlorpyrifos for controlling horn flies on cattle: laboratory model of insecticide release and field trial

A highly viscous formulation containing chlorpyrifos (RET) was evaluated under laboratory, pre-field, and field conditions, and compared against ear tags with organophosphorus insecticides. Laboratory bioassays were performed using Musca domestica L. and a thin layer chromatography (TLC) plate of reversed phase silica gel modeling a lipophilic surface. Insects were exposed to the insecticide vapors of both formulations, directly and indirectly to test for lateral diffusion. Knockdown time 50% (KT(50)) values were determined as toxicological indicators of insecticide release. Minimum KT(50) values of the direct effect of both formulations on horn flies were reached 4 weeks after being applied. The KT(50) effect of migrated insecticides showed that RET formulation had a maximal effectiveness between the fourth and tenth last week. The KT(50) effect of the insecticide migrating from ear tags decreased during the last 2 weeks of the experiment, and the KT(50) effect of the laterally migrated insecticide was significantly higher for the RET formulation during this period. A pre-field bioassay was performed by exposing pieces of rabbit leather with both formulations and recording the KT(50). At the end of the experiment, the KT(50) effect of laterally migrated insecticide was significantly higher for the RET formulation. Regarding vapor emission, as a general trend the KT(50) effect of ear tags was greater than for the RET formulation. To evaluate the horn fly infestation in the field bioassays, photographs of the animal were taken. The results shows that RET provided significant control for 11 weeks while the ear tags provided protection until the 12th week.

Insecticide resistance in the horn fly: alternative control strategies

The horn fly, Haematobia irritans (Linnaeus 1758) (Diptera: Muscidae) is one of the most widespread and economically important pests of cattle. Although insecticides have been used for fly control, success has been limited because of the development of insecticide resistance in all countries where the horn fly is found. This problem, along with public pressure for insecticide-free food and the prohibitive cost of developing new classes of compounds, has driven the investigation of alternative control methods that minimize or avoid the use of insecticides. This review provides details of the economic impact of horn flies, existing insecticides used for horn fly control and resistance mechanisms. Current research on new methods of horn fly control based on resistant cattle selection, semiochemicals, biological control and vaccines is also discussed.