The crystal structure of ivermectin hemihydrate ethanolate has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. Ivermectin hemihydrate ethanolate crystallizes in space group C2 (#5) with a = 40.9374(10), b = 9.26951(6), c = 14.9488(2) Å, β = 73.047(1)°, V = 5426.12(8) Å3, and Z = 4. The structure consists of layers of ivermectin molecules parallel to the bc-plane. The water and ethanol molecules reside in small voids in the structure. The water molecule, the ethanol molecule, and hydroxyl groups act as donors in O–H⋯O hydrogen bonds. Several C–H⋯O hydrogen bonds were detected. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™.

An electrophysiological preparation of Ascaris suum pharyngeal muscle suitable for recording changes of input conductance using a 2-microelectrode current clamp and pharmacological study is described. The preparation is shown to contain a glutamate-gated Cl (ion sensitive) channel sensitive to the avermectin analogue, milbemycin D. The application of glutamate produces a dose-dependent increase in Cl conductance and the effect of glutamate is potentiated by milbemycin D. Milbemycin D also produced a dose-dependent increase in input conductance.

Very few studies have examined, at the field scale, the potential for faecal residues in the dung of avermectin-treated cattle to affect dung-breeding insects. The current study examined populations of dung beetles (Scarabaeidae: Aphodius) using pitfall traps baited with dung from untreated cattle on 26 fields across eight farms in southwest Scotland. The fields were grazed either by untreated cattle or by cattle treated with an avermectin product, i.e. doramectin or ivermectin. During the two-year study, significantly more beetles were trapped in fields grazed by treated cattle (n=9377 beetles) than in fields where cattle remained untreated (n=2483 beetles). Additional trials showed that beetles preferentially colonised dung of untreated versus doramectin-treated cattle. This may explain the higher captures of beetles in traps baited with dung of untreated cattle, which were located in fields of treated cattle. Given that Aphodius beetles avoided dung of treated cattle in the current study, the potential harmful effects of avermectin residues in cattle dung could be reduced through livestock management practices that maximise the availability of dung from untreated livestock in areas where avermectins are being used.

The mechanism underlying the ability of the anthelmintic avermectin to paralyse the nematode Ascaris is not yet fully understood. Using conventional two-electrode electrophysiological recording techniques we have demonstrated that micromolar concentrations of ivermectin block the inhibitory GABA response on the muscle cells of the parasitic nematode Ascaris. The ability of a number of avermectin derivatives to act as receptor antagonists for the Ascaris muscle GABA receptor has been determined. This provides useful information to compare with the in vivo anthelmintic potency of these compounds. Abamectin, the most potent anthelmintic, was the most potent compound at inhibiting the GABA response whilst octahydroavermectin, a compound which lacks anthelmintic activity, did not block the GABA receptor. This is consistent with the notion that the GABA receptor antagonist properties of the avermectins could contribute to their anthelmintic action.

In this chapter we summarize the available data on a novel class of ligand-gated anion channels that are gated by the neurotransmitter glutamate. Glutamate is classically thought to be a stimulatory neurotransmitter, however, studies in invertebrates have proven that glutamate also functions as an inhibitory ligand. The bulk of studies conducted in vivo have been on insects and crustaceans, where glutamate was first postulated to act on H-receptors resulting in a hyperpolarizing response to glutamate. Recently, glutamate-gated chloride channels have been cloned from several nematodes and Drosophila. The pharmacology and electrophysiological properties of these channels have been studied by expression in Xenopus oocytes. Studies on the cloned channels demonstrate that the invertebrate glutamate-gated chloride channels are the H-receptors and represent important targets for the antiparasitic avermectins.

Ligand-gated chloride channels, including the glutamate-(GluCl) and GABA-gated channels, are the targets of the macrocyclic lactone (ML) family of anthelmintics. Changes in the sequence and expression of these channels can cause resistance to the ML in laboratory models, such as Caenorhabditis elegans and Drosophila melanogaster. Mutations in multiple GluCl subunit genes are required for high-level ML resistance in C. elegans, and this can be influenced by additional mutations in gap junction and amphid genes. Parasitic nematodes have a different complement of channel subunit genes from C. elegans, but a few genes, including avr-14, are widely present. A polymorphism in an avr-14 orthologue, which makes the subunit less sensitive to ivermectin and glutamate, has been identified in Cooperia oncophora, and polymorphisms in several subunits have been reported from resistant isolates of Haemonchus contortus. This has led to suggestions that ML resistance may be polygenic. Possible reasons for this, and its consequences for the development of molecular tests for resistance, are explored.

The effects of avermectin exposure on natural populations of the yellow dung fly, Scathophaga stercoraria Linnaeus, were investigated at the field scale on farms in south-west Scotland. Pastures forming the focus of the study were grazed with either untreated cattle or cattle receiving standard, manufacturer-recommended treatment regimes of an avermectin product. Flies were sampled between April and July in 2002 and 2003 using dung-baited pitfall traps. Abundance and wing asymmetry in S. stercoraria populations were examined in relation to a range of environmental and management variables (including avermectin exposure, pasture management intensity, weather and season). Data used for abundance analyses were collected in fields where treated cattle had been dosed with either doramectin or ivermectin, while the data for the asymmetry analyses were from a subset of fields where treated cattle had been dosed with doramectin only. While abundance of S. stercoraria varied significantly between years and with season, there was no difference in their abundance between fields grazed by avermectin-treated or untreated cattle. Asymmetry was significantly higher in fly populations in fields grazed by doramectin-treated cattle, suggesting that exposure to doramectin during development could have imposed some degree of environmental stress. While these results suggest that exposure to doramectin residues in dung on grazed pastures may have sublethal effects on the insects developing in that dung, there was no evident avermectin effect on the abundance of adult S. stercoraria occurring in the pastures.