Geometric means provide a biased efficacy result when conducting a faecal egg count reduction test (FECRT)

The process of conducting a faecal egg count reduction test was simulated to examine whether arithmetic or geometric means offer the best estimate of efficacy in a situation where the true efficacy is known. Two components of sample variation were simulated: selecting hosts from the general population which was modelled by the negative binomial distribution (NBD), and taking an aliquot of faeces from the selected host to estimate the worm egg count by assuming a Poisson distribution of sample counts. Geometric mean counts were determined by adding a constant (C) to each count prior to log transformation, C was set at 25, 12 or 1. Ten thousand Monte Carlo simulations were run to estimate mean efficacy, the 2.5% (lower) and the 97.5% (upper) percentile based on arithmetic or geometric means. Arithmetic means best estimated efficacy for all different levels of worm aggregation. For moderate levels of aggregation and with C = 1 the geometric mean substantially overestimated efficacy. The bias was reduced if C was increased to 25 but the results were no better than those based on arithmetic means. For very high levels of aggregation (over-dispersed populations) the geometric mean underestimated efficacy regardless of the size of C. It is recommended that the guidelines on anthelmintic resistance be revised to advocate the use of arithmetic means to estimate efficacy

Anthelmintic resistance

Since the introduction of broad-spectrum anthelmintics these drugs have been the major means of managed control of nematode parasites. Anthelmintics have been used particularly in livestock for the control of economically important parasites. Because of heavy reliance on these drugs and their widespread use, anthelmintic-resistant parasites have been selected and anthelmnintic resistance has emerged as a problem amongst parasites of livestock. The same compounds have also been used to treat parasitic infections in human populations and there are fears that resistance will develop in these parasites too. Although resistance is defined formally below, a brief definition is: the ability of helminths to survive doses of drug that would normally remove them. Resistance is genetic, that is, resistant worms carry alleles for resistance which are inherited by the next generation

Kinetics of expulsion of Haemonchus contortus from sheep and jirds after treatment with closantel

Experiments were conducted in sheep after intramuscular treatment with closantel and in jirds after oral treatment with closantel to determine when expulsion of established H. contortus commences. Expulsion starts at about 8 h in sheep and coincides with the onset of reduced motility in worms recovered from the abomasum. In jirds, expulsion starts by 2 h after treatment. Experiments also conducted in jirds showed that infective larvae are first killed by circulating closantel 3 days after infection, when blood feeding starts, and that by 8 days 80% of larvae are lost

Characterisation of an acetylcholine receptor gene of Haemonchus contortus in relation to levamisole resistance

The anthelmintic drug levamisole is thought to bind to nicotinic acetylcholine receptors of nematodes. It is possible that resistance to this drug is associated with either a change in binding characteristics or a reduction in the number of nicotinic acetylcholine receptors. Therefore, the molecular mechanism of levamisole resistance in the parasitic nematode Haemonchus contortus was studied by isolating and characterising cDNA clones encoding a putative ligand binding nicotinic acetylcholine receptor subunit, HCA1, of two susceptible and one levamisole resistant population. HcA1 is related to unc-38, a nicotinic acetylcholine receptor subunit gene associated with levamisole resistance in Caenorhabditis elegans. Although extensive sequence analyses of hca1 sequences revealed poly-morphism at amino acid level, no association with levamisole resistance could be detected. Restriction fragment length polymorphism analyses confirmed that, although polymorphism was detected, no selection of a specific allele of heal has taken place during selection for levamisole resistance in various levamisole resistant populations